PHY (N)-220 & PHY (N)-220L

B. Sc. III Semester

Optics
&
LABORATORY COURSE

DEPARTMENT OF PHYSICS
SCHOOL OF SCIENCES
UTTARAKHAND OPEN UNIVERSITY
HALDWANI
PHY (N)-220 Optics &
& PHY (N) - 220L LABORATORY COURSE

Board of Studies and Programme Coordinator

Board of Studies
Prof. Manoj Harbola
Prof. P.B. Bisht Department of Physics, I.I.T Kanpur
Department of Physics, I.I.T. Madras
Prof. P.S. Bisht
Prof S.R. Jha Department of Physics, SSJ University,
Professor of Physics , School of Sciences
Almora, Uttarakhand
I.G.N.O.U, Maidan Garhi, New Delhi
Dr. Kamal Devlal
Prof. P D Pant Associate Professor and Programme
Director, School of Sciences
Coordinator
Uttarakhand Open University, Haldwani
Department of Physics
Prof. G.C. Joshi
Uttarakhand Open University
Department of Physics, G.B. Pant University of
Dr. Vishal Sharma
Agriculture & Technology
Assistant Professor, Department of Physics
Dr. Gauri Negi Uttarakhand Open University
Assistant Professor, Department of Physics
Dr. Rajesh Mathpal
Uttarakhand Open University
Assistant Professor (AC), Department of
Dr. Meenakshi Rana Physics
Assistant Professor (AC), Department of Physics
Uttarakhand Open University
Uttarakhand Open University

Department of Physics (School of

Sciences) Dr. Meenakshi Rana (Assistant Professor
Dr. Kamal Devlal (Associate Professor)
(AC))
Dr. Vishal Sharma (Assistant Professor)
Dr. Rajesh Mathpal (Assistant Professor (AC))
Dr. Gauri Negi (Assistant Professor)

Unit writing and Editing (Optics)

Editing Writing
Dr. M. Sharma 1. Dr. Piyush Sinha
Department Physics Department Physics
Pt. L.M.S. Govt. P.G. College. HNB University Campus, Pauri, Uttarakhand
Rishikesh, Uttarakhand 2. Dr. Kamal Devlal
School of Sciences, Uttarakhand Open University,
Dr. Meenakshi Rana Haldwani
School of Sciences 3. Dr. M. Sharma
Uttarakhand Open University Department Physics
Haldwani Pt. L.M.S. Govt. P.G. College. Rishikesh, Uttarakhand
4. Dr. Santosh K. Joshi
Department Physics
University of Petroleum and Energy Studies,
Prem Nagar, Dehradun-248007, Uttarakhand
5. Dr. C. C. Dhondiyal
Department Physics
M.B. G.P.G.C. Haldwani, Nainital, Uttarakhand
6. Dr. B S Tiwari
Department Physics
University of Petroleum and Energy Studies,
Prem Nagar, Dehradun-248007, Uttarakhand

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Unit writing and Editing (LABORATORY COURSE)

Editing Writing

Dr. Kamal Devlal Prof. H M Agrawal

Associate Professor, Department of Physics Department of Physics
School of Sciences, College of Basic Sciences and Humanities
Uttarakhand Open University Govind Ballabh Pant University of Ag &
Technology Pantnagar, Uttrakhand-263145
Dr. Meenakshi Rana Dr. D K Upreti
School of Sciences Asstt. Professor, Department Physics
Uttarakhand Open University G.P.G.C. Ranikhet, Almora, Uttarakhand
Haldwani

Course Title and Code : Optics (PHY (N)-220) &

LABORATORY COURSE (PHY (N)-220L)
ISBN :
Copyright : Uttarakhand Open University
Edition : 2024
Published By : Uttarakhand Open University, Haldwani, Nainital- 263139
Printed By : htpp://uou.ac.in

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PHY (N)-220 Optics &
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PHY (N)-220 & PHY (N)-220L

Optics
&
LABORATORY COURSE

DEPARTMENT OF PHYSICS
SCHOOL OF SCIENCES
UTTARAKHAND OPEN UNIVERSITY

Phone No. 05946-261122, 261123

Toll free No. 18001804025
Fax No. 05946-264232, E. mail info@uou.ac.in
htpp://uou.ac.in

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PHY (N)-220 Optics &
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Contents
Course 2: Optics
Course code: PHY (N)-220
Credit: 3

Unit Block and Unit Title Page

number number
Block 1: Geometrical optics 1-59
1 1-22
Fermat's principle and its Applications: Optical Path, Fermat's principle of
Extreme Path,applications of Fermat's principle, to deduce law of reflection and
refraction etc., Gauss’s general theory of image formation.
2 Refraction by spherical surfaces and Image formation: Sign Convention, 23-41
Coaxial Optical System , Cardinal Points of an Optical System , Rules of Image
Formation Magnification of a lens system, Helmholtz Lagrange’s Magnification
Formula Some Important Relations, Newton’s Formula
3 42-59
Thick lens, lenses combinations and telescopes: Cardinal Points of a Thick
Lens, Focal Length of a Thick Lens, Variation of Focal Length of a Thick Bi-
convex Lens, Power of a Thick Lens, Telescopes, Astronomical Telescope,
Terrestrial Telescope, Newtonian Reflecting Telescope, Cassegrain Reflecting
Telescope, Advantages of Reflecting Type Telescope over Refracting Type
Telescope
BLOCK 2 Interference of light waves 60-118
4 60-84
Interference of light: Wave Nature of Light, Principle of Superposition,
Introduction to Interference of light, Classification of Interference, Young’s
experiment, coherence Length and Coherence Time, spatial and temporal
coherence; Interference Due to Thin Sheet ,intensity distribution, biprism and
Fresnel’s biprism, Experimental Arrangement of Biprism Apparatus, Lateral
Shift, Measurement of Wave Length of Light (λ) by Fresnel’s Biprism,
Interference with White Light
5 85-107
Interference in thin film and Newton’s rings : Interference by division of
amplitude, interference in thin film, wedge shaped film, Necessity of Extended
Source for Interference Due to Thin Films , Colours of Thin Films, Classification
of Fringes, Fringes of Equal Thickness, Fringes of Equal Inclination, Newton’s
rings in reflected and refracted light, Experiment Arrangement for Reflected
Light, Formation of Bright and Dark Rings, Diameter of Bright and Dark Rings,
Determination of Wavelength of a Monochromatic Light Source, Determination
of Refractive Index of a Liquid by Newton’s Rings Experiment, Newton’s Rings
in Case of Transmitted Light
6 108-118
Multiple beam interferometry: Interferometery, Fringes of Equal Inclination
(Haidinger Fringes), Michelson’s Interferometer, Construction, Working,
Formation of Fringes, Determination the Difference between Two Neighboring
Wavelengths, Determination of Refractive Index of a Material, Michelson-
Morley Experiment and Its Result

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BLOCK3 Diffraction of light waves 119-174

7 119-146
Diffraction of light waves and Fresnel diffraction: Diffraction of Light,
Difference between Interference and Diffraction,Types of diffraction, Fresnel
and Fraunhofer class, Fresnel’s half period zones; explanation of rectilinear
propagation of light; Rectilinear Propagation of Light, zone plate, comparison of
zone plate with lens, wave front, diffraction at a straight edge.
8 Fraunhofer diffraction: Condition for Fraunhofer diffraction, Fraunhofer 147-162
diffraction due to a single slit, Fraunhofer diffraction due to double slit,
Fraunhofer diffraction at circular aperture (qualitative), Plane diffraction grating
(transmission), diffraction due to a grating of N parallel slits, Maximum number
of order available in a grating, missing orders.
9 Resolution and resolving power: Introduction, resolving power, Rayleigh 163-174
criterion of resolution; resolving power of transmission grating, resolving power
of prism, resolving power of telescope, resolving power of microscope.
BLOCK 4: Polarization of light waves 175-250
10 175-192
Polarization: Introduction to Polarization, Types of Polarization, Concept of
plane polarized light, Pictorial Representation of Plane Polarized Light , Plane of
Vibration and Plane of Polarization of Plane Polarized Light, Methods of
Production of Plane Polarized Light, Plane Polarized Light by Reflection,
circularly and elliptically polarized light, Malus law, Brewster law.
11 Double refraction: Double Refraction or Birefringence, Geometry of Calcite 193-216
Crystal, Optic Axis, Principal Section and Principal Plane, Optic Axis,
Principal Section and Principal Plane, Huygen’s Explanation of Double
Refraction in Uniaxial Crystal, Positive and Negative Crystals, Nicol Prism
Action of Polarizer on Light of Different Type of Polarizations, Huygen’s
construction for uniaxial crystals; Double Images Polarizing Prisms, Dichroism
or Selective Absorption, polaroids and their uses, Polarization by Scattering.
12 Plane, circularly and elliptically Polarized light: Introduction, retardation 217-233
plates, Production and analysis of plane, circularly and elliptically polarized light
by retardation plates, Experimental Arrangements for the Production of
Polarized Light , 6Application of Retardation Plates: Analysis of Polarized Light
,and Rotatory polarization.
13 234-250
Optical activity: Introduction, rotatory polarization, optical activity; Laevo-
rotatory Substances, Dextro-rotatory Substances, Fresnel’s explanation of optical
activity; Specific Rotation, Polarimeters, Biquartz and Laurent’s half shade
polarimeter.

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Course 1: Laboratory Course

Course code: PHY (N)-220L

Credit: 1

EXPERIMENT EXPERIMENT NAME PAGE

NUMBER NUMBER
1 STUDY OF MALUS LAW 1-5
2 FOCAL LENGTH BY NODAL SLIDE 6-14
3 LOCATION OF CARDINAL POINTS BY NODAL SLIDE 15-25
METHOD
4 HARTMANN’S FORMULA USING PRISM SPECTOMETER 26-31
5 VERIFICATION OF CAUCHY’S DESPERSION FORMULA 32-39
6 DISPERSIVE POWER OF A PRISM 40-48
7 DISPERSIVE POWER OF A GRATING 49-56
8 ZONE PLATE EXPERIMENT 57-66

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UNIT 1: FERMAT’S PRINCIPLE AND ITS

APPLICATION
CONTENTS
1.1 Introduction
1.2 Objectives
1.3 Optical Path
1.3.1 Optical path Time Interval
1.4 Fermat’s Principle of Extreme Path
1.5 Application of Fermat’s Principle
1.5.1 Laws of Reflection
1.5.2 Laws of Refraction
1.5.3 Refractive Index
1.5.4 Total Internal Reflection
1.6 Gauss General Theory of Image Formation
1.7 Summary
1.8 Glossary
1.9 References
1.10 Suggested Readings
1.11 Terminal Questions
1.12 Answers

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1.1 INTRODUCTION
Light is a form of energy that enables us to see and perceive objects with our eyes.
Scientifically, light is an electromagnetic wave of wavelength belonging to visible part
(wavelength of 400 nm to 750 nm) of electromagnetic spectrum. We see objects either by the
light they produce or by the light they reflect from other objects. Objects that produce their
own light are said to be luminous. Examples are the sun, electric bulbs, candle light etc.,
whereas, non-luminous objects do not produce their own light. We can see these objects only
when light fall on them from other sources and it is thrown back or reflected into our eyes.
An important example is that of the moon, which shines in the night because it reflects light
coming from the sun and not because it is luminous.
Optics is the science or more specifically a branch of physics in which we study the
behavior and properties of light. The study also includes the interaction of light with matter
and construction of instruments that use or detect it. For the sake of convenience the subject
of optics can be divided into two parts: (i) physical or wave optics, which deals with the wave
nature of light. It accounts for optical effects such as diffraction and interference etc., and (ii)
geometrical or ray optics, which deals with the formation of images by lenses and mirrors and
their combinations on the basis of certain geometrical laws obeyed by light.
The present block of the course ‘optics’ is dedicated to the geometrical optics only
hence, in the following sections, we will concentrate on its learning in detail.
Geometrical optics describes light propagation in terms of rays. The rays are the
approximate paths along which light propagates under certain circumstances. The basic
assumptions of geometrical optics include, that light rays:
 Propagate in straight line paths in a homogeneous medium, called as rectilinear
propagation.
 Bend, and in particular circumstances may split into two, at the interface between two
dissimilar media
 Follow curved paths (iterative bending) in a medium in which refractive index changes
 May be absorbed or reflected at glossy surfaces
There are certain laws which explain the above assumptions. These laws form the basis
of geometrical optics and are called fundamental laws. The fundamental laws are
1. The laws of rectilinear propagation of light
2. The laws of reflection of light
3. The laws of refraction of light
A general principle which covers all these laws is known as Fermat’s principle of least time.

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1.2 OBJECTIVES
After studying this unit, you will be able to:
 know Fermat’s principle of least time
 familiarize with incident ray, reflected ray and refracted ray
 familiarize with angle of incidence, reflection and refraction
 state laws of reflection
 state laws of refraction- Snell’s law
 define refractive index
 explain total internal reflection as a special case of refraction
 understand Gauss general theory of image formation
Before we discuss the Fermat’s principle of least action, let us know about the terms –
optical path and optical path time interval.

1.3 OPTICAL PATH

Optical path is the path taken by light ray through an optical system. It is also known as
the product of refractive index (see section 1.5.3) of the medium, i.e. 𝜇 and the distance
travelled S by the light ray in medium, i.e.,
𝜇 S = Optical path
It is the path travelled by the light ray in air, in the same time, it takes to traverse the distance
S of medium or we can say it to be the equivalent air path.
If v is the velocity of light and t is the time taken for covering the distance S, then,
S=vt ……. (1.1)
But we know that the refractive index (section 1.5.3)
𝑐 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑢𝑢𝑚
𝜇= = ……. (1.2)
𝑣 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚

Thus
𝑐
𝑣= ……. (1.3)
𝜇

Substituting equation (1.3) in equation (1.1), we get,

𝑐
S= 𝑡 ……. (1.4)
𝜇

or 𝜇 𝑆 = 𝑐 𝑡 = ∆ = 𝑜𝑝𝑡𝑖𝑐𝑎𝑙 𝑝𝑎𝑡ℎ ……. (1.5)

1.3.1 Optical Path Time Interval

When a ray travels S1, S2, S3, S4 etc. distances in media of refractive indices
𝜇1 , 𝜇2 , 𝜇3 , 𝜇4 etc., then the optical path is given by
∆ = 𝜇1 S1 + 𝜇2 S2 + 𝜇3 S3 + 𝜇4 S4 + …..

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= ∑𝑖 𝜇𝑖 𝑆𝑖 ……. (1.6)
If dS is the small distance covered by light between two points P and Q in a medium and v is
its velocity in that particular medium, then the mathematical form of Fermat’s principle of
extreme path is defined as
𝑄 𝑑𝑆
∫𝑃 = maximum or minimum or stationary
𝑣
𝑄 𝜇𝑑𝑆 𝑐
or ∫𝑃 = maximum or minimum or stationary [𝐻𝑒𝑟𝑒 𝜇 = 𝑣]
𝑐

Since velocity of light in vacuum (c) is constant, the Fermat’s principle of extreme path takes
the form
𝑄
∫𝑃 𝜇 𝑑𝑆 = maximum or minimum or stationary ……. (1.7)

Where 𝜇dS is the optical path in a medium of refractive index 𝜇.

1.4. FERMAT’S PRINCIPLE OF LEAST TIME

In 1658 Pierre De Fermat, a French mathematician enunciated the principle of least
time in the following way:
A ray of light in passing from one point to another through a set of media by any
number of reflections or refractions chooses a path along which the time taken is
minimum or the least.
Based on this principle, the laws of rectilinear propagation, the laws of reflection and
refraction can be derived (see section 1.5). However in some cases, it has been found that the
time taken by light is not minimum but maximum or else it is neither maximum nor minimum
but it is stationary. This is found in case of image formation by lenses, in which all rays
starting from an object point, reaching to the image point; choose the path of maximum or
minimum time. Therefore, the modified form of Fermat’s principle of least time is known as
Fermat’s principle of stationary time or Fermat’s principle of extreme path, which may be
stated as follows:
A ray of light in passing from one point to another through a set of media by any
number of reflections or refractions chooses a path for which the time taken is either
minimum or maximum or stationary. The mathematical verification of this law is provided
in the later sections.

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1.5 APPLICATION OF FERMAT’S PRINCIPAL

On the basis of Fermat’s principle you can derive laws of reflection and refraction.
1.5.1. Laws of Reflection
When light ray falls on a smooth polished surface separating two media, it comes back
in the same medium, the phenomenon is called reflection and the boundary is called
reflecting surface. The light obeys following two laws of reflection.
First law:
The incident ray, reflected ray and the normal to the surface at the point of
incidence all lie in one plane. You can prove this law in the following way.
Let the plane ABCD be normal to the plane mirror shown in figure 1.1. P is point object
imaged by mirror as P'. Consider a point M' on the plane mirror; but not on plane ABCD. Let
a ray PM' be reflected as M'P'. Draw a normal M'M on plane ABCD. Point M is the foot of
the normal on ABCD.

Fig. 1.1
Now, PMM' and P'MM' are right angle triangles. PM' and P'M' are respective hypotenuse.
Therefore, we have
PM' > PM and P'M' > P'M
But Fermat’s principle demands that the path followed must be the shortest, i.e., the light
would not travel along PM'P'. As we shift M' towards M the path of light ray becomes
shorter. It is seen that the shortest possible path is PM P', where the point of incidence M lies
on plane ABCD. PM and MP' are the incident and reflected rays. This proves the first law of
reflection.
Second law:
For a smooth surface, the angle of incidence is equal to the angle of reflection. You
can prove the second law as follows.
Assuming DD' is a reflecting plane shown in figure 1.2. Object P is imaged as P' and M is the
point of incident. The normal to the plane at this point is MN and is shown by dotted line.

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Fig. 1.2
PM and MP' are the incidence and reflected rays. Let I and r are the angles of incidence and
reflection respectively. Let us suppose distances
PD = a, P'D' = b, DM = x, DD' = c.
The ray of light travels in air from P to P'. Let the path PMP' be s, then the total distance
covered by light ray be
s = P MP' = PM + MP'
= (PD2 + DM2) + (D'M2 + D'P'2)
= (a2 + x2) + {(c – x)2 + b2} ……. (1.8)
It is evident that the path from P to P' remains the same even if the point of incidence M
shifts. Shifting of M changes x only. According to Fermat’s principle the path PMP' must be
either minimum or maximum. It means that the differential coefficient of s with respect to x
must be zero, i.e.
𝑑𝑠 1 2𝑥 1 2 (𝑐−𝑥)
= − =0
𝑑𝑥 2 √(𝑎2 + 𝑥 2 ) 2 √{(𝑐−𝑥)2 + 𝑏2

𝑥 (𝑐−𝑥)
or =
√(𝑎2 +𝑥 2 ) √{(𝑐−𝑥)2 + 𝑏2 }

From figure 1.2, we have,

𝑥
= sin 𝑖, (c – x) /√{(c – x)2 + b2} = sin r
√(𝑎2 + 𝑥 2 )

∴ sin i = sin r ……. (1.9)

or i=r
Hence you can see that the second law of reflection is derived from Fermat’s principle.
𝑑2𝑠
Further the second differential co-efficient of s, i.e., comes out to be positive, which
𝑑𝑥 2
proves that the path is minimum (or path of least time).
1.5.2. Laws of Refraction
When a ray of light passes from one homogenous medium to another, the phenomenon
of bending of light ray towards or away from the normal is called refraction. Again there are
following two laws of refraction.

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Fig. 1.3
First Law:
The incident ray, refracted ray and the normal at the point of incidence all lie in
one plane.
Let us assume XY be a plane surface dividing two media shown in figure 1.3. A ray
starting from point P is incident on M. It is refracted as MP' in the other medium, ∠𝑖 and ∠r
are the angles of incidence and refraction. Let us assume that the ray follows path PM'P'
instead of PMP'. It is evident that
PM' + M'P' > PM + MP'
Therefore, path PM'P' is not possible. If you shift M' towards M, the path from P to P'
through M shortens. It is shortest when M' is coincident with M which is in accordance with
Fermat’s principle and proves the first law.
Second Law:
The ratio of the sine of angle of incidence to the sine of angle of refraction is a
constant for a given pair of media.
You can further prove that the ratio of the sin i to sin r is equal to the refractive index of
second medium with respect to the first medium which is also known as Snell’s law.

Fig. 1.4
In figure 1.4, XY is a plane surface dividing two media of refractive indices 𝜇 1 and 𝜇 2.
Consider a point object P in the first medium, PM and MP' are the incident and refracted
rays, i and r are the angle of incidence and refraction.

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PD = a, DM = x, DD' = c, D'P' = b
If a ray of light travels a distance S in a medium of refractive index 𝜇, than product 𝜇S is
called the optical path in the medium. The optical path from P to P' is given by
S = PMP' = 𝜇 1PM + 𝜇 2 MP'

= 𝜇 1 √(𝑃𝐷2 + 𝐷𝑀2 ) + 𝜇 2 √(𝐷′𝑀2 + 𝐷′𝑃2 )

= 𝜇 1 √(𝑎2 + 𝑥 2 ) + 𝜇 2 √{(𝑐 − 𝑥)2 + 𝑏 2 }

Now, for S to be minimum dS/dx must be zero and d2S/dx2 positive. Differentiating S with
respect to x, we get
𝑑𝑆 𝜇1 2𝑥 𝜇2 (𝑐−𝑥)
= . − =0
𝑑𝑥 2 √(𝑎2 + 𝑥 2 ) √{(𝑐− 𝑥)2 + 𝑏2 }
𝜇1 𝑥 𝜇2 (𝑐−𝑥)
or =
√(𝑎2 + 𝑥 2 ) √{(𝑐− 𝑥)2 + 𝑏2 }

Using triangles PMD and P'MD', you can write the above relation as
𝜇1 sin i = 𝜇2 sin r
sin 𝑖 𝜇2
or = = 12 ……. (1.10)
sin 𝑟 𝜇1

Where 1𝜇 2 is the refractive index of the second medium with respect to the first medium. This
is the Snell’s law of refraction.
𝑑2 𝑠
You can show that the second differential coefficient of S, i.e., ( ) for the plane
𝑑𝑥 2
surface comes out to be positive. It proves that the second law of refraction is in accordance
with Fermat’s principle, i.e., the actual path is minimum or path of least time. But you will
see that, this condition is satisfied in the case of plane surfaces only and not in the case of
curved surfaces. For curved surfaces, the path may be a maximum or minimum.
Case I:
When the reflecting surface is more curved than the ellipse passing through the point of
reflection O and having points A and B as foci (figure 1.5)
Let us consider two fixed points A and B and a curved mirror MON (figure 1.5). Let AOB be
the actual path of a ray of light traveling from A to B.

Fig. 1.5

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Let us draw an ellipse having A and B as foci and passing through O. By the property of
ellipse, (AO + OB) is a constant for all positions of O on the ellipse. For example,
AO + OB = AO' + O'B
An alternative neighboring path for the ray of light by reflection at the mirror is ANB. The
difference between the actual path AOB and the neighboring path ANB is,
∆ = AOB – ANB = (AO + OB) – (AN + NB)
= (AO + OB) – (AO' – NO' + NB)
= (AO' + O'B) – (AO' – NO' + NB)
= O' B + NO' – NB
This is positive because O'B and NO' are the two sides of a triangle of which NB is the
third side. Thus, in this case, any neighboring path ANB is smaller than the actual path AOB.
Thus from above discussion, we can conclude that the actual path is maximum compared
with all neighboring paths. Hence Fermat’s law is a law of extremum path.
Case II:
When the reflecting surface is less curved than the ellipse passing through the point of
reflection O and having points A and B as foci (figure 1.6).Let us consider a reflecting
surface MON (figure 1.6),which is less curved than the ellipse passing through O and having
points A, B as foci. In this case, the difference between the actual path AOB and aneighboring
path ANB is given by,

Fig. 1.6
∆ = AOB – ANB = (AO + OB) – (AN + NB)
= (AO + OB) – (AO' + O'N + NB)
= (AO' + O'B) – (AO' + O'N + NB) (by property of ellipse)
= – (O'N + NB – O'B).
This is negative because O'N and NB are the two sides of a triangle of which O'B is the third
side. Thus, in this case, any neighboring path ANB is longer than the actual path AOB. You
can say that, the actual path is minimum among all neighboring paths.

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Hence the curvature of the reflecting surface relative to that of the ellipse through the
point of reflection decides whether the path of light from the focus of the ellipse to the other
focus through reflection at the surface will be a maximum or a minimum.

1.5.3. Refractive Index

The refractive index is a relative property of two media. The refractive index of any
medium with respect to free space (or air) is called the absolute refractive index. The absolute
refractive index of any medium depends on its nature, wavelength of incident light and the
temperature. The frequency of refracted ray remains the same as that of incident light, but its
velocity and wavelength change. Let us now consider the following cases.
Case I:
If 𝜇1 and 𝜇2 are the absolute refractive indices of first and second media respectively
and light ray enters from rarer medium to the denser medium then, we have, 𝜇2 > 𝜇1
sin 𝑖 𝜇2
∴ = >1
sin 𝑟 𝜇1

or sin 𝑖 > sin r

or 𝑖>𝑟
This simply shows that refracted ray is deviated towards the normal.
Case II:
If light ray enters from denser medium to rarer medium, then 𝜇2 < 𝜇1
sin 𝑖 𝜇2
∴ = <1
sin 𝑟 𝜇1

or sin 𝑖 < sin 𝑟

or 𝑖<𝑟
Thus the refracted ray is deviated away from the normal
.

Fig.1.7

Case III:

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If i = 0, then r = 0, i.e., the incident light ray is along normal then the refracted ray
passes undeviated, but its velocity gets changed.
The absolute refractive index can be defined in other ways also. The absolute refractive
index of a medium is defined as the ratio of speed of light in the free space to the speed of
light in medium. Accordingly, if c is speed of light in free space and v the speed of the light
in the medium, then the refractive index
𝑐
𝜇= ……. (1.11)
𝑣

In refraction the frequency of wave () remains unchanged, therefore, c = 𝜆 and 𝑣 =

𝜆m. Where, 𝜆m is wavelength of light in medium. Thus refractive index of a medium may
also be given by the expression,
𝜆
𝜇= ……. (1.12)
𝜆𝑚

1.5.4. Total Internal Reflection

When a ray of light moves from denser to rarer medium, then the refracted ray is
deviated away from normal (figure 1.8 (a)). With increase in angle of incidence, the angle of
refraction increases. For a certain angle of incidence in denser medium, the corresponding
angle of refraction in rarer medium is 900 (figure 1.8(b)).

Fig. 1.8
This particular angle of incidence for which the corresponding angle of refraction is 900
is called the critical angle and is denoted by C. The value of the critical angle depends on the
nature of the two media.
If angle of incidence is increased beyond its critical value, the incident ray is not
refracted but returns back to denser medium [figure 1.8(c)]. This phenomenon is called total
internal reflection.
Conditions of Total Internal Reflection
(i) The ray must pass from denser medium to rarer medium.
(ii) The angle of incidence in the denser medium must be greater than the critical angle for
the given pair of media.
Relation between Refractive Indices of Media and Critical Angle

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If 𝜇d and 𝜇r are the refractive indices of denser and rarer media respectively then from
Snell’s law, we have,
sin 𝑖 𝜇𝑟
= ……. (1.13)
sin 𝑟 𝜇𝑑

For critical angle of incidence, i = C and r = 900

sin 𝐶 𝜇𝑟 1
∴ = ⇒ sin 𝐶 = ……. (1.14)
sin 900 𝜇𝑑 𝑟𝜇𝑑

Where r𝜇d = refractive index of denser medium with respect to rarer medium. In most of the
problems the rarer medium is chosen to be air with refractive index 1.
Example 1: The absolute refractive indices of glass and water are 4/3 and 3/2 respectively. If
the speed of light in glass is 2 × 108 m/s, calculate the speed of light in (i) vacuum and (ii)
water.
4
Solution: Refractive index of glass, 𝜇𝑔 = 3
𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑐𝑢𝑚
∴ 𝜇𝑔 = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑔𝑙𝑎𝑠𝑠

4 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑐𝑢𝑚

or =
3 2×108

4×2×108
Thus, speed of light in vacuum = = 2.67 × 108 𝑚/𝑠
3

3
Refractive index of water (given), 𝜇𝑤 = 2

𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑐𝑢𝑚

But we know that 𝜇𝑤 = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑤𝑎𝑡𝑒𝑟

3 2.6×108
∴ = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑤𝑎𝑡𝑒𝑟
2

Therefore, speed of light in water = 1.73 × 108 m/s

Example 2: Refractive index of water with respect to air is 4/3 and glass is 3/2. What is the
refractive index of glass with respect to water?

Solution: For three media air, water and glass, we have

𝑎 𝑤 𝑔
𝜇𝑤 × 𝜇𝑔 × 𝜇𝑎 = 1
𝑎
𝑤 1 𝜇𝑔 3/2 9
∴ 𝜇𝑔 = 𝑎𝜇 𝑔 = 𝑎𝜇 = 4/3 = 8
𝑤× 𝜇𝑎 𝑤

Thus, refractive index of glass with respect to water is 9/8.

Example 3: If the angle of incidence (i) for a light ray in air be 450 and the angle of
refraction (r) in glass be 300. Find refractive index of glass.

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sin 450 1
Solution: Refractive index of glass, n = sin 300 = × 2 = √2
√2

Self Assessment Question (SAQ)

1. What is total internal reflection?
2. What is critical angle for a medium of refractive index √2?
3. Using Fermat principle, establish condition of total internal reflection.

1.6. GAUSS’S GENERAL THEORY OF IMAGE FORMATION

In a coaxial symmetric system, Gauss’s theory deals with ideal image formation. This
includes the cases where there is a point-for-point, line-for-line and plane-for-plane
relationship between the object and its image. With the various reflecting or refracting
coaxial surfaces, the common axis is taken as X-axis. Due to symmetry about this axis, it is
enough to deal with any one plane through the X-axis, and the transverse distances is given
along Y – axis.
With arbitrary choice of origins for the object and image space, let (x, y) be the co-
ordinates of an object point and (x', y') those for its image. Then the most general linear
relationships of x', y' and x, y would be
𝑎1 𝑥+𝑏1 𝑦+𝑑1 𝑎2 𝑥+𝑏2 𝑦+𝑑2
x' = and y' =
𝑎𝑥+𝑏𝑦+𝑑 𝑎𝑥+𝑏𝑦+𝑑

We state this without a rigorous mathematical proof. We can see that out of the nine
constants involved, five are redundant.
Firstly, if y changes sign, x' should remain unaffected. This condition makes
b = b1 = 0
Again if y changes sign, y' should only change in sign, not in magnitude. This makes a2=d2=
0. Finally dividing all coefficients by a, and expressing the new values by the corresponding
capital letters, we get
𝐴1 𝑥+𝐷1 𝐵2 𝑦
x' = and y' =
𝑥+𝐷 𝑥+𝐷

Thus, we reach a very important conclusion that in any co-axial symmetrical system forming
ideal images the co-ordinates x', y' of an image point corresponding to the object point x, y are
uniquely determined by four and only four independent constants.
In actual practice, two of these constants are used to specify the origins for object space
and image space on the x-axis for measurement of distances. Then only two other constants
specify the object to image conjugate relations uniquely. There are several alternative ways of
choosing the origins.

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1.7. SUMMARY
In this unit you have studied the following
1. Fermat’s principle.
2. Before explaining Fermat’s principle it is necessary to know about optical path. It has
been defined as, when a ray of light travels a distance d in a medium of refractive index 𝜇,
the product 𝜇d is called the optical path.
3. In 1960 Fermat’s principle was stated as “Fermat’s principle of least time, later it was
stated in a more general form, based on the number of cases observed practically. This is
known as “Fermat’s principle of stationary time” or Fermat’s principle of extreme path (a
maximum or a minimum or stationary).
4. The laws of reflection and refraction have been deduced with the help of Fermat’s
Principle.
5. Condition of total internal reflection is also discussed. You have seen, this condition
occurs when a ray of light moves from denser medium to rarer medium. One of the
examples of total internal reflection is the formation of rainbow, which we see usually in
our day to day life.
6. Many solved examples are given in this unit to make concepts clear. In the last “Gauss
General theory of image formation” is explained. You will use this theory in the next unit.
7. To check your progress Self Assessment Questions (SAQs) are also given.

1.8 GLOSSARY
Beam – group of rays
Extreme – maximum or minimum
Angle of Incidence – angle between a beam striking a surface and the normal to surface
Angle of Reflection – angle formed between the normal to a surface and reflected ray
Angle of Refraction – angle formed between a refracted ray and the normal to the surface
Homogeneous – of the same kind, alike
Iterative – Frequentative

1.9 REFERENCES
1. A text Book of Optics – Brijlal, S.Chand Publishing, N.Delhi
2. Optics-Satya Prakash, Pragati Prakashan, Meerut
3. Introductory University Optics, PHI Learning, New Delhi, 1998.

1.10 SUGGESTED READINGS

1. Milton Katz (1994), Introduction to Geometrical Optics.
2. B.K. Mathur (2015), Optics, Edition 2, Digital Library of India Item.462758.
3. Ajoy Ghatak (2012), OPTICS, Tata Mc Graw Hill, New Delhi.

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1.11. TERMINAL QUESTIONS

1.11.1. Short Answer Type
1. Write a short note on optical path.
2. Write a note on Fermat’s principle.
3. Using Fermat’s principle, establish the condition of total internal reflection.
4. Define critical angle. State the relation between refractive index of medium and critical
angle.
1.11.2. Long Answer Type
1. Discuss Fermat’s principle in brief and prove laws of reflection and refraction with its
help.
2. Give examples to show that the path of reflected ray is
i. Maximum in some case and
ii. Minimum in other
3. What is Fermat’s principle? Prove that Snell’s law follows the Fermat’s principle.
1.11.3. Numerical Questions
1. A man walks on the hard ground with a speed of 5 ft/s, but he has speed of 3 ft/s on the
sand ground. Suppose he is standing at the border of sandy and hard ground and wishes to
reach a tree situated on the sandy ground. The man can reach the tree by walking 100ft. along
the border and 120 ft. on the sandy ground normal to the border. Find out the value of path
which requires minimum time to reach the tree.
2. In fig. 1.9, two stations A and B in different territories are separated by a border line CD. A
messenger can travel in the upper territory with a speed Va and in the lower territory with
speed Vb. Several messengers start from A and follow different paths like APB having
different position of P specified by a distance x from M. It is found that the messenger who
chooses x as 4.0 km reaches B in the minimum time.

Fig. 1.9

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Answer the following

(a) What is the relation between Va and Vb?
(b) If the speeds Va and Vb are interchanged, what will be the new value of x to give the
fastest path?
(c) Explain the formation of a ray of light on the basis of Fermat’s Principle of extremum
path.
3. In fig. 1.10, light starts from point A and after reflection from the inner surface of the
sphere reaches the diametrically opposite point B. Calculate the length of the hypothetical

Fig.1.10
path APB and using Fermat’s Principle find the actual path of light. Is the path minimum?
4. In fig. 1.14, P is a point source of light. If the distance of P from the centre O of the
spherical reflecting surface is 0.8r and if the light ray starting from P and after being reflected

Fig.1.11
𝜃 3
at A reaches at a point Q. Using Fermat’s principle show That cos 2 = 4

5. A man walks with a speed of 1.8 m/s when he walks

on the hard ground, but has a speed of only 1 m/s when
he walks on sandy area. Suppose he is standing at the border of sandy and hard
ground and he wishes to reach a tree deep
inside the sandy area. He can reach the tree by walking 30m along the border and followed by
36m walk on the sandy area normal to the border. Find the suitable path which requires
minimum time to reach.

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1.11.4. Objective Answer Type

1. The product of 𝜇 of the medium and the distance travelled by the light in medium is known
as
a. Ray path b. Fermat’s path
c. Optical path d. Actual path
2. A ray of light traveling from one point to another by any number of reflections and
refractions follows that particular path for which time taken is
a. highest b. least
c. maximum d. depends upon path
3. The incident, reflected and normal ray at the point of incidence are in same plane, is the
statement for
a. First law of reflection b. Second law of reflection
c. First law of refraction d. All above
4. The optical path of monochromatic light is same as it goes through 200cm of glass or
2.25cm of water. If the refractive index of water is 1.33, the refractive index of glass is
a. 1.00 b. 1.23
c. 1.33 d. 1.50
5. Fermat Principle is the principle of:-
a. Maximum path only b. Extreme path only
c. Minimum path only d. None above
6. Refractive indices of water, sulphuric acid, glass and carbon disulphide are 1.33, 1.43, 1.53
and 1.63respectively. The light travels slowest in:

a. Sulphuric acid b. Glass

c. Water d. Carbon Disulphide
3
7. The refractive index of glass with respect to air is and the refractive index of water with
2
4
respect to air is 3. The refractive index of glass with respect to water will be:

a. 1.525 b. 1.225
c. 1.425 d. 1.125

1.12 ANSWERS
1.12.1 Self Assessment Questions (SAQs):
1. Refer Article 1.5.4

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1 1
2. Sin C = or Sin C = ⇒ ∠C = 450
𝜇2 √2
1

3. In the last question, we have proved, using Fermat’s principle that

sin 𝑖
= 1𝜇 2.
sin 𝑟

If medium 2 is rarer (1𝜇 2 < 1), then,

sin i < sin r or i<r
For r = 900, sin r = 1, and we have
sin i = 1𝜇 2
At angles of incidence larger than this limiting angle, the light is reflected back into the
denser medium.
1.12.2 Terminal Questions
Numerical Questions
If the man follows the path followed by a light ray he will take minimum time to reach the
tree. This is in accordance with Fermat’s principle. Let M be the starting position of the man
on the border line AB (Fig. 1.12). Distance MM" = 100 ft. on the border. T is the tree on the
sandy ground so that the distance M"T = 120 ft. Let the man follow the path from M to M'
and M' to T in order to reach the tree in minimum time. If distance MM' = x then M'M" =
(100 – x) ft. From fig. 1, we have

1. M'T = √[(𝑀′𝑀")2 + (𝑀"𝑇)2 ] = √{(100 – x)2 + (120)2}

Fig. 1.12: Hard Ground

Total time taken by the man to reach from M to M' and M' to T is given by,
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑀𝑀′ 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑀′𝑇
t = +
𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑛 𝑏𝑜𝑟𝑑𝑒𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑛 𝑠𝑎𝑛𝑑𝑦 𝑔𝑟𝑜𝑢𝑛𝑑

𝑀𝑀′ 𝑀′𝑇 𝑥 √[(100−𝑥)2 + (120)2 ]

= + = +
5 3 5 3
Since the value of t depends upon the distance x, hence for t to be minimum, we have
𝑑𝑡 1 1 1
=0= + . [(100 − 𝑥)2 + (120)2 ]−1/2. 2 (100 – x) (-1)
𝑑𝑥 5 3 2
1 (100−𝑥) 1
or . =
3 √[(100−𝑥)2 + (120)2 5

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1 (100−𝑥)2 1
Squaring it, we get, . =
9 (100−𝑥)2 + (120)2 25

or (100 – x)2 . 25 = 9 [(100 – x)2 + (120)2]

or (100 – x)2 [25 – 9] = 9. (120)2
360
∴ x = 100 + = 100 + 90 = 190 or 10
4
As per data, the value 190 is out of question; hence the man should walk 10 ft. along the
border and then make a head-way towards the tree from M'.
2. (a). See fig. 2 The path AP transverse with a speed Va and the path PB with a speed Vb.
The path PA is given by :
AP = √{(AM)2 + (MP)2 = √{(6)2 + x2}
The time taken to traverse this path
𝐴𝑃 √{(6)2 + 𝑥 2 }
ta = =
𝑉𝑎 𝑉𝑎

Similarly path PB and time taken to traverse it are given by

PB = √{(BN)2 + (PN)2} = √{(3)2 + (12 – x)2}
𝑃𝐵 √{(3)2 + (12−𝑥)2 }
tb = 𝑉 =
𝑏 𝑉𝑏

Hence total time taken to travel down the path APB is given by:
√{(6)2 + 𝑥 2 } √{(3)2 + (12−𝑥)2 }
t = ta + tb = +
𝑉𝑎 𝑉𝑏

For t to be minimum, we have,

𝑑𝑡 1 1 1 1
= 0 = 𝑉 . 2 [62 + x2]-1/2 . 2x + 𝑉 . 2 [(12 – x)2 + 32]-1/2 × 2 (12 – x) (-1)
𝑑𝑥 𝑎 𝑏

𝑥 12−𝑥
or =
𝑉𝑎 √(62 + 𝑥 2 ) 𝑉𝑏 √(12−𝑥 2 + 32 )

But we are given that x = 4 km, for t to be minimum, hence

4 12−4
=
𝑉𝑎 √(36+16) 𝑉𝑎 √[(12−4)2 +9]

4 8
or = ,
𝑉𝑎 √52 𝑉𝑎 √73

𝑉𝑎 4 73 1 73 73
or = √( ) = √( ) = √( )
𝑉𝑏 8 52 2 52 208

(b) If the speeds are interchanged, then we have

𝑃𝐵 √(62 + 𝑥2 ) {32 +(12−𝑥)2 }

t' = 𝐴𝑃 + = + √
𝑉𝑏 𝑉𝑎 𝑉𝑏 𝑉𝑎

For the path to be the fastest, t' should be minimum, hence

𝑑𝑡′
=0
𝑑𝑥

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1
1 1 1 1
∴ (62 + 𝑥 2 )−2 . 2x + 𝑉 [32 + (12 − 𝑥)2 ]−1/2 (12 – x) (-1) = 0
𝑉𝑏 2 𝑎 2

𝑥 12−𝑥
or =
𝑉𝑏 √(62 + 𝑥 2 ) 𝑉𝑎 √{32 + (12−𝑥)2 }

𝑉𝑎 (12−𝑥)√(36+𝑥 2 )
or =
𝑉𝑏 √{9+ (12−𝑥)2 }

Squaring and solving, we get

x = 10.2km.
(c) See figure 2 for the solution of this part.
3. See figure 3. Let r be the radius of the sphere. If L be the hypothetical length of the path
followed by light, that is,
L = AP + PB
If angle PAB be 𝜃, then the length L is given by :
L = AB cos 𝜃 + 𝐴𝐵 sin 𝜃, [∵ ∠𝐴𝑃𝐵 = 900]
or L = 2r (cos 𝜃 + sin 𝜃), [∵ AB = 2r]
Following Fermat’s principle, the path length L should be maximum or minimum. But in
both cases, we have
𝑑𝐿
= 0 = 2𝑟 (− sin 𝜃 + cos 𝜃) ……(1)
𝑑𝜃

or sin 𝜃 = cos 𝜃,
𝜋
∴ 𝜃= 4

Substituting the value of 𝜃, the actual path length which light shall follow :
1 1
L = 2r (cos 450 + sin 450) = 2r ( + )
√2 √2

= 2r × √2 = √2 × diameter of the sphere.

Differentiating equation (1) again to see whether the path is a maximum, or
minimum, we get
𝑑2 𝐿
= 2𝑟(−𝑐𝑜𝑠𝜃 − 𝑠𝑖𝑛𝜃) = −2𝑟 (cos 𝜃 + sin 𝜃)
𝑑𝜃2
1 1
= – 2r ( + ) = −√2 × 2r.
√2 √2

Which is negative quantity; hence the actual path is maximum in this case.
4. Let PQ = ar, we have
(PA)2 = (PO)2 + (OA)2 – 2 (PO) (OA) cos 𝜃
= (ar)2 + r2 – 2 (ar) r cos 𝜃
= r2 [a2 + 1 – 2a cos𝜃];

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PA = r (1 + a2 – 2a cos𝜃]1/2
Draw a perpendicular OM on AQ from point O. In ∆𝐴𝑂𝑄,
𝑂𝐴 = 𝑂𝑄 = 𝑟
𝜃
∠𝑂𝐴𝑄 = ∠𝑂𝑄𝐴 = 2.
𝜃 𝜃 𝜃
AQ = AM + MQ = r cos + r cos 2 = 2r cos 2.
2

If the length of the total path is l, then

𝜃
L = PA + AQ = r [1 + a2 – 2a cos 𝜃]1/2 + 2r cos 2 . ..….(1)
𝑑𝑙
But according to Fermat’s Principle, =0
𝑑𝜃

∴ Differentiating equation (1) w.r.t. 𝜃 and equating to zero, we get

𝑑𝑙 1 𝜃 1
= 𝑟. 2 [1 + 𝑎2 − 2𝑎 𝑐𝑜𝑠𝜃]-1/2 (2a sin 𝜃) – 2r sin 2 . (2) = 0
𝑑𝜃
𝑎𝑟 sin 𝜃 𝜃
or − 𝑟 sin = 0
[1+ 𝛼2 − 2𝑎 cos 𝜃]1/2 2
𝜃 𝜃
𝑎𝑟 .2 sin cos 𝜃
2 2
or 1 − 𝑟 sin = 0
2
[1+ 𝛼2 − 2𝛼 cos 𝜃]2
𝜃
𝜃 2𝛼 cos
2
or r sin [ − 1] = 0 …..(2)
2 [1+𝑎2 − 2𝛼 cos 𝜃]1/2

𝜃
𝜃 2𝛼 cos
2
∴ Either sin =0 or − 1=0 …..(3)
2 {1+𝑎2 − 2𝛼 cos 𝜃}1/2

Equation (3) represents that the light ray PQ' starting from P, after being reflected at Q'
reaches at Q. But we are given that the light ray is PA which after reflection at A reaches
at Q. Therefore, equation (3) does not give satisfactory solution or equation (2). Equation
(4) gives
𝜃
2𝛼 cos
2
=1
{1+𝑎2 − 2𝛼 cos 𝜃}1/2

𝜃 𝜃
or 4𝑎2 cos2 2 = 1 + 𝑎2 – 2𝑎 cos 𝜃 = 1 + 𝑎2 - 2𝑎 (2 cos2 2 − 1)
𝜃
or 4𝑎2 cos2 2 (1 + 𝑎) = (1 + 𝑎)2
𝜃 1+𝑎
cos2 2 = 4𝑎
𝜃 3
In this example, 𝑎 = 0.8, therefore, cos 2 = 4

5. Let the positions of the man and tree be A and T respectively. Given AC = 30 m and CT
= 36m. Let us suppose that the man walks on the hard ground along the border from A to

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B and then from B to T on the sandy area to reach the tree T in minimum time.

Fig. 1.13
Let AB = x m, then
BC = (100 – x) m.

∴ BT = √[(𝐵𝐶)2 + (𝐶𝑇)2 ] = √[(30 − 𝑥)2 + (36)2 ]

The man traverse the distance AB with speed of 1.8 m/s and the distance BT with a speed
of 1m/s. Therefore, time taken by the man to reach the tree
𝐵𝑇 𝐵𝑇 𝑥 √(30−𝑥)2 + (36)2
t= + = + ...….(1)
1.8 1 1.8 1
𝑑𝑡
If t is minimum, then 𝑑𝑥 = 0, so differentiating equation (1) w.r.t. time t, we get,
1
𝑑𝑡 1 1
= 1.8 + 2 [30 − 𝑥)2 + (36)2 ]−2 × 2(30 − 𝑥)(−1) = 0
𝑑𝑥
1 30−𝑥
= 1
1.8
[(30−𝑥)2 + (36)2 ]2

Solving we get
x ≈ 30 + 24 = 54m or 6m
From figure it is obvious that for minimum time x cannot be 54m;
Therefore, x = 6m
Thus, to reach the tree in minimum time the man should walk 6m along the border on the
hard ground and then he should walk on the sandy area along the line BT.

1.12.3 Objective Type Questions

1. (b), 2. (b), 3. (a), 4. (d), 5. (b) 6. (d) 7. (d)

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UNIT 2: REFRACTION BY SPHERICAL SURFACES

CONTENTS
2.1 Introduction
2.2 Objectives
2.3 Sign Convention: Object and Image Spaces
2.3.1 Axial or Longitudinal Distances
2.3.2 Lateral or Transverse Distance
2.3.3 Angles
2.4 Coaxial Optical System
2.5 Cardinal Points of an Optical System
2.5.1 Focal Points
2.5.2 Principal Points
2.5.3 Nodal Points
2.6 Rules of Image Formation
2.7 Magnification of a lens system
2.7.1 Relation between the three Magnifications
2.8 Helmholtz Lagrange’s Magnification Formula
2.9 Some Important Relations
2.10 Newton’s Formula
2.11 Summary
2.12 Glossary
2.13 References
2.14 Suggested Readings
2.15 Terminal Questions
2.16 Answers

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2.1 INTRODUCTION
In the previous unit you have studied about the Gauss’s theory of image formation for a
coaxial system. In the present unit we will apply this theory to understand the image
formation by a lens or combination of lenses. Gauss has shown that if in an optical system the
positions of certain specific points are known, the system may be treated as a single unit. The
position and size of the image of an object may then directly be obtained by same relations as
used for thin lenses or single surface, however, complicated the system may be. These points
are called Cardinal points or Gauss points of an optical system.
This simplifies the understanding and processing of image formation to a great deal.
The cardinal points and their use in the image formation have formed the basis of the present
unit. Before dealing with the cardinal points, it is imperative to know the sign convention
used in the optical system. Hence you are made accustomed to it, as well, in the initial part of
this unit. An important aspect of image magnification in terms of lateral, axial and angular
magnifications is discussed and a relation between different types of magnifications is
established in the later part of the unit. Finally an important relation in the form of Newton’s
formula is derived, which will enhance your learning about the optical system of thin lenses.

2.2 OBJECTIVES
After studying this unit you will be able to
 Draw the ray diagrams for the image formation in a coaxial lens system
 Locate the six cardinal points in a lens system
 Apply the sign convention to solve various problems
 Determine the different types of magnifications produced in an optical system
 Derive Newton’s formula

2.3 SIGN CONVENTION: OBJECT AND IMAGE SPACES

Before studying the different cases of image formation in an optical system, you must
know the sign convention. In figure 2.1, XX' represents the optical axis and the dotted line
represents the normal to it. The region on the left of optical system in which the object is
placed is known as object Space and the region on the right of the optical system where the
image is formed is called as image space. You can divide the sign convention into following
three parts.
2.3.1 Axial or Longitudinal Distances
The distances measured along optical axis XX' or parallel to it are taken positive (+ve)
or negative (-ve) with respect to the direction of incident ray. Conventionally, all distances

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measured in the direction of incidence are taken as positive while distances opposite in
direction to the direction of incidence are taken as negative.

Fig. 2.1
This is clearly depicted in the figure 2.1. All distances are measured from the point C
i.e. the centre of the reflecting or refracting surface known as optical centre. Thus distance
OC is –ve while distance CI is +ve.

2.3.2 Lateral or Transverse Distance

The distances at right angle to the optical axis XX' are known as lateral or transverse
distances. In figure 2.1, OO' and II' are the lateral distances of object and image respectively.
Conventionally, lateral distance above XX' is taken as +ve while below it is taken as –ve.
Hence OO' is +ve and II' is –ve.
2.3.3 Angles
Conventionally, anticlockwise angles with respect to XX' axis are taken as +ve and
clockwise angles with respect to XX' axis are taken as –ve. In figure 2.1, the light ray in
object space forms an angle 𝜃1 in clockwise direction (-ve) while light ray in the image space
forms an angle 𝜃2 in anticlockwise direction (+ve) with respect to XX' axis respectively.

2.4 COAXIAL OPTICAL SYSTEM

A lens system having common optical axis is known as co-axial lens system. Generally,
it consists of a number of lenses placed apart with a common optical axis known as Principal
axis. We can determine the position and size of the image of an object formed by such a
system by considering refraction at each lens separately. This process is however very
tedious. Gauss showed that, if in an optical system the positions of certain specific points be
known, the system may be treated as a single unit. The position and size of the image of an
object may then directly be obtained by same relations as used for thin lenses or single
surfaces.

2.5. CARDINAL POINTS

An optical system consists of six cardinal points which are as follows
(i) Two focal point

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(ii) Two principal point

(iii) Two nodal point

2.5.1 Focal points and Focal Planes

These points lie on the principal axis of the optical system. Figure 2.2 (a) shows a
convergent coaxial optical system having its axis XX'. An incident ray parallel to the axis,
after refraction through the system, passes through a point F2 on the axis, whatever is the path
inside the system.

Fig. 2.2
This would be true for all the incident rays parallel to the axis (For divergent system,
the emergent rays would appear to diverge from a point on the axis of the system). The point
F2 is called the second focal point of the system. You can define it as the image-point on the
axis for which the object-point lies at infinity.
Similarly, an incident ray b passing through a point F1 on the axis, after refraction
through the system, becomes parallel to the axis (figure 2.2 (b)). The point F1 is called the
first focal point of the system. You can define it as the object-point on the axis for which the
image-point lies at infinity.
The planes through F1 and F2 and perpendicular to the axis are called first focal plane
and second focal plane of the system respectively. The first focal plane of an optical system
is also called principal focal plane of the object space and second focal plane is called as
principal focal plane of the image space.
2.5.2 Principal Points and Principal Planes
The Principal points are a pair of conjugate points on the principal axis of the optical
system having unit positive linear magnification.
In figure 2.3, an incident ray A parallel to the principal axis, after refraction through the
optical system, passes through the second focal point F2. Produce the incident ray A onwards
and emergent ray A backwards such that they intersect at A2. The plane through A2 and
perpendicular to the axis XX' is called the second principal plane, and its point of
intersection H2 with the axis is called the second principal point.
Similarly, the incident ray B passing through the first focal point F1, after refraction
through optical system, emerges parallel to the axis. Produce the emergent ray B backward

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and incident ray B forward such that they intersect at A1. The plane through A1 and
perpendicular to the XX' axis is called the first principal plane, and its point of intersection
H1 with the axis is called the first principal point.

Fig. 2.3
The incident ray B has been so chosen that the corresponding emergent ray lies at the
same distance above the axis as the incident ray A. It is thus seen that the incident rays A and
B are converging towards the point A1 and the corresponding emergent rays appear to diverge
from the point A2. Hence A2 is the image of A1, where H1A1 = H2A2. Thus, A1 and A2 are
conjugate points. This is true for all such pairs on the principal planes. Hence if an object be
placed in the first principal plane, an erect image of the same size would be formed in the
second principal plane, that is, the linear magnification is +1.
Finally, it follows that if an incident ray passes through the first principal plane at a
certain height from the principal axis, the corresponding emergent ray will pass through the
second principal plane at the same height and on the same side of the axis.
Focal Lengths
The distance H1F1 is called the first focal length f1 and the distance H2F2 the second
focal length f2 of the system. f1 and f2 are also known as focal lengths in object space and
image space respectively. If the medium be same on the two sides of the system then f1 = f2
(numerically).
2.5.3 Nodal Points and Nodal Planes
The nodal points are a pair of conjugate points on the principal axis of the system,
having unit positive angular magnification. They are such that an incident ray directed
towards the one nodal point emerges parallel to its original direction through the other nodal
point. In figure 2.4, N1 and N2 are the nodal points, and the emergent ray N2B is parallel to
the incident ray AN1. The nodal points therefore do for angles what the principal planes do
for transverse distance.

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Fig. 2.4
The planes through N1 and N2 and perpendicular to the axis are called nodal planes.
The distances of these nodal points are measured from the focal points. Also the distance
between two nodal points is equal to the distance between Principal Points.

2.6 RULES OF IMAGE FORMATION

When the positions of cardinal points are known, you can draw the image of an object
formed by a lens system using the following rules which obeys the properties of the cardinal
points:
(i) An incident ray parallel to the principal axis, after refraction, passes or appears to pass
through the second focal point F2.
(ii) An incident ray passing through the first focal point F1 or directed towards F1 becomes
parallel to the axis after refraction.
(iii) An incident ray passing through the first principal plane at a certain height from the
principal axis, emerges through the second principal plane at the same height and on the
same side of the axis.
(iv) An incident ray directed towards the first nodal point N1, emerges parallel to its original
direction through the second nodal point N2.
Usually, the principal points H1, H2 and the focal points F1, F2 are enough to trace the
image formed by a system. Hence, we can replace an actual optical system by a skeleton
consisting simply of the axis and these four points.

Fig. 2.5

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Figure 2.5 shows the construction of the image of an object OO' by a convergent
system. O'A1 is ray parallel to the axis meeting the first principal plane in A1. It must emerge
from the system through A2 on the second principal plane such that H1A1 = H2A2, and must
also pass through F2. O'F1 is another ray through the first focal point F1 and meeting the first
principal plane in B1. It must emerge parallel to the axis through B2 such that H1B1 = H2B2.
The two emergent rays meet at I', which is the image of O'. A third ray through N1 and its
conjugate parallel ray through N2 may also be drawn, but it is not necessary. The
perpendicular II', drawn from I' on the axis, is the image of the object OO'.

2.7 MAGNIFICATION OF A LENS SYSTEM

All optical systems are used to form a magnified image of an object. The magnification
produced by an optical system is defined as its ability to enlarge the size of image with
respect to the size of the object. It is denoted by m and expressed as:
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒
m=
𝑠𝑖𝑧𝑒 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡

There are three types of magnifications associated with coaxial lens systems:
(i) Lateral or Transverse Magnification: The lateral magnification of a lens-system is
defined as the ratio of the length of the image to the length of the object, both being
measured perpendicular to the axis of the system. As the distances above the axis are
taken as positive and those below the axis negative, the lateral magnification is positive
for an erect image and negative for an inverted image.

ℎ𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒
m=
ℎ𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡
(ii) Axial or Longitudinal Magnification: If the object and the image have small
extensions dx1 and dx2 respectively along the axis of the system, the ratio of dx2 and dx1
is called the longitudinal magnification of the system. It is given by
𝑑𝑥2
mx =
𝑑𝑥1

(iii) Angular Magnification: If 𝜃1 and 𝜃2 are the angles which the incident and the
corresponding emergent rays make with the principal axis of the system, then the ratio
of tan 𝜃2 to tan 𝜃1 is called the angular magnification of the system

tan 𝜃2
𝑚𝜃 =
tan 𝜃1
2.7.1 Relation among Three Magnifications
Consider a convergent coaxial lens-system as shown in figure 2.6, where XX' represents
the principal axis. H1, H2 be the principal points, and F1, F2 the focal points of the system.
Then H1F1 = f1 and H2F2 = f2, where f1 and f2 are the focal lengths of the system.

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Fig. 2.6
Let OO' be a small object of size y1 placed perpendicular to the axis. An incident ray
O'A1, parallel to the axis, emerges through A2 (where H1A1 = H2A2) and passes through F2.
Another ray O'B1 passing through F1 emerges through B2 (where H1B1 = H2B2) and becomes
parallel to the axis. The two emergent rays meet at I', which is the image of O'. The
perpendicular II' is the complete image of OO'.
Let x1 and x2 be the distances of the object OO' and the image II' respectively from F1
and F2. According to the sign convention, (Section 2.3) y1, f2 and x2 are positive while y2, f1
and x1 are negative. In similar triangles OO' F1 and H1B1F1, we have,
𝐻1 𝐵1 𝐻1 𝐹1
=
𝑂𝑂′ 𝐹1 𝑂

𝑦2 −𝑓1 𝑓1
or − = = . [∵ H1B1 = II' = - y2]
𝑦1 −𝑥1 𝑥1

Hence, the lateral magnification, my is,

𝑦2 𝑓1
my = = − ..….. (2.1)
𝑦1 𝑥1

Also, in similar triangles A2H2F2 and II' F2, we have,

𝐼𝐼′ 𝐹2 𝐼
=
𝐻2 𝐴2 𝐻2 𝐹2
𝑦2 𝑥1
or − = [∵ H2A2 = OO' = y1]
𝑦1 𝑓2

Hence, the lateral magnification, my, is given by

𝑦2 𝑥2
my = =− …….. (2.2)
𝑦1 𝑓2

Comparing equations (2.1) and (2.2), we have,

−𝑓1 𝑥2
=–
𝑥1 𝑓2

or x1 x2 = f1 f2 …….. (2.3)
This is known as Newton’s formula. Differentiating above equation, we have,
x1 (dx2) + x2 (dx1) = 0.
Hence the longitudinal magnification mx is given by

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𝑑𝑥2 𝑥2
mx = =− ...…. (2.4)
𝑑𝑥1 𝑥1

Let us now consider an incident ray OA1 and its corresponding emergent ray A2I. Let 𝜃1 and
𝜃2 be the angles which these rays make with the principal axis. According to sign convention,
𝜃1 is positive and 𝜃2 negative. Now, from the figure 2.7, we can write
𝐴1 𝐻1 𝑦1 −𝑦1
tan 𝜃1 = = =
𝐻1 𝑂 −𝑓1 − 𝑥1 𝑓1 + 𝑥1
𝐴2 𝐻2 𝑦1
and tan (-𝜃2) = =
𝐻2 𝐼 𝑓2 + 𝑥2
−𝑦1
or tan 𝜃2 =
𝑓2 + 𝑥2

Hence the angular magnification 𝑚𝜃 can be written as

tan 𝜃2 −𝑦1 / (𝑓2 + 𝑥2 ) 𝑓1 + 𝑥1
𝑚𝜃 = = =
tan 𝜃1 −𝑦1 / (𝑓1 + 𝑥1 ) 𝑓2 + 𝑥2

Using Newton’s formula x1x2 = f1f2

𝑓1 + 𝑥1 𝑓1 + 𝑥1 𝑥1
𝑚𝜃 = 𝑓 𝑓 = 𝑓2 =
𝑓2 + ( 1 2 ) (𝑥1 + 𝑓1 ) 𝑓2
𝑥1 𝑥1

𝑥1 𝑓1
∴ 𝑚𝜃 = 𝑓2
= 𝑥2
....... (2.5)

We can write various magnifications by using the expressions given by equations 2.1, 2.2,
2.3, 2.4 and 2.5 as
𝑓1 𝑥2
Transverse magnification (Lateral Magnification) my = − =−
𝑥1 𝑓2
𝑥2
Longitudinal magnification mx = −
𝑥1
𝑥1 𝑓1
Angular magnification 𝑚𝜃 =
𝑓2
= 𝑥2
From these, we get the following relations:
Relation I:
𝑥2 𝑥1 𝑥2
mx × 𝑚𝜃 = – × = − =my
𝑥1 𝑓2 𝑓2

That is, longitudinal magnification × angular magnification = lateral magnification.

Relation II:
𝑓1 𝑥1 𝑓1
my × 𝑚𝜃 = – × = − ,
𝑥1 𝑓2 𝑓2

That is, the product of lateral and angular magnifications is a constant and equal to the ratio
of the focal lengths of the system.
Relation III:
𝑥2
mx = –
𝑥1

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Substituting values of x2 and x1 from equation 2.1 and 2.2, we get,

−𝑚𝑦 𝑓2 𝑓
mx = – = –𝑚𝑦2 ( 2)
−𝑓1 /𝑚𝑦 𝑓1

But f2/f1 is a constant

∴ mx ∝ 𝑚𝑦2 ,
That is, the longitudinal magnification is proportional to the square of the lateral
magnification.

2.8 HELMHOLTZ - LAGRANGE’S MAGNIFICATION

FORMULA
This is also known as Helmholtz’s magnification formula. This formula gives the inter
relationship between angular magnification, lateral magnification and axial magnification.
Let SPS' (figure 2.7) be the convex spherical refracting surface separating media of refractive
indices 𝜇 1 and 𝜇 2 respectively, 𝜇 2 being denser. Let P be the pole and C the centre of
curvature of the surface.

Fig. 2.7
Let OO' be an object placed on the axis perpendicular to it. An incident ray OA, after
refraction at A, bends towards the normal CN (drawn on spherical surface) and goes in the
direction AI. Another ray OP meets the surface normally and passes undeviated. The two
refracted rays meet at I, which is the image of O. To find the image of O', let us take a ray
O'C. As it passes through the centre of curvature C, it strikes the surface normally and goes
undeviated. The image of O' lies somewhere on this line. Also it lies on the perpendicular to
the axis at I. Hence I', the point of intersection of the two, is the image of O' and II' is the
complete image of OO'.
Now if y1 and y2 be the sizes of the object and the image respectively then by sign
convention y1 is positive and y2 is negative. Let 𝜃1 and 𝜃2 be the angles made by the
conjugate rays OA and IA respectively with the axis. By sign convention, 𝜃1 is positive and
𝜃2 is negative. Let i and r be the angles of incidence and refraction respectively at A.
Since triangle COO' is similar to CII', therefore,

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𝐼𝐼′ 𝐶𝐼
=
𝑂𝑂′ 𝐶𝑂
−𝑦2 𝐶𝐼 𝐶𝐴
or = ( ) ....... (2.6)
𝑦1 𝐶𝐴 𝐶𝑂

Now, in ∆𝐶𝐼𝐴, we have

𝐶𝐼 ∠𝐶𝐴𝐼 sin 𝑟 sin 𝑟
= = = −
𝐶𝐴 ∠𝐶𝐼𝐴 sin (−𝜃2 ) sin 𝜃2

and in ∆𝐶𝑂𝐴, we have

𝐶𝐴 ∠𝐶𝑂𝐴 sin 𝜃1 sin 𝜃1
= = =
𝐶𝑂 ∠𝐶𝐴𝑂 sin (1800 − 𝑖) sin 𝑖

Substituting these values in equation (2.6), we get,

𝑦2 sin 𝑟 sin 𝜃1
= ( )
𝑦1 sin 𝜃2 sin 𝑖

sin 𝑖 𝜇2
According to Snell’s law, =
sin 𝑟 𝜇1

𝑦2 𝜇1 sin 𝜃1
∴ =
𝑦1 𝜇2 sin 𝜃2

or 𝜇1 𝑦1 sin 𝜃1 = 𝜇2 𝑦2 sin 𝜃2 .
For paraxial rays, (the rays which make small angles with the principal axis) 𝜃1 and 𝜃2 are
small, so that we may put,
sin 𝜃1 = 𝜃1 = tan 𝜃1
and sin 𝜃2 = 𝜃2 = tan 𝜃2 .
Then, 𝜇1 𝑦1 tan 𝜃1 = 𝜇2 𝑦2 tan 𝜃2 ....... (2.7)
This relation can also be written as
𝜇1 𝑦1 𝜃1 = 𝜇2 𝑦2 𝜃2 ....... (2.8)
This formula, which was first given by Lagrange, relates the linear transverse magnification
(y2/y1) and the angular magnification (tan 𝜃2 / tan 𝜃1 or 𝜃2/𝜃1) conjugate planes OO' and II'.
Let us now consider a coaxial system having (n–1) refracting surfaces separating n
media of refractive indices 𝜇 1, 𝜇 2, 𝜇 3,..……𝜇 n respectively. Let a small object of linear size y1
perpendicular to the axis be placed in the first medium and a ray from it make an angle 𝜃1
with the axis. After refraction at the first surface this ray makes an angle 𝜃2 with the axis and
appears to form an image of linear size y2. After refraction at the second surface, it makes an
angle 𝜃3 with the axis and appears to form an image of linear size y3, and so on. Hence
applying equation 2.7 to each surface in turn, we obtain,
𝜇 1 y1 tan 𝜃1 = 𝜇 2 y2 tan 𝜃2 = 𝜇 3 y3 tan 𝜃3 = ……… = 𝜇 n yn tan 𝜃n
This is the Helmholtz Lagrange’s equation of magnification for refraction at a system of
coaxial surfaces. If the angles 𝜃1, 𝜃2, 𝜃3………. 𝜃n are small, then it can also be written as
𝜇 1 y1 𝜃1 = 𝜇 2 y2 𝜃2 = ………………= 𝜇 n yn 𝜃n

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Example 1: A convex lens of focal length 25cm and made of glass ( 𝑎𝜇𝑔 = 1.5) is immersed
in water ( 𝑎𝜇𝑤 = 4/3). Calculate the change in the focal length of the lens.

Solution: Refractive index of glass with respect to water is

𝑎𝜇𝑔 1.5 9
𝑤 𝜇𝑔 = = 4/3 = 8.
𝑎𝜇𝑤

∴ Focal length of glass lens in water is

( 𝑎𝜇𝑔 − 1) (1.5−1)
𝑤𝑓𝑔 =
( 𝑤𝜇𝑔 − 1)
× 𝑎𝑓𝑔 = (9/8 −1)
× 25 = 100𝑐𝑚.

2.9 SOME IMPORTANT RELATIONS

Consider a convergent coaxial lens-system (figure 2.8), having principal axis XX',
separating two media of refractive indices 𝜇 1 and 𝜇 2. Let H1, H2 be the principal points and
F1, F2 the focal points of the system.
Suppose OO' is a small object placed perpendicular to the axis. An incident ray O'A 1 parallel
to the principal axis meets the first principal plane at A1. It emerges through A2 in the second
principal plane such that H1A1 = H2A2 and passes through F2. Another incident ray O'B1,
passing through F1, meets the first principal plane at B1. It emerges through B2 in the second
principal plane such that H1B1 = H2B2 and becomes parallel to the axis. The two emergent
rays meet at I' which is the image of O'. II', which is the perpendicular from I' on the axis, is
the complete image of OO'.

Fig. 2.8
Let y1 and y2 be the sizes of the object and the image and u and v their distances from the first
and the second principal points respectively. Let f1 and f2 be the focal lengths of the lens-
system. Thus, according to the sign convention, we have
OO' = y1, II' = – y2, H1O = – u, H2I = +v, H1F1 = – f1 and H2F2 = +f2
Now proceeding in four steps:
Step-1: In similar triangles F1H1B1 and O'A1B1, we have
𝐻1 𝐹1 𝐻1 𝐵1 𝐻1 𝐵1
= =
𝐴1 𝑂 ′ 𝐴1 𝐵1 𝐻1 𝐴1 + 𝐻1 𝐵1

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𝐻1 𝐹1 𝐼𝐼′
or =
𝐻1 𝑂 𝑂𝑂′ + 𝐼𝐼′

Putting the values according to the sign convention we get

−𝑓1 −𝑦2
= ....... (2.9)
−𝑢 𝑦1 − 𝑦2

Again, in similar triangles A2H2F2 and A2B2I', we have

𝐻2 𝐹2 𝐻2 𝐴2 𝐻2 𝐴2
= =
𝐵2 𝐼′ 𝐵2 𝐴2 𝐻2 𝐴2 + 𝐻2 𝐵2
𝐻2 𝐹2 𝑂𝑂′
or =
𝐻2 𝐼 𝑂𝑂′ + 𝐼𝐼′

Putting the values according to sign convention, we get

𝑓2 𝑦1
= ....... (2.10)
𝑣 𝑦1 − 𝑦2

On adding equations (2.9) and (2.10), we get,

−𝑓1 𝑓2 −𝑦2 + 𝑦1
+ =
−𝑢 𝑣 𝑦1 − 𝑦2
𝑓1 𝑓2
or + =1 ....... (2.11)
𝑢 𝑣

Step-2: Dividing equation (2.10) by (2.11), we obtain,

𝑓1 /𝑢 𝑦2
= −
𝑓2 /𝑣 𝑦1

∴ The linear transverse magnification is

𝑦2 𝑣 𝑓
m= = − ( 1) ....... (2.12)
𝑦1 𝑢 𝑓2

Step-3: Let us now consider a ray OA1. The corresponding emergent ray meet the second
principal plane at A2, where H1A1 = H2A2. It also passes through I. Since I is the image of O,
A2I is the emergent ray. If 𝜃1 and 𝜃2 be the angles which the rays OA1 and A2I make with the
principal axis then by sign convention, 𝜃1 is positive and 𝜃2 is negative. Now, from figure
2.8, we have,
𝐴1 𝐻1 𝑦1
tan 𝜃1 = =
𝐻1 𝑂 −𝑢
𝐴2 𝐻2 𝑦1
and tan (– 𝜃2) = =
𝐻2 𝐼 𝑣

tan 𝜃1 𝑣
∴ =
tan 𝜃2 𝑢

But, by Helmholtz’s law of magnification (𝜇 1y1 tan 𝜃1 = 𝜇 2y2 tan 𝜃2), we have
tan 𝜃1 𝜇2 𝑦2
=
tan 𝜃2 𝜇1 𝑦1
𝜇2 𝑦2 𝑣
∴ =
𝜇1 𝑦1 𝑢

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Hence the linear transverse magnification is

𝑦2 𝜇1 𝑣
= ( )
𝑦1 𝜇2 𝑢

Comparing this with equation 2.12, we get

𝑣 𝑓 𝜇1 𝑣
– ( 1) = ( )
𝑢 𝑓2 𝜇2 𝑢

𝑓 𝜇1
or ( 1) = − ....... (2.13)
𝑓2 𝜇2

Step-4: If the medium on the two sides of the system is the same, that is, 𝜇1 = 𝜇2, then
equation 2.13 can be written
f2 = – f1 ....... (2.14)
Now, putting f2 = – f1 = f (say) in equation (2.10), we get,
1 1 1
− = ....... (2.15)
𝑣 𝑢 𝑓

Similarly, putting f2 = – f1 in equation (2.11), we get,

𝑦2 𝑣
m= = ....... (2.16)
𝑦1 𝑢

These formulae are similar to those for a thin lens.

Thus, when the medium on both sides of lens-system is the same and, u and v are
measured from the first and the second principal planes respectively then the formula for
conjugate distances is exactly similar to that for a thin lens.
Self Assessment Questions
1. Define Cardinal points of a lens system.
2. Show that the distance between Principal points is the same as distance between nodal
points.
3. Show that principal points coincide with nodal points if the medium is same on both side
of the system.
4. Define lateral, axial and angular magnifications of a lens system. Establish relations
between them.

2.10 NEWTON’S FORMULA

If the distances of two conjugate points (object and image) on the principal axis from
the respective focal points be x1 and x2 then from Newton’s formula (equation 2.3), we have,
x1x2 = f1f2
This Newton’s formula can be easily derived by making use of cardinal points of the lens
system. Figure 2.10 shows a convergent coaxial lens-system in which XX' is the principal

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axis, H1 and H2 are the principal points, and F1, F2 are the focal points of the system. Then
H1F1 = f1 and H2F2 = f2, where f1 and f2 are the focal lengths of the system.
Let OO' be a small object placed perpendicular to the principal axis. An incident ray
O'A1 parallel to the axis meets the first principal plane at A1. It emerges through a point A2 in
the second principal plane such that H1A1 = H2A2, and passes through F2. Another ray O'B1
passing through F1 meets the first principal plane at B1. It emerges through B2 in the second
principal plane such that H1B1 = H2B2 and becomes parallel to the axis. The two emergent
rays meet at I' which is the image of O'. The perpendicular II', drawn from I' on the axis, is
the complete image of OO'.

Fig. 2.9
Let OO' = y1, II' = y2, F1O = x1 and F2I = x2. According to the sign convention y1, x2 and f2
are positive while y2, x1 and f1 are negative.
Using similar triangles H1B1F1 and OO' F1, we have
𝐻1 𝐵1 𝐻1 𝐹1
=
𝑂𝑂′ 𝐹1 𝑂

Putting the values with proper signs, we get,

𝑦2 −𝑓1 𝑓1
− = = ....... (2.17)
𝑦1 −𝑥1 𝑥1

Again from similar triangles triangles A2H2F2 and II' F2, we have,
𝐼𝐼′ 𝐹2 𝐼
=
𝐻2 𝐴2 𝐻2 𝐴2
−𝑦2 𝑥2
or = ....... (2.18)
𝑦1 𝑓2

Comparing equations (2.17) and (2.18), we obtain,

𝑓1 𝑥2
=
𝑥1 𝑓2

x1x2 = f1f2 ....... (2.19)

This is Newton’s formula. If the medium on both sides of the system is the same, then we
have, f2 = – f1 = f. Hence the Newton’s formula (equation 2.19) becomes
x1 x2 = – f 2 ....... (2.20)

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2.11 SUMMARY
1. The distances are measured from optical center of the lens. The distances measured along
the optical axis in the direction of incident ray are positive while those in opposite
direction are negative.
2. The distances measured above optical axis are positive while those below are negative.
3. The angles measured anticlockwise with respect to optical axis are positive while those
measured clockwise are negative.
4. A lens has six cardinal points: a pair of focal points, a pair of principal points and a pair
of nodal points. The planes passing through these points and normal to optical axis are
known as focal-, principal- and nodal- planes respectively.
5. The ray diagram for the image formation in lens follows the rules given below-
a. An incident ray parallel to optical axis passes or appears to pass through second focal
point after refraction.
b. An incident ray through first focal point or directed towards it emerges parallel to
optical axis after refraction.
c. An incident ray passing through the first principal plane at a certain height from the
principal axis emerges through the second principal plane at the same height and on
the same side of the axis.
d. An incident ray directed towards the first nodal point emerges parallel to its original
direction through the second nodal point.
6. Magnification is the ability to enlarge the size of image relative to the size of the object.
It is expressed as the ratio of size of image to that of object. It is of three kinds:
a) Lateral Magnification: It is the ratio of the length of the image to the length of object,
both being measured perpendicular to the axis of the system
b) Axial Magnification: It is the ratio of dx2 and dx1, where dx1 and dx2 are small
extensions of the object and the image respectively along the axis of the system.
c) Angular Magnification: It is the ratio of tan 𝜃2 and tan 𝜃1, where 𝜃1 and 𝜃2 are the
angles made by incident ray and the corresponding emergent ray with the optical axis.
7. The three types of magnification can be related as-
𝑓1 𝑓
a) mx × 𝑚𝜃 = my, (b) my × 𝑚𝜃 = − , (c) mx =– 𝑚𝑦2 (𝑓2 )
𝑓2 1

Where, mx = axial magnification, 𝑚𝜃 = angular magnification, my = lateral

magnification, f1= first focal length and f2 = second focal length
8. Helmholtz Lagrange’s magnification equation gives the interrelationship between angular
magnification, lateral magnification and axial magnification as
𝜇 1 y1 tan 𝜃1 = 𝜇 2 y2 tan 𝜃2 = 𝜇 3 y3 tan 𝜃3 = ……… = 𝜇 n yn tan 𝜃n
9. Some important relations in coaxial lens system are as follows-
𝑓 𝜇1 𝑣 𝑓 𝑓1 𝑓2
a) ( 1) = − , (b) m = − 𝑢 (𝑓1 ), (c) + =1
𝑓2 𝜇2 2 𝑢 𝑣
10. Newton’s Formula states that, if the distances of two conjugate points (object and image)
on the principal axis from the respective focal points be x1 and x2 then x1x2 = f1f2, where,
f1 and f2 are the first and second principal focal lengths of the system respectively.

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2.12 GLOSSARY
Pole: The midpoint of a spherical mirror.
Radius of curvature: The linear distance between the pole and the centre of curvature.
Centre of curvature: The centre of the sphere of which the spherical mirror is a part.
Optical Centre: The point on the optical axis of a lens where all rays passing through it
remain unrefracted.
Principal axis: The imaginary line passing through the pole and the centre of curvature of a
spherical mirror.
Paraxial rays: The rays that makes small angle (θ) to the optical axis of the system, and lies
close to the axis throughout the system.
Geometrical Centre: Physical centre of a lens as determined by measurement

2.13 REFERENCES
4. Optics – Ajoy Ghatak, Mc Graw Hill Publications, New Delhi
5. A Textbook of Optics – N. Subrahmanyam and Brij Lal, S. Chand and Company Ltd., New Delhi
6. Introductory University Optics, PHI Learning, New Delhi

2.14 SUGGESTED READINGS

4. Introduction to Geometrical Optics; Milton Katz 1994
5. Principles of Optics- B.K. Mathur, Digital Library of India
6. Introduction to Optics, The Indian Press, Allahabad

2.15 TERMINAL QUESTIONS

2.15.1 Short Answer Type
1. What are cardinal points of a coaxial optical system?
2. What are paraxial rays?
3. What are object space and image space?
4. Write down the rules of formation of image in a coaxial lens system.
2.15.2 Long Answer Type
1. Define cardinal points of a coaxial optical system and give their characteristics. Draw a
ray diagram to clarify each.
2. Define Axial, Lateral and Angular magnifications. How they are interrelated with each
other?
3. Deduce Helmholtz Lagrange equation for image formation for paraxial rays.

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4. Deduce Newton’s formula x1x2 = f1f2 for a coaxial optical system of two thin lenses.
Where x1 and x2 are the distances of the object and the image from the first and second
focal points respectively.
5. Show that the distance between two principal points is same as the distance between two
nodal points. Also, show that the principal points coincide with nodal points if the
medium is same on both sides of the system.
6. Prove the following relations for a coaxial lens-system separating two media of refractive
indices 𝜇 1 and 𝜇 2 :
𝑓1 𝑓2 𝑣 𝑓 𝑓1 𝜇1
(i) + =1, (ii) m = – ( 1) and (iii) =– ,
𝑢 𝑣 𝑢 𝑓2 𝑓2 𝜇2

where u and v are the distances of the object and the image from the first and the
second principal points respectively, f1 and f2 the focal lengths of the system and m is
linear transverse magnification.
7. What form do the three expressions, given in question number 6, take when the medium
on the two sides of the system is the same?

2.15.3 Numerical Questions

1. Calculate the axial and angular magnification for a coaxial optical system in which the
initial media is air and final media is water. The lateral magnification of the object is -3.0.
(Take refractive index of water = 4/3)

2.15.4 Objective Type Questions

1. The points lying on the principal axis of the optical system and conjugate to points at
infinity are
(a) Principal points (b) Focal points
(c) Nodal points (d) Cardinal points
2. Conjugate points on the principal axis of the optical system having unit positive linear
magnification are called
(a) Principal Points (b) Focal points
(c) Nodal points (d) Cardinal point
3. Two Principal planes on a thin lens coincide and pass through
(a) First Principal Point (b) First focal point
(c) First Nodal Point (d) The optical centre of the lens
4. If h be the transverse distance of the point from the axis at which the ray meets the lens and
f the focal length of the lens, then the angular deviation of the ray will be
(a) h / f (b) f / h
(c) h2 / f2 (d) f2 / h2
5. L1 and L2 be two thin convergent lenses of focal lengths f1 and f2 placed coaxially at a
distance d apart. If H1, H2 be two principal points, then

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𝐹𝑑 𝐹𝑑
(a) L1H1 = + (b) L2H2 = -
𝑓1 𝑓1

𝐹𝑑 𝐹𝑑
(c) L1H1 = - (d) L2H2 =
𝑓1 𝑓1

6. If a thin lens is placed such that refractive indices on the two sides are n 1 and n2. The ratio
of focal length f1 / f2 is
(a) – n1 / n2 (b) – n2/n1
(c) n1 / n2 (d) n2 / n1
7. An incident ray directed towards the first nodal point emerges
(a) Perpendicular to its original direction through first principal point
(b) Parallel to its original direction through second nodal point
(c) Opposite to its original direction through first focal point
(d) None of these

2.16 ANSWERS
Numerical Questions
4
Given, 𝜇1 = 1, 𝜇2 = 3 and my = 3.0
𝑓
Axial magnification, mx =– 𝑚𝑦2 ( 2)
𝑓1
𝑓1 𝜇1
Also, ( )=−
𝑓2 𝜇2
𝜇2
mx = – 𝑚𝑦2 (− ) = 12
𝜇1
Angular magnification, 𝑚𝜃 = my /mx = - 0.25

Objective Type Questions

1. (b), 2. (a), 3. (d), 4. (a), 5. (b), 6. (a), 7. (c)

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UNIT 3: THICK LENS, LENSES COMBINATIONS

AND TELESCOPE

3.1 Introduction
3.2 Objectives
3.3 Cardinal Points of a Thick Lens
3.3.1 Focal Points
3.3.2 Principal Points
3.3.3 Nodal Points
3.4 Focal Length of a Thick Lens
3.4.1 Position of Cardinal Points
3.5 Variation of Focal Length of a Thick Bi-convex Lens
3.6 Power of a Thick Lens
3.7 Telescope
3.7.1 Astronomical Telescope
3.7.2 Terrestrial Telescope
3.7.3 Newtonian Reflecting Telescope
3.7.4 Cassegrain Reflecting Telescope
3.7.5 Advantages of Reflecting Type Telescope over Refracting Type Telescope
3.8 Summary
3.9 Glossary
3.10 References
3.11 Suggested Reading
3.12 Terminal Questions
3.13 Answers

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3.1 INTRODUCTION
So far we have discussed the image formation of an optical system consisting of thin
lenses. However, most practical applications require the use of thick lenses. In order to
produce sufficient illumination of the image for virtual observation the optical instruments
such as telescopes and photographic objectives require wide apertures. Practically, the lenses
thus produced are thick in nature.
In this unit, we will discuss the image formation by thick lens. The thickness of a lens is
defined as the separation between the poles of its spherical refracting surfaces. When this
thickness is comparable to its focal length then the lens is said to be thick. The distances in
case of thick lens cannot be measured from a single optical centre as in the case of thin lens;
however the distances can be referred to from the poles on the two surfaces. It was shown by
C.F. Gauss that the formulae of thin lenses are applicable to thick lenses too if the position of
certain specific points for a lens called cardinal points are known. In this way, a thick lens is
treated as a combination of thin lenses. We have already studied that there are six cardinal
points of a lens namely two principal points, two focal points and two nodal points. We will
also discuss the variation of focal length of different types of lenses with its thickness by
considering different examples.

3.2 OBJECTIVES
After studying this unit, you will be able to,
 Know the difference between thin and thick lenses
 Locate the cardinal points of a thick lens
 Classify the lens on the basis of their shape
 Calculate power of a lens
 Know about the optical devices like different types of telescopes
 Understand the advantages of reflecting telescope over refracting telescope

3.3 CARDINAL POINTS OF A THICK LENS

There are six cardinal points of a thick lens; two focal points F1, F2; two principal
points H1, H2 and two nodal points N1, N2. You can see the cardinal points and the
corresponding cardinal planes for a thick convex lens in figure 3.1. When the lens is placed in
air, the nodal points N1, N2 coincide with the principal points H1, H2 respectively. The
formation of the image II' of an object OO' is also shown in this figure. Although, the cardinal
points are explained in the previous unit but we are again discussing them briefly, in this unit,
with reference to thick lenses.

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3.3.1 Focal Points

The focal points F1, F2 are a pair of points lying on the principal axis and conjugate to
points at infinity. An incident ray O'A1 parallel to the principal axis, after refraction through
the lens, passes through the second focal point F2, while an incident ray O'B1 through the first
point F1, after refraction, emerges parallel to the principal axis XX'.

Figure 3.1
3.3.2 Principal Points
The principal points H1, H2 are a pair of conjugate points on the principal axis having
unit positive linear transverse magnification. An incident ray meeting the first principal plane
at a certain height from the principal axis emerges through the second principal plane at the
same height and on the same side of the axis.
You can see O'A1 is a ray parallel to the axis meeting the first principal plane at A1. It
will emerge from the lens through A2 on the second principal plane such that H1A1 = H2A2,
and also pass through F2. O'F1 is another ray through the first focal point F1 and meeting the
first principal plane in B1. It will emerge parallel to the axis through B2 such that H1B1 =
H2B2.
3.3.3 Nodal Points
The nodal points N1, N2 are a pair of conjugate points on the principal axis having unit
positive angular magnification. They are such that an incident ray directed towards N1
emerges through N2 parallel to itself. An incident ray O'N1 and its conjugate parallel
emergent ray N2I' are shown in figure 3.1.

3.4 FOCAL LENGTH OF A THICK LENS

Let us consider a convex lens of thickness t and refractive index 𝜇 placed in air. Let R1
and R2 be the radii of curvature of the faces of the lens. The lens is a combination of two
refracting surfaces with poles P1 and P2 (Figure 3.2). Draw a ray PQ parallel to the principal
axis, incident on the first surface at a height h1 above the axis. After refraction at the first
surface, it follows the path QR in the lens and meets the second surface of the lens at a height
h2 above the axis. This ray, if produced forward, will meet the axis at point S, which acts as

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virtual object for the second surface. After refraction at the second surface, the emergent ray
intersects the principal axis at F2 which is the second focal point of the lens.
Let us produce the incident ray PQ forward and the emergent ray RF2 backward such
that they meet at A2. The plane through A2 and perpendicular to the axis is the second
principal plane, and its point of intersection with the principal axis, H2 is the second principal
point. H2F2 is the focal length f of the lens.

Figure 3.2
The refraction formula for a single surface is
𝜇 1 𝜇−1
− =
𝑣 𝑢 𝑅

For refraction at the first surface (from air to lens) we can write
𝜇 = ∞, 𝑣 = 𝑃1 𝑆 and R = R1
𝜇 1 𝜇−1
∴ − =
𝑃1 𝑆 ∞ 𝑅1
1 𝜇−1
or = ....... (3.1)
𝑃1 𝑆 𝜇𝑅1

For refraction at the second surface (from lens to air), we can write
u = P2S, v = P2F2 and R = R2.
Also in this case, from lens to air (i.e., from denser to rarer), 𝜇 will be replaced by 1/𝜇. Hence
1
1/𝜇 1 ( )− 1
𝜇
− =
𝑃2 𝐹2 𝑃2 𝑆 𝑅2
1 𝜇 1− 𝜇
or = + ....... (3.2)
𝑃2 𝐹2 𝑃2 𝑆 𝑅2

Now, from similar triangles A2F2H2, RF2P2 and from similar triangles QSP1 and RSP2, we
have,
𝐻2 𝐹2 ℎ1 𝑃1 𝑆
= =
𝑃2 𝐹2 ℎ2 𝑃2 𝑆
1 1 𝑃1 𝑆 1 𝑃1 𝑆
or = = ....... (3.3)
𝑃2 𝐹2 𝐻2 𝐹2 𝑃2 𝑆 𝑓 𝑃2 𝑆
1
Substituting the value of in equation (3.2), we get,
𝑃2 𝐹2

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1 𝑃1 𝑆 𝜇 1− 𝜇
= +
𝑓 𝑃2 𝑆 𝑃2 𝑆 𝑅2
1 𝜇 𝑃2 𝑆 1− 𝜇
or = +
𝑓 𝑃1 𝑆 𝑃1 𝑆 𝑅2

You can see from figure 3.2 that, P2S = P1S – P1P2 = P1 S – t.
1 𝜇 𝑃1 𝑆−𝑡 1− 𝜇
∴ = +
𝑓 𝑃1 𝑆 𝑃1 𝑆 𝑅2

𝜇 𝑡 1− 𝜇
= + (1 − )
𝑃1 𝑆 𝑃1 𝑆 𝑅2
1
Putting the value of from equation 3.1 in this expression, we obtain,
𝑃1 𝑆

1 𝜇−1 𝑡(𝜇−1) 1− 𝜇
= + {1 − }
𝑓 𝑅1 𝜇𝑅1 𝑅2

𝜇−1 𝜇−1 𝑡(𝜇−1)2

= − +
𝑅1 𝑅2 𝜇𝑅1 𝑅2

1 1 1 (𝜇−1)𝑡
or = (𝜇 − 1) { − + } ....... (3.4)
𝑓 𝑅1 𝑅2 𝜇 𝑅1 𝑅2

This is the expression for compute focal length of a thick lens.

3.4.1 Position of Cardinal Points
Let us now compute the positions of cardinal points.
Second Focal Point (𝜷𝟐 ): The distance of the second focal point F2 from the second
surface of the lens is P2F2. Using equation 3.3, we can write,
𝑃2 𝑆 𝑃1 𝑆−𝑡 𝑡
𝑃2 𝐹2 = 𝑓 =𝑓 = 𝑓 (1 − )
𝑃1 𝑆 𝑃1 𝑆 𝑃1 𝑆
1
Substituting the value of 𝑃 𝑆 from equation 3.1 we get
1

(𝜇−1) 𝑡
P2F2 = +f [1 − ]
𝜇𝑅1

(𝜇−1) 𝑡
or β2 = +f [1 − ] ....... (3.5)
𝜇𝑅1

Second Principal Point (∝𝟐): The distance of the second principal point H2 from the
second surface P2 is
P2H2 = F2H2 – F2P2
= – H2F2 + P2F2
Substituting the value of P2F2 from equation 3.5 we get
(𝜇−1) 𝑡
P2H2 = –f + f [1 − ]
𝜇𝑅1

(μ−1) t
=–f
μR1

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(𝜇−1) 𝑡
or ∝𝟐 = – f ....... (3.6)
𝜇𝑅1

First Focal Point (𝜷𝟏 ): If we consider the incident ray PQ as shown in figure 3.2, coming
from right, then R1 and R2 will interchange and the signs of f, R1 and R2 will become
opposite. Now, the distance of the first focal point F1 from the first surface P1 is (from
equation 3.5)
(𝜇−1) 𝑡
P1F1 = – f [1 − ]
𝜇(−𝑅2 )

(𝜇−1) 𝑡
= –f [1 + ]
𝜇𝑅2

(𝜇−1) 𝑡
𝛽1 = –f [1 + ] ....... (3.7)
𝜇𝑅2

First Principal Point (∝𝟏 ): The distance of the first principal point H1 from the first
surface P1 is (from equation 3.6)
(𝜇−1) 𝑡
P1H1 = +f
𝜇(−𝑅2 )

(𝜇−1) 𝑡
=–f
𝜇𝑅2

(𝜇−1) 𝑡
∝𝟏 = – f ....... (3.8)
𝜇𝑅2

Nodal Points: Since the medium on both sides of the lens is same (air), the nodal points N1
and N2 are the same as the principal points H1 and H2 respectively.

3.5 VARIATION OF FOCAL LENGTH OF A THICK BI-

CONVEX LENS
We have calculated the focal length of a thick lens in a previous section as
1 1 1 (𝜇−1)𝑡
= (𝜇 − 1) { − + } ....... (3.9)
𝑓 𝑅1 𝑅2 𝜇 𝑅1 𝑅2

Let us consider a biconvex lens having R1 = +r1 and R2 = –r2 where r1, r2 > 0, we have,
1 1 1 (𝜇−1)𝑡
= (𝜇 − 1) { + − } ....... (3.10)
𝑓 𝑟1 𝑟2 𝜇 𝑟1 𝑟2

For a thin lens i.e. t = 0 the focal length fo can be obtained as

1 1 1
= (𝜇 − 1) { + } ....... (3.11)
𝑓0 𝑟1 𝑟2

Substituting equation 3.11 in equation 3.10, we get,

1 1 (𝜇−1)2 𝑡
= − ....... (3.12)
𝑓 𝑓0 𝜇 𝑟1 𝑟2

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From this equation we can infer that if the value of t increases from zero to upwards, the
1
value of decreases which means the focal length of a lens increases with the increase in
𝑓
thickness. We know that the power of a lens is inversely proportional to the focal length;
therefore the power of a thick lens is less than the power of a thin lens for the same refracting
surfaces. When we increase the thickness of a biconvex lens continuously a situation arises
1
when becomes zero and on further increasing thickness, the lens becomes a concave lens
𝑓
(diverging). This is the critical value of thickness, denoted by tc and can be obtained by
equation 3.10. Therefore, we can write
1 1 1 (𝜇−1)𝑡𝑐
= (𝜇 − 1) { + − }=0
𝑓 𝑟1 𝑟2 𝜇 𝑟1 𝑟2

1 1 (𝜇−1)𝑡𝑐
or + − =0
𝑟1 𝑟2 𝜇 𝑟1 𝑟2

(𝜇−1)𝑡𝑐 1 1
or = +
𝜇 𝑟1 𝑟2 𝑟1 𝑟2
𝜇
or tc = (𝑟1 + 𝑟2 ) ....... (3.13)
(𝜇−1)

If the radii of curvature of both the surfaces of biconvex lens is same then we can write r1 = r2
= r, so
𝜇
tc = . 2𝑟 ....... (3.14)
(𝜇−1)

3.6 POWER OF A THICK LENS

The focal length of a thick lens is given by
1 1 1 (𝜇−1) 𝑡
= (𝜇 − 1) [ − + . ]
𝑓 𝑅1 𝑅2 𝑅1 𝑅2 𝜇

𝜇−1 𝜇−1 (𝜇−1)2 𝑡

= − + .
𝑅1 𝑅2 𝑅1 𝑅2 𝜇

If P is the power of the lens then we have,

1 𝜇−1 𝜇−1 (𝜇−1)2 𝑡
P= = − + .
𝑓 𝑅1 𝑅2 𝑅1 𝑅2 𝜇

The power of the first refracting surface is given by

𝜇−1
P1 =
𝑅1

Similarly, the power of second refracting surface is given by

1− 𝜇 𝜇−1
P2 = =− .
𝑅2 𝑅2
𝑡
∴ P = P1 + P2 – P1 P2 . ....... (3.15)
𝜇

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Self Assessment Questions

1. What is a thick lens?
2. How many cardinal points a thick lens has? Name them.
3. Obtain an expression to show the variation of focal length of thick lens with thickness.
4. What is critical thickness of a lens?

Example 3.1: A convergent thick lens has radii of curvature 10.0cm and – 6.0cm, 𝜇 = 1.60
and thickness t = 5.0cm. Deduce its focal length. At what value of t will the lens become
divergent?
Solution: The focal length of a lens of thickness t is given by
1 1 1 (𝜇−1)𝑡
= (𝜇 − 1) [ − + ].
𝑓 𝑅1 𝑅2 𝜇𝑅1 𝑅2

Here, 𝜇 = 1.60, R1 = +10.0 cm, R 2 = −6.0 cm and t = 5.0 cm.

1 1 1 (1.60−1)×5.0
∴ = (1.60 − 1) [ + + ]
𝑓 10.0 6.0 1.60×10.0×(−6.0)
480
or f= = +7.08 cm.
67.8

At critical value of thickness tc the lens will become convergent. Using equation 3.13 we can
write
𝜇(𝑟1 +𝑟2 )
tc =
𝜇−1

Putting 𝜇 = 1.60, r1 = 10.0 cm and r2 = 6.0 cm, we get

1.60 (10.0+6.0)
tc = = 42.7 cm
(1.60−1)

Beyond a thickness of 42.7cm, the lens will become divergent.

Example 3.2: Show that for a thin lens the two principal planes coincide.
Solution: The positions of first and second principal planes of a thick lens are
−𝑓 (𝑛−1)𝑡
𝛼1 = 𝑃1 𝐻1 =
𝑛𝑅2

−𝑓 (𝑛−1)𝑡
𝛼2 = 𝑃2 𝐻2 =
𝑛𝑅1

For a thin lens t = 0 and P1 and P2 coincide, so P1H1 = P2H2 = 0, i.e., H1 and H2 coincide and
lie at the optical centre of the lens.

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3.7 TELESCOPES
A telescope is an optical device which enables us to see the distant objects clearly. It
provides angular magnification of the distant objects. There are two types of telescopes.
1. Refracting Telescopes: These types of telescopes work on refraction phenomenon of
light and therefore make use of lenses to view distant objects. As its name suggests this
device works on refraction phenomenon that is why lenses are used here. These are of
two types :
(a) Astronomical Telescope: It is used to see heavenly objects like sun, stars, planets,
etc. The final image formed by this telescope is inverted but it does not make any
difference in the case of heavenly bodies because of their round shape.
(b) Terrestrial Telescope: It is used to see distant objects on the surface of the earth.
The final image formed by this telescope is erect. This is an essential condition of
viewing the objects on earth’s surface correctly.
2. Reflecting Telescopes: These make use of converging mirrors to view the distant
objects. For example, Newtonian and Cassegrain telescopes.
3.7.1 Astronomical Telescope
Astronomical telescope uses refraction phenomenon of light to see heavenly bodies like
sun, stars, planets, satellites etc. It consists of two converging lenses mounted coaxially at the
outer ends of two sliding tubes. One of which is used as an eyepiece while the other is used as
objective.
1. Objective: It is a convex lens of large focal length and a much larger aperture. It faces
the distant object. In order to form bright image of the distant object, the aperture of the
objective is taken large so that it can gather sufficient light from the distant objects.
2. Eyepiece: It is also a convex lens but of short focal length. It faces the eye. The aperture
of the eyepiece used is also taken small so that whole light of the telescope may enter the
eye for distinct vision. It is mounted in a small tube which can slide inside the bigger tube
carrying the objective.
Working
1. When the Final Image is Formed at the Least Distance of the Distinct Vision:
The parallel beam of light coming from the distant objects falls on the objective at some
angle 𝛼 as shown in figure 3.3. The objective focuses the beam in its focal plane and forms a
real, inverted and diminished image A'B'. This image A'B' acts as an object for the eyepiece.
The distance of the eyepiece is so adjusted that the image A'B' lies within its focal length.

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Fig. 3.3
The eyepiece magnifies this image so that final image A"B" is magnified and inverted with
respect to the object. The final image is seen distinctly by the eye at the least distance of the
distinct vision.
Magnifying Power: The magnifying power of a telescope is defined as the ratio of the
angle subtended at the eye by the final image formed at the least distance of the distinct
vision to the angle subtended at the eye by the object at infinity, when seen directly.
Since the distance of the object from the telescope is very large therefore the angle
subtended by it at the eye is practically equal to the angle 𝛼 subtended by it at the objective.
Hence,
∠A'OB' = 𝛼
Suppose ∠A"EB" = 𝛽
𝑡𝑎𝑛 𝛽 𝛽
Magnifying power, m= = (∵ 𝛼 and β are small angles)
𝑡𝑎𝑛 𝛼 𝛼

According to the sign convention

OB' = +fo = focal length of the objective
B'E = -ue = distance of A'B' from the eyepiece acting as an object for it
𝑓𝑜
∴ m=−
𝑢𝑒
1 1 1
Again, for the eyepiece, u = -ue and v = – D. Thus from equation − = , we have,
𝑣 𝑢 𝑓
1 1 1
∴ − =
−𝐷 −𝑢𝑒 𝑓𝑒

1 1 1 1 𝑓
or = + = (1 + 𝑒)
𝑢𝑒 𝑓𝑒 𝐷 𝑓𝑒 𝐷

Substituting this in above expression of m, we get,

𝑓𝑜 𝑓
m=− (1 + 𝑒) ....... (3.16)
𝑢𝑒 𝐷

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As we can see, for large magnifying power, fo >> fe. The negative sign of the magnifying
power indicates that the final image is real and inverted.
2. When the Final Image is Formed at Infinity: Normal Adjustment:
When a parallel beam of light is incident on the objective as shown in figure 3.4, it
forms a real, inverted and diminished image A'B' in its focal plane. The eyepiece is so
adjusted that the image A'B' exactly lies at its focus. Therefore, the final image is formed at
infinity and is highly magnified and inverted with respect to the object.
Magnifying Power: It is defined as the ratio of the angle subtended at the eye by the final
image as seen through the telescope at the eye to the angle subtended by the object seen
directly when both the image and object lie at infinity at the eye. As the distance of the object
from the telescope is very large, the angle subtended by it at the objective is
∠A'OB' = 𝛼
Also, let ∠A'EB' = 𝛽
𝑡𝑎𝑛 𝛽 𝛽
∴ Magnifying power, m= = (∵ 𝛼 and β are small angles)
𝑡𝑎𝑛 𝛼 𝛼
𝐴′ 𝐵′/𝐵 ′ 𝐸 𝑂𝐵′
= =
𝐴′ 𝐵′ /𝑂𝐵′ 𝐵′𝐸

Fig. 3.4
Applying the cartesian sign convention.
OB' = +fo = Distance of A'B' from the objective along the incident light
and B'E = -fe = Distance of A'B from the eyepiece against the incident light
𝑓𝑜
∴ m=− ....... (3.17)
𝑓𝑒

As we can see, for large magnifying power, fo >> fe. The negative sign of m indicates that the
image is real and inverted.
3.7.2 Terrestrial Telescope
It is a refracting telescope which is used to see erect images of distant earthy objects. It
uses an additional convex lens between objective and eyepiece for obtaining an erect image.

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Fig. 3.5
In this telescope the objective forms a real, inverted and diminished image A'B' of the
distant object in its focal length from the focal plane of the objective as shown in figure 3.5.
This lens forms a real, inverted and equal size image A"B" of A'B'. This image is now erect
with respect to the distant object. The eyepiece is so adjusted that the image A"B" lies at its
principal focus. Hence the final image is formed at infinity and is highly magnified and erect
with respect to the distance object. As the erecting lens does not cause any magnification, the
angular magnification of the terrestrial telescope is the same as that of the astronomical
telescope.
If the image is formed at infinity the
𝑓𝑜
m=
𝑓𝑒

and if image is formed at the least distance of distinct vision then

𝑓𝑜 𝑓
m= (1 + 𝐷𝑒 ) ....... (3.18)
𝑓𝑒

Drawbacks
1. The length of the terrestrial telescope is much larger than the astronomical telescope. In
normal adjustment, the length of a terrestrial telescope = fo + 4f + fe’ where f is the focal
length of the erecting lens.
2. Due to extra reflection at the surfaces of the erecting lens, the intensity of the final image
decreases.

3.7.3 Newtonian Reflecting Telescope

The first reflecting telescope was set up by Newton in 1668. It consists of a large
concave mirror of large focal length as the objective, made of an alloy of copper and tin as
shown in figure 3.6.

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Fig. 3.6
A beam of light from the distant star is incident on the objective. Before the rays are
focused at F, a plane mirror inclined at 450 intercepts them and turns them towards an
eyepiece adjusted perpendicular to the axis of the instrument. The eyepiece forms a highly
magnified, virtual and erect image of the distant object.

3.7.4 Cassegrain Reflecting Telescope

It consists of a large concave parabolic (primary) mirror having a hole of its centre. It is
a small convex (secondary) mirror near the focus of the primary mirror. The eyepiece is place
on the axis of the telescope near the hole of the primary mirror (figure 3.7).
The parallel rays from the distant object are reflected by the large concave mirror.
Before these rays come to focus at F, these are reflected by a small convex mirror and are
converged to a point I, just outside the hole. The final image formed at I is viewed through
the eyepiece. As the first image at F is inverted with respect to the distant object and the
second image I is erect with respect to the first image F, hence the final image is inverted
with respect to the object.

Fig. 3.7
Let fo be the focal length of the objective and fe that of the eyepiece then for the final image
formed at the least distance of distinct vision, we have,
𝑓𝑜 𝑓
m= (1 + 𝑒)
𝑓𝑒 𝐷

For the final image formed at infinity

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𝑓𝑜 𝑅/2
m= = ....... (3.19)
𝑓𝑒 𝑓𝑒

3.7.5 Advantages of a Reflecting Telescope over Refracting Telescope

A reflecting–type telescope has the following advantages over a refracting–type
telescope.
 A concave mirror of large aperture has high gathering capacity and absorbs very less
amount of light than the lenses of large apertures. The final image formed in reflecting
telescope is very bright. So even very distant or faint stars can be easily viewed.
 Due to the large aperture of the mirror used, the reflecting telescopes have high resolving
power.
 As the objective is a mirror and not a lens, it is free from chromatic aberration (formation
of coloured image of a white object).
 The use of parabolic mirror reduces the spherical aberration (formation of non-point
blurred image of a point object).
 A mirror requires grinding and polishing of one surface only. So it costs much less to
construct a reflecting telescope than a refracting telescope of equivalent optical quality.
 A lens of large aperture tends to be very heavy, and therefore, difficult to make and
support by its edges. On the other hand, a mirror of equivalent optical quality weighs less
and can be supported over its entire back surface.
Currently, reflecting and refracting telescopes have their own roles to play. Reflecting
telescopes are used more and more in astronomy due to their ability to see much farther and
much clearer. On the other hand, refracting telescopes are used more in everyday items like
binoculars and camera lens system due to their straightforward designs and lower
construction costs.
Example 3.3: A refracting astronomical telescope uses objective lens and eyepiece of focal
lengths 60 cm and 3 cm. Find the magnifying power of telescope and also distance between
the objective and eye piece, if the final image formed at infinity.
Solution: Given fo= 60 cm and fe = 3 cm
The magnifying power of telescope when the final image formed at infinity is

Also the distance between objective and eye piece is given by L =

Thus magnifying power of telescope is 20 and distance between objective and eye piece is 63
cm.
Example 3.4: The magnifying power of a telescope in normal adjustment position is 30 and
its length is 93 cm. Find the focal length of the objective and the eye piece.
Solution: The magnifying power of telescope when the final image formed at infinity is

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Also the length of telescope is given by L=

Given M = 30 and L = 93

Thus,

Hence or

Also as

Thus the focal lengths of objective lens and eye lens are 90 cm and 3 cm respectively.

3.8 SUMMARY
1. Thickness of a lens is a separation between the poles of its spherical refracting surfaces.
2. When the thickness of a lens is comparable to its focal length, the lens is said to be thick.
3. A thick lens is considered as a combination of thin lenses.
4. A thick lens has six cardinal points: two focal points F1, F2; two principal points H1, H2
and two nodal points N1, N2.
5. The focal length increases with the increase in a thickness of a lens.
6. A telescope is an optical device which enables us to see the distant objects clearly. It
provides angular magnification of the distant objects.
7. Reflecting telescopes use converging mirrors to show distant objects. Examples are:
Newtonian and Cassegrain telescope.
8. Refracting telescopes use lenses to see heavenly objects like Sun, Stars, Planets etc. from
the surface of the earth. Example: Astronomical and Terrestrial telescopes.
9. Generally all telescopes consist of two converging lenses mounted coaxially at the outer
ends of two sliding tubes. One lens is called objective, facing the distant object and
another lens is eyepiece, facing the eye. Both lenses are convex in nature.

3.9 GLOSSARY
Aperture – Opening which allows light to reach the lens.
Erect – upright or strait
Inverted – opposite position
Distance of distinct vision – a minimum comfortable distance between the naked human eye
and a visible object
Infinity – endless, limitlessness
Terrestrial – something relating to the earth

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Intercept – obstruct
Parabolic mirror – it is a reflective surface used to collect or project light rays (energy)
Principal axis – the straight line joining the centres of curvature of two bounding surfaces is
called the principal axis of the lens.

3.10 REFERENCES
1. Optics – Ajoy Ghatak, Mc GrawHill Publications, New Delhi
2. A Textbook of Optics – N. Subrahmanyam and Brij Lal, S. Chand and Company Ltd., New Delhi
3. Introductory University Optics, PHI Learning, New Delhi

3.11 SUGGESTED READINGS

1. Introduction to Geometrical Optics; Milton Katz 1994
2. Principles of Optics- B.K. Mathur, Digital Library of India
3. Introduction to Optics, The Indian Press, Allahabad

3.12 TERMINAL QUESTIONS

SHORT ANSWER TYPE QUESTIONS
1. Write formula for the focal length of a thick lens.
2. What is meant by chromatic aberration of a lens?
3. What is achromatism?
4. What is the condition of achromatism of two thin lenses of same material placed at
distance d apart?
5. What is the condition for minimum spherical aberration for two lens placed at a distance
d apart?
OBJECTIVE–TYPE QUESTIONS
1. When the medium on the two sides of a lens-system is same, the principal points coincide
with:
(a) Focal points (b) nodal points (c) centres of curvature (d) none of these
2. If mx, my and 𝑚𝜃 be the longitudinal, lateral and angular magnifications respectively, then
choose the correct relation (s):
(a) mx × my = 𝑚𝜃 (b) mx × 𝑚𝜃 = my
(c) mx ∝ my (d) mx ∝ 𝑚𝜃2
3. A convex lens of focal length f1 and a concave lens of focal length f2 are placed at a
distance d apart. The focal length of the combination is:
𝑓 𝑓 𝑓 𝑓
(a) 𝑓 +1𝑓 2− 𝑑 (b) 𝑓 +1𝑓 2+ 𝑑
1 2 1 2

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𝑓 𝑓 𝑓 𝑓
(c) 𝑑+ 1𝑓 −2 𝑓 (d) 𝑑− 1𝑓 +2 𝑓
1 2 1 2

4. When an extended white object is placed before a convex lens, coloured images are
formed. The image of the least size will be of the colour:
(a) red (b) yellow (c) green (d) violet
5. For a parallel incident white beam, the longitudinal chromatic aberration of a lens of unit
focal length is numerically equal to:
(a) its focal length (b) its dispersive power
(c) 1 (d) ∞
6. Two lenses of focal lengths f1 and f2, made of glasses of dispersive powers 𝜔 and 2 𝜔
respectively, form an achromatic combination when placed in contact. Then:
(a) f2 = f1/2 (b) f2 = – f1/2
(c) f2 = – 2f1 (d) f2 = 2f1
𝜔1 𝜔2
7. The condition of achromatism + = 0 holds for:
𝑓1 𝑓2

(a) longitudinal and lateral chromatic aberrations

(b) longitudinal chromatic aberration only
(c) lateral chromatic aberration only
(d) neither longitudinal nor lateral chromatic aberration
8. Two lenses in contact form an achromatic doublet. Their focal lengths are in the ratio 2:
3. The dispersive powers of their material must be in the ratio:
(a) 2 : 3 (b) 1 : 3 (c) 3 : 1 (d) 3 : 2
9. A convex crown-glass lens of focal length 20cm and dispersive power 0.018 forms an
achromatic doublet when placed in contact with a flint-glass lens of dispersive power
0.036. The focal length of the combination is:
(a) – 20cm (b) – 40 cm (c) +20cm (d) +40cm
10. Two lenses of same material and focal lengths f1 and f2 show achromatism when the
distance between them is:
𝑓1 + 𝑓2
(a) zero (b) f1 ~ f2 (c) (d) f1 + f2
2

11. Parallel paraxial rays incident on a convex lens are converged by the lens at an axial
points F. The marginal rays incident on the same lens will be converged at :
(a) a points further away than F (b) a point nearer the lens than F
(c) F only
(d) a point further or nearer than F depending on the focal length of the lens.

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12. The variation in focal length of a lens when we pass from the central portion to the
periphery is called:
(a) spherical aberration (b) astigmatism
(c) comma (d) chromatic aberration
13. Spherical aberration of a lens may be reduced by designing the lens so that the deviation
of a ray is:
(a) maximum at the first surface (b) minimum at the first surface
(c) equally shared by the two surfaces (d) reduced to a minimum
NUMERICAL QUESTIONS
1. The focal length of objective lens and eyepiece of a telescope is 72 cm and 1.2 cm. Find
its angular magnification and length for relaxed eye. (Ans. angular magnification = 60,
length for relaxed eye = 73.2 cm.)
2. The magnifying power of a telescope for relaxed eye is 24 and its length is 75 cm. Find
the focal length of the objective and the eye piece. (Ans. focal length of the objective = 72
cm and the focal length of eye piece = 3 cm)

3.13 ANSWERS
SHORT ANSWER TYPE QUESTIONS
1 1 1 (𝜇−1)𝑡
1. Ans. 𝑓 = (𝜇 − 1) {𝑅 − 𝑅 + }
1 2 𝜇 𝑅1 𝑅2
2. The image of a white object formed by a lens is usually coloured and blurred due to
different refractive indices of the lens material for different wavelengths of light. This
defect of image is called ‘chromatic aberration’.
3. The process of elimination of chromatic aberration by combining two or more lenses is
called achromatism.
𝑓1 + 𝑓2
4. d = , where f1 and f2 are the focal lengths of the two lenses.
2
5. d = f1 – f2.

OBJECTIVE TYPE QUESTIONS:

1 (b) 5 (b) 9 (d) 13 (c)

2 (b) and (c) 6 (c) 10 (c)

3 (d) 7 (a) 11 (b)

4 (d) 8 (a) 12 (a)

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UNIT 4: INTERFERENCE OF LIGHT WAVES

Structure
4.1 Introduction

4.2 Objective

4.3 Wave Nature of Light

4.3.1. Monochromatic Light and Ordinary Light

4.3.2. Plane Wave

4.3.3. Polarized and Unpolarized Light

4.3.4. Phase Difference and Coherence

4.3.5. Optical Path and Geometric Path

4.4 Principle of Superposition

4.5 Interference

4.5.1 Theory of Superposition

4.5.2 Condition for Maxima or Bright Fringes

4.5.3 Condition for Minima or Dark Fringes

4.5.4 Intensity Distribution

4.6 Classification of Interference

4.6.1 Division of Wavefront

4.6.2 Division of Amplitude

4.7 Young’s Double Slit Experiment

4.8 Coherence Length and Coherence Time

4.8.1 Coherence Length

4.8.2 Coherence Time

4.8.3 Spatial Coherence

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4.8.4 Temporal Coherence

4.9 Conditions for Sustainable Interference

4.10 Interference Due to Thin Sheet

4.11 Fresnel’s Biprism

4.11.1 Experimental Arrangement of Biprism Apparatus

4.11.2 Lateral Shift

4.11.3 Measurement of Wave Length of Light (λ) by Fresnel’s Biprism

4.12 Interference with White Light

4.13 Solved Examples

4.14 Summary

4.15 Glossary

4.16 References

4.17 Suggested Reading

4.18 Terminal Questions

4.18.1 Short Answer Type Questions

4.18.2 Long Answer Type Questions

4.18.3 Numerical Questions

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4.1 INTRODUCTION
In 1680 Huygens proposed the wave theory of light. But at that time, it was not clear
about the nature of light wave, its speed and way of propagation. In 1801 Thomas Young
performed an experiment called Young’s double slit experiment and noticed that bright and
dork fringes are formed which is called inference pattern. At that time it was a surprising
phenomenon and is to be explained.

After the Maxwell’s electromagnetic theory it was cleared that light is an

electromagnetic wave. In physics, interference is a phenomenon in which two waves
superimpose on each other to form a resultant wave of greater or lower or of equal amplitude.
When such two waves travel in space under certain conditions the intensity or energy of
waves are redistributed at certain points which is called interference of light and we observe
bright and dark fringes.

4.2 OBJECTIVES
After reading this unit you will able to understand

 The wave nature of light

 Phase and phase changes in light wave
 Coherence and coherent source of light
 Principle of superposition
 Young’s double slit experiment and explanation
 Interference
 Interference phenomena in biprism and thin sheets

4.3 WAVE NATURE OF LIGHT

Light wave is basically an electromagnetic wave. Electromagnetic wave consists of
electric and magnetic field vectors. The directions of electric and magnetic vectors are
perpendicular to direction of propagation as shown in the figure 4.1. The electric and
magnetic vectors are denoted by E and H and vary with time.

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Figure 4.1

In light, electric vectors (or magnetic vectors) vary in sinusoidal manner as shown in
figure 4.1. Therefore the electric vectors can be given as

E= E0 sin (kz - ωt)

Where E = Electric field vector, E0 = maximum amplitude of field vector, k = wave number
(= 2π/λ), z = displacement along the direction of propagation (say z axis), ω = angular
velocity and t = time.

Before understanding the interference we should understand some terms and properties of
light which are related to interference.

4.3.1 Monochromatic Light

The visible light is a continuous spectrum which consist a large number of wavelengths
(approximately 3500Å to 7800Å). Every single wavelength (or frequency) of this continuous
spectrum is called monochromatic light. However, the individual wavelengths are sufficiently
close and indistinguishable. Some time we consider very narrow band of wave lengths as
monochromatic light.

Ordinary light or white light, coming from sun, electric bulb, CFL, LED etc. consists a
large number of wave lengths and hence non-monochromatic. But some specific sources like
sodium lamp and helium neon laser emit monochromatic lights with wave lengths 589.3 nm
and 632.8 nm respectively. It should be noted that sodium lamp, actually emits two spectral
lines of wavelengths 589.0 nm and 589.6 nm which are very close together, and source is to
be consider monochromatic.

4.3.2 Plane Wave

A plane wave is a wave whose wave front remains in a plane during the propagation of
wave. In light wave, the maximum amplitude of electric vector E0 remains constant and
confined in a plane perpendicular to direction of propagation. Such type of wave called plane
wave.

4.3.3 Polarized and Unpolarized Light

Light coming from many sources like sun, flame, incandescent lamp produce
unpolarized light in which electric vector are oriented in all possible directions perpendicular
to direction of propagation. But in polarized light electric vector are confined to only a single
direction. The detail about polarized light will be discussed in the next block.

4.3.4 Phase Difference and Coherence

Wave is basically transportation of energy by mean of propagation of disturbance or
vibrations. In wave motion through a medium, the particles of medium vibrate but in case of
electromagnetic wave the electric or magnetic vectors vibrate form its equilibrium position.

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The term phase describes the position and motion of vibration at any time. For example if
y= a sin (ωt +θ) represents a wave, then the term (ωt +θ) represents the phase of wave. The
unit of phase is degree or radium. After completion of 3600 or 2𝜋, the cycle of wave or phase
repeats.

Phase difference

If there are two waves have some frequency then the phase difference is the angle (or
time) after which the one wave achieves the same position and phase as of first wave. In the
figure 4.2, two waves with phase different θ are shown.

Figure 4.2

Coherence
If two or more waves of same frequencies are in same phase or have constant phase
difference, those waves are called coherent wave. Figure 4.3 shows coherent wave with same
phase (zero phase difference) and with constant phase difference.

Figure 4.3

4.3.5 Optical path and Geometric Path

Optical path length (OPL) denoted by ∆ is the equivalents path length in the vacuum
corresponding to a path length in a medium. Path length in a medium can be considered as
geometric path length (L). Suppose a light wave travels a path length L in a medium of
refractive index µ and velocity of light is v in this medium, then for a time period t the
geometric path length L is given by

𝐿 = 𝑣𝑡

In the same time interval t, the light wave travel a distance ∆ in vacuum which is optical path
length corresponding to length L. Then

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𝐿
∆= 𝑐𝑡 = 𝑐 𝑣

Where, c is the velocity of light in vacuum.

or ∆ = μL

or The Optical path length = µ × (Geometrical path length in a medium).

In case of interference we always calculate optical path for simplification of understanding

and mathematical calculations.

4.4 PRINCIPLE OF SUPERPOSITION

According to Young’s principle of superposition, if two or more waves are travelling
and overlap on each other at any point then the resultant displacement of wave is the sum of
the displacement of individual waves (figure 4.4). If two waves are represented by y1 = a1 sin
ωt and y2 = a2 sin (ωt+δ). Then according to principle of superposition, the resultant wave is
represented by 𝑦 = 𝑦1 + 𝑦2

Figure 4.4

4.5 INTERFERENCE
When two light waves of some frequency, nearly same amplitude and having constant
phase difference travel and overlap on each other, there is a modification in the intensity of
light in the region of overlapping. This phenomenon is called interference.

The resultant wave depends on the phases or phase difference of waves. The
modification in intensity or change in amplitude occurs due to principle of superposition. In
certain points the two waves may be in same phase and at such point the amplitude of
resultant wave will be sum of amplitude of individual waves. Thus, if the amplitudes of
individual waves are a1 and a2 then the resultant amplitude will be a = a1+ a2. In this case,
the intensity of resultant wave increases (I ∝ a2) and this phenomena is called constructive
interference. Corresponding to constructive interference we observe bright fringes.

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Figure 4.5
On the other hand, at certain points the two waves may be in opposite phase as shown
in figure 4.4. In these points the resultant amplitude of waves will be sum of amplitude of
individual waves with opposite directions. If the amplitudes of individual waves are a1 and a2
then the resultant amplitude will be a = a1- a2 and the intensity of resultant wave will be
minimum. This case is called destructive interference. Corresponding to such points we
observe dark fringes. Figure 4.5 depicts two waves of opposite phase and their resultant.

4.5.1 Theory of Superposition

Let us consider two waves represented by y1 = a1 sin ωt and y2= a2 sin (ωt + δ).
According to Young’s principle of superposition the resultant wave can be represented by

y= y1 +y2

= a1 sin ωt + a2 sin(ωt+δ)

= a1 sin ωt + a2 (sin ωt cos δ + cos ωt sin δ)

= (a1+ a2 cos δ) Sin ωt + (a2 sin δ) cos ωt ....... (4.1)

Let a1+ a2 cos δ= A cos ∅ ....... (4.2)

and a2 sin δ= A sin ∅ ....... (4.3)

Where A and ∅ are new constants, then above equation becomes

y= A cos ∅ sin ωt + A sin ∅ cos ωt

or y= A sin (ωt + ∅) ....... (4.4)

This is the equation of the resultant wave. In this equation y represents displacement, A
represents resultant amplitude, Ø is the phase difference.

From equation (4.2) and (4.3) we can determine the constant 𝐴 and ∅. Squaring and adding
the two equations, we get,

A2 = a12 + a22 cos2 δ + 2 a1a2 cos δ+ a22 sin2 δ

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or A2 = a12 + a22 + 2 a1a2 cos δ ....... (4.5)

On dividing equation (4.3) by eq (4.2), we obtain,

𝑠𝑖𝑛 Ø 𝑎2 𝑠𝑖𝑛 𝛿
= tan ∅ = ....... (4.6)
𝑐𝑜𝑠Ø 𝑎1 +𝑎2 𝑐𝑜𝑠 𝛿

4.5.2 Condition for Maxima or Bright Fringes

If cos δ = +1 then δ = 2nπ where n= 0, 1, 2, 3……(positive integer numbers).

Then, A2 = a12 + a22+ 2a1a2 = (a1+a2)2

Intensity, I= A2 = (a1+a2)2 ....... (4.7)

Therefore, for δ = 2nπ = 0, 2π, 4π……, we observe bright fringes.

In term of path difference ∆

𝜆 𝜆
∆= × phase difference = 2𝑛𝜋
2𝜋ᴫ 2𝜋ᴫ

or ∆ = 𝑛𝜆 = λ, 2λ, 3λ… etc. ....... (4.8)

4.5.3 Condition for Minima or Dark Fringes

If cos δ = − 1 or 𝛿 = (2𝑛 − 1)𝜋 = 𝜋, 3𝜋, 5𝜋 … …

Then A2 = a12 + a22 − 2 a1a2 = (a1−a2)2

Intensity, I = A2 = (a1- a2)2 ....... (4.9)

Therefore if phase difference between two waves is δ = (2𝑛 − 1)𝜋 = 0, 3 𝜋, 5 𝜋… etc. is the
condition of minima or dark fringes.
𝜆
Now path difference, ∆ = 2𝜋 × 𝑃ℎ𝑎𝑠𝑒 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒

𝜆 (2𝑛−1) 𝜆 3𝜆 5𝜆
or ∆ = 2𝜋 × (2𝑛 − 1)𝜋 = 𝜆 = 2, , ,…… ....... (4.10)
2 2 2

Example 4.1. Two coherent resources whose intensity ratio is 81:1 produce interference
fringes. Calculate the ratio of maximum intensity and minimum intensity.

Solution: If I1 and I2 are intensities and a1 and a2 are the amplitudes of two waves then

𝐼1 81 𝑎12 81 𝑎1 9
= 𝑜𝑟 = 𝑜𝑟 =
𝐼2 1 𝑎22 1 𝑎2 1

Maximum intensity = a1+a2= 9 a2+ a2 = 10 a2

Minimum intensity = a1−a2 = 9 a2− a2 = 8 a2

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The ratio of maximum intensity to minimum intensity

Imax/ Imin= (a1+a2)2 / (a1-a2)2 = 102/ 82 = 100/64=25/16

4.5.4 Intensity Distribution

The intensity (𝐼) of a wave can be given as 𝐼 = (½) ∈𝑜 𝑎2 where 𝑎 is the amplitude of
wave, and ∈0 is the permittivity of free space. If we consider two waves of amplitudes a1 and
a2 then at the point of maxima

Imax = (a1+a2)2 = a12+a22+2a1a2

If a1 = a2 = a then I = 4a2. Therefore, at maxima points the resultant intensity is more than
the sum of intensities of individual waves.

Similarly the intensity at points of minima

Imin= a12+a22 - 2a1a2 = (a1~a2)2

If a1= a2=a then Imin=0. Thus the intensity at minima points is less than the intensity of any
wave.

The average intensity Iav is given as

2𝜋 2𝜋
∫0 𝐼𝑑𝛿 ∫0 (𝑎12 +𝑎22 +2𝑎1 𝑎2 𝐶𝑜𝑠 𝛿)𝑑𝛿 (𝑎12 +𝑎22 )2𝜋ᴫ
Iav= 2𝜋 = 2𝜋 = = 𝑎12 + 𝑎22
∫0 𝑑𝛿 ∫0 𝑑𝛿 2𝜋ᴫ

If a1 = a2 = a then Iav = 2a2 = 2I

Therefore, in interference pattern energy (intensity) 2a1a2 is simply transferred from minima
to maxima points. The net intensity (or average intensity) remains constant or conserved.

4.6 CLASSIFICATION OF INTERFERENCE

The interference can be divided into two categories.

4.6.1 Division of Wavefront

In this class of interference, the wave front originating from a common source is
divided into two parts by employing mirror, prisms or lenses on the path. The two wave front
thus separated traverse unequal paths and are finally brought together to produce interference
pattern. Examples are biprism, Lloyd’s mirror, Laser etc.

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4.6.2 Division of Amplitude

In this class of interference the amplitude or intensity of incoming beam divided into
two or more parts by partial reflection and refraction. Examples are thin films, Newton’s
rings, Michelson interferometer etc.

4.7 YOUNG’S DOUBLE SLIT EXPERIMENT

In 1801, Thomas Young performed double slit experiment in which a light first entered
through a pin holes, then again divided into two pinholes and finally brought to superimpose
on each other and obtained interferences. Young’s performed experiment with sum light.
Now the experiments are modified with monochromatic light and efficient slits.

Fig. 4.6
Figure 4.6 shows the experimental setup of double slit experiment. S1 and S2 are two
narrow slits illuminated by a monochromatic light source. The distance between two slits S 1
and S2 is 2d. The two waves superimposed on each other and fringes are formed on the screen
placed at a distance D from the centre of slits M. Let us consider a point P on the screen
which is y distant from O. The two rays S1P and S2P meet at point P and produce interference
pattern on screen.

Mathematically, path difference between rays S1 P and S2P is given as

∆= S2P –S1P ....... (4.11)

S2P2 = D2 + (y+d)2 = D2[1+ (y+d)2 / D2]

S2P = D[1+ (y+d)2 / D2]1/2

1
= D[1+ 2 (y+d)2 / D2] [∵(1 + 𝑥)𝑛 = 1 + 𝑛𝑥 + ⋯ … ]

or S2P = D + (y+d)2 /2D ....... (4.12)

Similarly

S1P2 = D2 + (y-d)2

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S1P = D [1+ (y-d)2 / D2]1/2

1
= D [1+ 2(y-d)2 / D2]

= D + (y-d)2 /2D ....... (4.13)

Using equation (4.12) and (4.13), the path difference becomes

(y+d)2 (y−d)2 2yd

∆= 𝐷 + −𝐷− = ....... (4.14)
2𝐷 2𝐷 𝐷

For the position of bright fringes path difference

∆ = n𝜆 (where n=1, 2, 3……..)

2yd
or = n𝜆
𝐷

nDλ
or y=
2𝐷

Since the expression consists of integer n, i.e., y is a function of n. Thus it is better to use yn in
place of y and we can write,
nDλ
yn = ....... (4.15)
2𝐷

Where n = 1, 2 … etc. represents the order of fringe

Dλ 2Dλ
On putting the value of n=1, n=2 etc. we get the bright fringes at positions y1= , y2=
2𝐷 2𝐷
etc. Similarly for the position of dark fringes, the path difference should be
(2n−1)𝜆
∆= 2

2yd (2n−1)𝜆
or =
𝐷 2

(2n−1) Dλ
or yn = ....... (4.16)
2 2𝐷

1 Dλ
If we place the value of n = 1, 2, 3 … we get the positions of dark fringes at y1 = ,
2 2𝐷
3 Dλ 5 Dλ
y2= , y3 = …… etc.
2 2𝐷 2 2𝐷

Fringe Width: Distance between two consecutive bright or dark fringes is called fringe
width denoted by ω (sometimes β). In case of bright fringes, fringe width
Dλ Dλ Dλ
ω = yn+1 – yn = (n+1) –n =
2𝐷 2𝐷 2𝐷

Similarly, in case of dark fringes

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2(𝑛+1)−1 Dλ (2𝑛−1)−1 Dλ Dλ
ω = yn+1 – yn = - =
2 2𝐷 2 2𝐷 2𝐷

4.8 COHERENCE LENGTH AND COHERENCE TIME

In case of ordinary light source, light emission takes place when an atom leaves it
excited state and come to ground state or lower energy state. The time period for the process
of transition from an upper state to lower state is about 10-8 s only. Therefore an excited atom
emits light wave for only 10-8 s and wave remains continuously harmonic for this period.
After this period, the phase changes abruptly. But in a light source, there are innumerous
numbers of atoms which participate in the emission of light. The emission of light by a single
atom is shown in figure 4.7. After the contribution of a large number of atoms emitting light
photon, a succession of wave trains emits from the light source.

Figure 4.7

4.8.1 Coherence Length

Coherence length is propagation distance over which a coherent wave maintains
coherence. If the path of the interfering waves or path different is smaller than coherent
length, the interference is sustainable and we observe distinct interference pattern.

4.8.2 Coherence Time

Coherent time τc is defined as the average time period during which the wave remains
sinusoidal and after which the phase change abruptly.

4.8.3 Spatial Coherence

Spatial coherence describes the correlation between waves at different points on a plane
perpendicular to the direction of propagation. More precisely, the spatial coherence is the
cross-correlation between two points in a wave for all times. If a wave has only 1 value of
amplitude over an infinite length, it is perfectly spatially coherent.

4.8.4 Temporal Coherence

Temporal coherent describes the correlation between two points in the direction of
propagation. In other words, it characterizes how well a wave can interfere with itself at a
different time as direction of propagation indicates time line. The delay over which the phase
or amplitude wanders by a significant amount (and hence the correlation decreases by
significant amount) is nothing but coherence time τc

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4.9 CONDITIONS FOR SUSTAINABLE INTERFERENCE

As we studied the different aspects of interference it is clear that under which
conditions interference can take place. But for strong interference or sustained interference
some more condition may be summarized. The conditions are:

1. The interfering waves must have same frequencies. For this purpose we can select a single
source.
2. The interfering waves must be coherent. To maintain the coherence, the path difference of
two interfering waves must be less than coherence length.
𝐷𝜆
3. As fringe width is given by ω = . Thus to obtain reasonable fringe width the distance
2𝑑
between source and screen D should be large and distance 2d between two sources should
be small.
4. For good contrast we can prefer the interfering wave of same amplitude. If amplitude of
two waves, a1 and a2 are same or nearly same than we observe distinct maxima and
minima.
5. The back ground of screen should be dark.

4.10 INTERFERENCE DUE TO THIN SHEET

When a thin transparent sheet of mica of thickness t and refractive index µ is introduced
in the path of one of the interfering beam of light, then entire fringe system is displaced.
Suppose a thin sheet of mica of thickness t is place in the path of a light beam as shown in
figure 4.8 then suppose the fringe system is displaced by a distance x.

If t is the time taken by light to travel distance S1P, then

S1 P−t 𝑡
𝑡= +
𝑐 𝑣

where v is velocity of light in the thin sheet and c is the velocity of light in air.

S1 P − t 𝑡 𝑐
𝑡= + µ ∵µ=
𝑐 𝑐 𝑣
S1 P−t+µ 𝑡
𝑡=
𝑐

For light ray reaching to P from slit S1, the path travelled in air is S1P-t while in thin sheet is
t, the optical path can be written as

= S1P - t + µt = S1P + (µ-1) t

Now path difference between two interfering says S1P and S2P at P is given as

∆ = S2P-S1P = S2P- [S1P+ (µ-1)t]

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= S2P - S1P-(µ-1)t
2𝑦𝑑
= - (µ-1) t (Using equation 4.14)
𝐷

Figure 4.8
For nth maxima (bright fringe) path difference should be of the order of nλ, i.e.,
2𝑦𝑑
- (µ-1) t= nλ
∆

𝐷
Taking y as yn we get, yn= [nλ + (µ-1) t] ....... (4.17)
2𝑑

In the absence of thin sheet (t =0)

𝑛𝐷𝜆
yn =
2𝑑

Therefore, net displacement in the presence and absence of sheet is given by equations 4.18
and 4.19 respectively
𝐷 𝑛𝐷𝜆
x= [nλ+ (µ-1) t] − ....... (4.18)
2𝑑 2𝑑

𝐷
x= (µ-1) t ....... (4.19)
2𝑑

Therefore, on introducing a thin transparent sheet in the path of any interfering ray, the entire
fringe system will disposed by distance of x. By measuring the value of x we can calculate
the thickness of sheet.
𝑥 .2𝑑
t= ....... (4.20)
𝐷(µ−1)

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4.11 FRESNEL’S BIPRISM

Fresnel biprism consists of two acute angle prisms with their bases in contact. Generally
the angles are 1790, 30’ and 30’ as shown in figure 4.9. The light coming from a source is
allowed to fall symmetrically on a biprism as shown in figure 4.9. As we know, when a light
beam is incident on a prism, the light is deviated from its original path through an angle
called angle of deviations. Similarly in case of biprism, the light beam coming from source S,
is appeared to be coming from S1 and S2 as shown in figure 4.10. Thus we can say for prism
S1 and S2 behave as virtual sources for the biprism.

Fig. 4.9
In case of biprism, it can be considered that two cones of lights AS1Q and BS2P are
coming from S1 and S2 and superimposed on each other and produce interference fringes in
the region of superposition (between AB). The formation of interference fringes due to
Fresnel’s biprism is the same as due to Young’s double slit experiment.

Fig. 4.10
In this experiment point O is equidistance from both slits S1 and S2. If we consider
distance between source and screen is D and separation between two slits S1 and S2 is 2d the
fringe width can be given as
𝐷𝜆
𝜔=
2𝑑

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𝐷𝜆
The position of nth bright fringe is given by yn = n
2𝑑

2𝑛−1 𝐷𝜆
Similarly the position of nth dark fringe is given by yn = .
2 2𝑑

The wave length of the light source used in biprism experiment can be obtained by using
above relation as
2𝑑
𝜆=𝜔 ....... (4.21)
𝐷

4.11.1 Experimental Arrangement of Biprism Apparatus

The experiment is performed on an optical bench as shown in figure 4.11. In this
experiment we have an optical bench, which is an arrangement of two parallel metallic rods
which are horizontal at same label. The rods or optical bench carry upright on which optical
instruments are mounted. These upright are movable on the rods. In the first uprights, we
have a slit illuminated by a monochromatic light source S. The slit provides a linear
monochromatic light to the biprism which is mounted on the second upright. The biprism is
placed in such a way that its refracting edges parallel to the slit so that light falls
symmetrically on the biprism. In third upright there is a concave lens for conversing the light
coming from biprism. Finally on forth upright a micrometer eyepiece is mounted in which
interference fringes are observed.

For obtaining fringes, following adjustments are to be made.

(i) The optical bench is leveled with the help of spirit level.
(ii) Axis of slit is made parallel to edge of biprism.
(iii) The heights of all four uprights should be same so that line joining slit, biprism and
micrometer should be parallel to optical bench.

Figure 4.11

4.11.2 Lateral Shift

If the eyepiece of micrometer is moved away from the biprism, and fringes shift either
left or right of bench then it is called lateral shift. Simply, we can say the shift of fringes

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across the bench is called lateral shift. It indicates that the line joining the slit biprism and
eyepiece is not parallel to the optical bench.

To remove the lateral shift we put the eyepiece near the birprism and fix the vertical
crosswire on any fringe. Now micrometer eyepiece is moved some distance away from
biprism and direction of fringe shift is observed. Now biprism is moved in the direction
opposite to the fringe shift so that vertical crosswise again reached on same fringe. We repeat
this process again and again so that lateral shift removes compatibly.

4.11.3 Measurement of Wavelength of Light (λ) by Fresnel Biprism

By using the Fresnel biprism we can determine the wavelength of given source of light.
For this purpose we use the given light source in experimental arrangement. We adjust the
apparatus for fringes are to be observed on the eyepiece. We measure the fringe width on
apparatus and apply the formula for fringe width as
𝐷𝜆 2𝑑
𝜔= or λ = 𝜔.
2𝑑 𝐷

Fringe width 𝜔 can be measured with the help of micrometer on eyepiece. D is the
distance between eyepiece and slit, and can be measured with the help of optical bend. The
2d is the distance between two virtual sources (S1 and S2) and cannot be measured directly
with the help of any scale. We apply two methods for the measurement of distance 2d.

Magnification Method

To determine the distance 2d, we placed a convex lens of short focal length between biprism
and screen. We find out a position L1, of lens very near to biprism so that two sharp real
images are obtained in the field of view of eyepiece. In figure 4.12 the position of Lens L1 is
denoted by bold lines. In this position, we measure distance between two images d1, with the
help of micrometer of eyepiece.

For this position the magnification is given by

𝑣 𝑑1
=
𝑢 2𝑑

Now we move the lens some distance away from the biprism and obtain another position L2
so that two sharp images are seen again in the field of view. We again measure the distance
between two images, say d2 with the help of micrometer of eyepiece.

In this case of position L2 the magnification is given as

𝑢 𝑑2
=
𝑣 2𝑑

By using above two equations (10) and (11) we get:

𝑑1 𝑑2
1= .
2𝑑 2𝑑

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or 2𝑑 = √𝑑1 𝑑2 ....... (4.22)

By putting the value of d1 and d2 we can determine the value of 2d.

Figure 4.12

Refractive Index Method

In this method, we use the formula of angle of deviation for a prism. As shown in figure
4.13 the angle of deviation can be given as

𝛿 = (µ − 1) 𝛼 ....... (4.23)

Where µ is refractive index and α is angle of prism as shown in figure 4.13. Again the angle
of deviation can be given as.
𝑑
𝛿= or 𝑑 = 𝑎 𝛿 ....... (4.24)
𝑎

Using equations (4.23) and (4.24), we obtain, 2d = 2a 𝛿

Figure 4.13

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or 2d=2a (µ-1) α ....... (4.25)

By using any of the above mentioned methods, we can determine the value of 2d and then
putting this value is equation 4.21, we can determine the wavelength of given light source.

4.12 INTERFERENCE WITH WHITE LIGHT

Now let us discuss what happen when the monochromatic light source in a Young’s
double slit experiment is replaced by a white light. Since the white light consists innumerable
wavelengths from red to violet, when white light is used, all wavelengths have their own
fringe pattern and finally superimposed on each other. Since the path different for all colours
at center point is same then the waves of all colours reach at mid point without any path
difference and we observed a white fringe at Center point. This central fringe is called zero
order fringes. After central fringe, we observed few coloured fringes with poor contrast.
These fringes are due to superposition of different fringes of different colours. Thus the
interference pattern is not clear but the superposition of many colours.

Self Assessment Questions

1. What is difference between coherence and non coherence light?
2. Why non-coherent sources do not produce interference pattern?
3. What are the conditions for sustainable interference?
4. Young’s double slit experiment, why the central fringe is bright?
5. How can we arrange coherence sources in practical?
6. What is meant by interference of light?
7. Explain the principle of superposition of light wave?
8. How is the shape of fringes formed by biprism?

4.13 SOLVED EXAMPLES

Example 4.2: A monochromatic light of wave length 5100 Å from a slit is incident on a
double slit. If the overall separation of 30 fringes on a screen 200 cm always is 3cm, find the
distance between slits.
Dλ
Solution: The fringe width 𝜔 = 2𝐷

Where 𝜔 = fringe width, D = distance between slit and screen, 2d= distance between slits.
3
It is given that D =200 cm, 𝜔 = 30 = 0.1𝑐𝑚

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5100 ×10−8 ×200

Therefore, 2d= 𝐷𝜆/𝜔 = = 0.025 cm
0.1

Example 4.3: In Young’s double slit experiment the two slits are 0.05 mm apart and screen is
located 2m away from the slit. The third bright fringe from the slit is displaced 8.3 cm apart
from the central fringe. Determine the wavelength of incident light.

Solution: For the third bright fringe n= 3

𝑛𝐷𝜆 𝑥𝑛 .2𝑑 8.3×10−2 ×0.05×10−3

xn = 2𝑑
or 𝜆= = = 6.91 × 10-7 m = 6910 Å
𝑛𝐷 3×2

Example 4.4: In Fresnel’s biprism experiment, a light of wavelength 6000 Å falls on biprism.
The distance between source and screen is 1m and distance between source and birprism is
10 cm. The angle of biprism is 10. If the fringe width is 0.03cm, find out the refractive index
of the material of biprism.
𝐷𝜆
Solution: The fringe width 𝜔 =
2𝑑

If the refractive index of material is µ and angle of prism is 𝛼 then

𝐷𝜆
2d = 2a (µ-1) 𝛼. Then 𝜔 =
2𝑎 𝜔(𝜇−1) 𝛼

𝜋
Here, D = 1m = a+b and a= 10 cm, b= 90cm, 𝜆= 6000× 10-8 cm, 𝛼 = 10 = radian and 𝜔
180
= 0.03 cm

𝐷𝜆 100×6000×10−8
Thus, µ-1 = = 𝜋 = 0.57
2𝑎 𝜔𝛼 2×10×0.03×
180

∴ µ = 1+0.57 =1.57

Example 4.5: A light of wavelength 6900 Å is incident on a biprism of refracting angle 10

and refractive index 1.5. Interference fringes are observed on a screen 80 cm away from the
biprism. If the distance between source and the biprism is 20 cm, calculated the fringe width.
Dλ
Solution : The fringe width is given by 𝜔= and 2d =2( µ-1)a𝛼
2𝑑

𝜋
Here λ=6900 Å =6900 ×10-8 cm, 𝛼 =10= redius, µ = 1.5, D = a+b = (20+80) cm =100cm
180

Dλ 100×6900×10−8
ω= = 𝜋 = 0.02 cm.
2𝑎( µ−1)α 2×20×(1.5−1)×
180

Example 4.6: A thin sheet of a transparent material of refractive index µ =1.60 is placed in
the path of one of the interfering beam in a biprism experiment. The wave length of the light
used is 5890Å. After placing the sheet, the central fringe shifted to a position originally
occupied by 12th bright fringe. Calculate the thickness of the sheet.

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Solution: On introducing a thin transparent sheet in the path of one interfering say, the
interfering system is shifted by a distance x and
𝐷
x= (µ − 1)𝑡
2𝑑

In this case the fringe shifted by 12th bright fringe.

𝐷 𝐷𝜆
x= y12 =12 [∵ yn = n ]
2𝑑 2𝑑

𝐷𝜆 𝐷 12𝜆 12×5890×10−8
Therefore, 12 = (µ − 1)𝑡 or t= (µ−1) = = 1.18 × 10-3 cm
2𝑑 2𝑑 (1.6−1)

4.14 SUMMARY
1. When two light waves of same frequency and nearly some amplitude and having constant
phase difference traverse in a medium and cross each other, there is redistribution in the
intensity of light which is called interference of light.
2. If y1 = a1 sin ωt and y2= a2 sin (ωt+δ) are two waves, then resultant wave is given by
𝑎2 𝑠𝑖𝑛𝛿
y = A sin (ωt+∅). Where A=√𝑎12 + 𝑎22 + 2𝑎1 𝑎2 𝐶𝑜𝑠𝛿 and ∅ = 𝑡𝑎𝑛−1 [ ]
𝑎1 +𝑎2 𝑐𝑜𝑠𝛿
3. For constructive interference or bright fringes, path difference ∆ = nλ where n=1, 2, 3…
2𝑛−1
4. For destructive interference or dark fringes, path difference ∆ = ( ) nλ
2
5. For sustainable interference the two waves should be coherent. If two or more waves of
same frequency are in the same phase or have constant phase difference then there waves
are called coherent.
6. In interference pattern, the component of energy (intensity) 2a1 a2 is simply transfer from
minima to maxima point. The net intensity or average intensity remains constant or
conserved.
7. Interference is of two types, known as division of wave front and division of amplitude.
8. Division of wave front is a class of interference in which the light from original common
source is divided into two parts by employing mirror, prism, lens, biprism etc.
9. In case of division of amplitude, the incoming beam is divided into two or more parts by
partial reflection or refraction. Interference due to thin film, Newton’s rings, Michelson
interferometer are the examples of division of amplitude.
𝐷𝜆
10. In Young’s double slit experiment fringe width is given by ω = . Sometimes symbol β
2𝑑
𝐷𝜆
is to be used for fringe width. Position of nth bright fringe is given by yn = n . Similarly
2𝑑
2𝑛−1 𝐷𝜆
position of nth dark fringe 𝑦𝑛 = , where D distance between slit and screen and
2 2𝑑
2d is the separation between slits S1 and S2.

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11. On introducing a thin transparent sheet of thickness t in the path any interfering ray, the
𝐷
entire fringe system will be displaced by a distance x given as x = (µ-1) t.
2𝑑
Where µ is refractive index of material of sheet, 2d is distance between two slits.
𝐷𝜆
12. In Fresnel’s biprism the fringe width is given by ω = and 2d = 2a (µ-1)𝛼 where a is
2𝑑
distance between source and biprism and α is the angle of biprism and µ is refractive
index of material of biprism.

4.15 GLOSSARY
Interference: Redistribution of energy due to superposition of waves.

Interference fringes: Pattern of dark and bright bands due to interference.

Superposition: Combining the displacements of two or more waves to produce a resultant

displacement.

Coherence: Property of two or more waves with equal frequency and constant phase
difference.

Coherent light: Light in which all wave trains have same frequency and its crests and
troughs aligned in same directions which have constant phase difference.

Biprism: Combination of two prisms with their bases in contact.

Slit: A narrow opening for light.

4.16 REFERENCES
1. N Shubramanyam and Brij Lal, Optics (S. Chand and Company, Delhi)

2. CL Arora and PS Hemne, Physics For Degree students, S. Chand and company, Delhi.

3. Stya Prakash, Optics,

4. http:// Wikipedia. org.

5. http://ocw.mit.edu

6. nptel.ac.in

7. http:// books. google.co.in

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4.17. SUGGESTED READING

1. Frank S.J. Pedrotti, Introduction to optics Prentice Hall of India (1993)

2. Ajay Ghatak, Optics, McGraw Hill Company, New Delhi.

4.18 TERMINAL QUESTION

4.18.1 Short Answer Type Questions
1. What is interference of light? Give some example of interference of light.
2. What are the necessary conditions for interference of light?
3. What are coherent sources of light?
4. Discuss why two independent sources of some frequency are not coherent?
5. State the principle of superposition of waves.
6. Explain the optical path of light in a medium.
7. What is the difference between ordinary prism and biprism? How can we distinguish?
4.18.2 Long Answer Type Questions
1. What is interference of light? Obtain the condition for constructive and distractive
interference.
2. What is Young’s double slit experiment? Find out the position of bright fringes, dark
fringes and fringe width.
3. Derive an expression for the resultant intensity of two coherent beam of light which are
superimposed.
4. Explain the construction and working of biprism.
5. Calculate the displacement of fringe system when a transparent thin film is introduced in
the path of an interfering beam in the double slit experiment.

4.18.3 Numerical Questions

1. A biprism is placed 5 cm from the slit and 75cm from the screen. The biprism is
illuminated by sodium light of wavelength 5890Å. The fringe width is observed 424 ×
10−2 cm. Calculate the distance between two coherent sources. [Ans. 0.5mm]
2. A biprism form interference fringes with monochromatic light of wave length 5450Å. On
introducing a thin glass plate of refractive index 1.5 in the path of one of the interfering
beam, the central fringe shifts to the position previously occupied by 6th bright fringe.
Find out the thickness of the plate.
3. The inclined faces of a biprism of refractive index 1.5 make angle 20 with base. A slit
illuminated by a monochromatic light is placed at a distance of 10cm from the biprism. If
the distance between two dark fringes observed at a distance of 1cm from the biprism is
0.18 mm, find out the wavelength of light used.

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4. The inclined faces of a glass biprism of refractive index 1.5 makes angle of 10 width base
of the prism. The distance between slit and biprism is 0.1m. The biprism is illuminated by
a light of wavelength 5900Å and fringes are observed at a distance 1m from the biprism.
find out the fringe width.

4.18.4 Objective Type Questions

1 . Phase difference Φ and path difference δ are related by Φ=
2𝜋 𝜆
(a) 𝛿 (b) 𝛿
𝜆 2𝜋
𝜋 2𝜆
(c) 𝛿 d) 𝛿
2𝜆 𝜋

2 . The condition for constructive interference is path difference should be equal to

(a) odd integral multiple of wavelength

(b) integral multiple of wavelength
(c) odd integral multiple of half wavelength
(d) Integral multiple of half wavelength

3. The ratio of intensities of two waves that produce interference pattern is 16:1 then the ratio
of maximum and minimum intensities in the pattern is

(a) 25:9 (b) 9:25 (c) 1: 4 (d) 4:1

4. Correlation between the a point in the field and the same point in the field at later time is
known as

(a) Temporal coherence (b) coherence

(c) Spatial coherence (d) none of these

5. The overlapping of waves into the regions of the geometrical shadow is

(a) Dispersion (b) polarization

(c) diffraction (d) interference

6. Interference occurs due to

(a) Wave nature of light (b) particle nature of light

(c) both a and b (d) none of these

7. Two interfering beams have their amplitudes ratio 2:1 then the intensity ratio of bright and
dark fringes is

(a) 2:1 (b) 1:2 (c) 9:1 (d) 4:1

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8. If a1 and a2 are the amplitudes of light coming from two slits in Young’s double slit
experiment then the minimum intensity of interference fringe is

(a) a1 + a2 (b) a1 - a2 (c) (a1 + a2)2 (d) (a1 - a2)2

9. Young’s double slit experiment is an example of division of

(a) amplitude (b) Wavelength (c) wave front (d) None

10. In Young’s double slit experiment, the fringe width ω is given by

𝐷𝜆 𝐷𝜆 2𝑑𝜆 𝑑𝜆
(a) (b) (c) (d) 2𝐷
2𝑑 𝑑 𝐷

4.18.5 Answers of Objective Type Questions

1. (a), 2. (b), 3(a), 4(a), 5(d), 6(a), 7(c), 8(c), 9(c), 10(a)

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UNIT 5: INTERFERENCE IN THIN FILMS AND

NEWTON’S RINGS

CONTENTS
5.1 Introduction
5.2 Objective
5.3 Interference Due to Plane Parallel Thin Film
5.3.1 Interference in Case of Reflected Light
5.3.2 Interference in Case of Refracted Light
5.4 Interference Due to Wedge Shaped Film
5.4.1 Properties of Fringes Due to Wedge Shaped Film
5.4.2 Applications of Wedge Shaped Film
5.5 Necessity of Extended Source for Interference Due to Thin Films
5.6 Colours of Thin Films
5.7 Classification of Fringes
5.7.1 Fringes of Equal Thickness
5.7.2 Fringes of Equal Inclination
5.8 Newton’s Rings
5.8.1. Experiment Arrangement for Reflected Light
5.8.2. Formation of Bright and Dark Rings
5.8.3 Diameter of Bright and Dark Rings
5.8.4. Determination of Wavelength of a Monochromatic Light Source
5.8.5. Determination of Refractive Index of a Liquid by Newton’s Rings Experiment
5.8.6 Newton’s Rings in Case of Transmitted Light
5.9. Summary
5.10. Glossary
5.11. References
5.12. Suggested Reading
5.13. Terminal Questions

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5.1. INTRODUCTION
In optics any transparent material in a shape of thin sheet of order 1µm to 10 µm is
simply called thin film. The material may be glass, water, air, mica and any other material of
different refractive index. When a thin film is illuminated by a light, some part of incident
light get refracted from the upper surface of film and some part of get transmitted into the
film. Some part of transmitted light gets reflected again from the lower surface of thin film.
Now the light reflected from upper and lower surface of thin may course interference.
In case of thin film, the maximum portion of incident light is transmitted and a very few
part of light reflected form the thin film. Therefore the intensity of reflected light is
significantly small. For example if we consider a light beam is reflected from a glass plate of
refractive index 1.5 then the reflection coefficient is given by
µ1− µ2 2 1.5−1 2 0.5
𝑟=( ) =( ) = ( ) = 0.04
µ1− µ2 1.5+1 2.5

Thus only 4% of incident light is reflected by the upper surface of glass film and 96%
of light is transmitted into the glass plate. Similarly nearly 4% of light is again reflected
through the lower surface of glass plate. If we consider the interference due to the light
reflected from upper and lower surface of glass plate, the intensity of light will be
significantly small.
When white light is incident of thin film, interference pattern is appeared as colourful
bands since white light consists different wavelengths, different wavelengths produce
interference bands of different colours and thicknesses. Interference in thin films also occurs
in nature. Thin wings of many insects and butterflies are layer of thin films. There thin films
are responsible for structural colourization which produce different colours by
microscopically structured surface, and suitable enough for interference of light.

5.2. OBJECTIVE
After reading this unit you will be able to understand
 Thin film
 Interference in thin film
 Interference in wedge shaped film
 Classification of fringes and its shapes
 Newton’s rings experiments and its applications

5.3. INTERFERENCE DUE TO PLANE PARALLEL THIN FILM

A plane parallel thin film is transparent film of uniform thickness with two parallel
reflecting surfaces. The example is a thin glass film. Light wave generally suffers multiple
reflections and refractions at the two surfaces. There are two cases of interference as given
below

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5.3.1 Interference in Case of Reflected Light

Let us consider a thin film of thickness t as shown in figure 5.1. A monochromatic light
ray SA is incident on a thin film with an angle of incident i as shown in figure. The film is
made of a transparent material (say glass) of refractive index µ. Some part of light ray
reflected at point A along the direction AB and some part of light transmitted into the film
along AC direction. The ray AC makes an angle of refraction r at point A, and the angle r
becomes angle of incident ACN at point C. Some part of light of ray AC again reflected in
the direction CD which comes out from the film along the direction DE. The light rays AB
and DE come together and they can produced interference pattern on superposition.

Fig 5.1
The path difference ∆ between rays AB and DE is given as
∆ = (AC+DC) in film- AL in air.
Since optical path in air = µ × optical path in a medium
Therefore, path difference ∆ can be given as
∆ = µ (AC+ DC) – AL
𝑡 𝑡 𝑡
From figure 5.1, we have, cos r = or AC = and DC =
𝐴𝐶 𝑐𝑜𝑠 𝑟 𝑐𝑜𝑠 𝑟

Again, AL = AD sin i = (AN+ ND) sin i

= (t tan r + t tan r) sin i = 2t tan r Sin i
µ 2t 2µ𝑡
∆= − 2𝑡. tan 𝑟 𝑠𝑖𝑛 𝑖 = − 2µ𝑡 (𝑠𝑖𝑛2 𝑟)
𝑐𝑜𝑠 𝑟 𝑐𝑜𝑠 𝑟
𝑡
= 2µ (1− 𝑠𝑖𝑛2 𝑟) = 2µt cos r
𝐶𝑜𝑠 𝑟

According to Stock’s treatment, if a wave is reflected form a denser medium it involves a

path difference of λ/2 or phase difference or 𝜋. Therefore, net path difference
𝜆
∆ =2µt Cos r − ....... (5.1)
2

Condition of Maxima: For maxima or bright fringes the path difference should be nλ
where n is integer number given as n= 0,1,2,3 ……..
𝜆
∆= 2µt Cos r − = nλ
2

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2𝑛+1
or 2µt Cos r = ( )𝜆 ....... (5.2)
2
2𝑛+1
Thus maxima occur when optical path difference is ( )𝜆.
2
2𝑛−1
Condition for minima: Minima occur when the path difference is order of ( )𝜆. Then
2
𝜆 2𝑛−1
∆= 2µt Cos r − = ( )𝜆
2 2

or 2µt Cos r = n𝜆 ....... (5.3)

5.3.2 Interference in Case of Refracted Light

A light ray SA is incident at point A on a film of refractive index  as shown in figure
5.2. Some part of light ray reflected at point A and some part of light transmitted into the film
along AB. In case of interference due to refracted light we are not interested in the reflected
light. At point B some part of light is again reflected along direction BC, then again reflected
at point C and finally refracted at point D and comes out form the medium along DF
direction. Now the light rays coming along BE and DF are coherent and can produce
interference pattern in the region of superposition.

Figure 5.2
In this case path difference ∆ is given as
∆ = (BC+ CD) in film – BN in air
As Calculated in case of reflection, the path difference comes out
∆= 2µt Cos r
In this case there is no correction according to Stoke’s treatment as no wave from rarer
medium is reflected back to denser medium. Therefore this is net path difference.
For maxima or bright fringes, ∆= 2µt Cos r = nλ
2𝑛−1
For minima or dark fringes, ∆= 2µt Cos r = ( )𝜆
2

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5.4 INTERFERENCE IN A WEDGE SHAPED FILM

In a wedge shape film, the thickness of the film at one end is zero and it increases
consistently towards another end. A glass wedge shaped film is shown in figure 5.3. Similarly
a wedge shaped air film can be formed by using two glass films touch at one end and
separated by a thin wire at another end.

Figure 5.3
The angle made by two surfaced at touching end of wedge is called angle of wedge as
shown Ө in figure 5.3. The angle is very small in order of less than 10. Path difference
between two reflected rays BE and DF is given by
∆ = (BC+CD) in film – BE in air
= µ (BC+CD) – BE
=µ (BC+CI) – BE ∵ CD= CI
= µ (BN+NI) – BE ....... (5.4)
𝐵𝐸
In right triangle ∆ BED, sin i =
𝐵𝐷
𝐵𝑁
Similarly in ∆ BND, sin 𝑟 =
𝐵𝐷

Refractive index µ can be given as

𝑆𝑖𝑛 𝑖 𝐵𝐸
µ= = or BE =  BN
𝑆𝑖𝑛 𝑟 𝐵𝑁

Putting this value in equation (5.4) we get

∆ = µ (BN+ NI) -µ BN = µ NI ....... (5.5)
𝑁𝐼
Now in ∆ DNI, cos(𝑟 + 𝜃) =
𝐷𝐼
𝑁𝐼
or cos (r + Ө) = ⇒ NI = 2t Cos (r + Ө)
2𝑡

Putting this value in equation (5.5)

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Path difference, ∆ = µ. 2t Cos (r + Ө) ....... (5.6)

Since the light is reflecting from a denser medium therefore according to stokes treatment a
path change of λ/2 occurs. Now net path difference
∆= 2t Cos (r + Ө) – λ/2 ....... (5.7)
For bright fringes the path difference should be in order of ∆ = nλ where n is an integer (n= 0,
1, 2…....).
2µt Cos (r + Ө) – λ /2 = n λ
2𝑛+1
or 2µt Cos (r + Ө) = ( )𝜆 where n= 0,1,2…..
2
2𝑛−1
or 2µt Cos (r + Ө) = ( )𝜆 ....... (5.8)
2

Where, n = 1, 2, 3…..
2𝑛−1
For dark fringes path difference should be in order of ∆ = ( ) 𝜆.
2
2𝑛−1
2µt Cos (r + Ө) – λ /2 = ( )𝜆
2

or 2µt Cos (r + Ө) = n𝜆 ....... (5.9)

Since the focus of points of constant thickness is straight line, therefore the fringes are
straight lined in shape.
According to equation (5.8), for bright fringes
(2𝑛−1)𝜆 𝜆 3𝜆
𝑡= = = = ⋯…… ....... (5.10)
4 µ cos(r+θ) 4 µ cos(r+θ) 4 µ cos(r+θ)

If xn is the distance of fringes from the edge (position of nth fringe) then,
𝑡
tan 𝜃 = 𝑥
𝑛

(2𝑛−1)𝜆
or 𝑥𝑛 = ....... (5.11)
4 µ cos(r+θ) tan θ

𝜆 3𝜆
Thus, x1 = , x2 = …………
4 µ cos(r+θ) tanθ 4 µ cos(r+θ) tanθ

Fringe width ω = xn+1 − xn

2𝜆
ω= ) ....... (5.12)
4 µ Cos(r+θ) tan θ

If Ө is very small then tan Ө≃ Ө, and cos (r + Ө) ≅ r. Further if we consider normal

incidence then r= 00 then cos 0 = 1 and equation (5.12) becomes
𝜆
ω= ....... (5.13)
2µθ

5.4.1 Properties of Fringes Due to Wedge Shaped Film

1. As the locus of the points of constant thickness is a straight line therefore the fringes are
straight lime and parallel.

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2. The fringe width ω is constant for a particular wave length or colour, therefore the fringes
are of equal thickness and equidistant.
3. Fringes are localized

5.4.2. Applications of Wedge Shaped Film

By observing the interference pattern, the thickness of a spacer or wire which is placed
between two films at one end can be determined. Suppose t is the thickness of a wire or
spaces and l is length of wedge shaped film as shown in figure 5.4 then we can calculate the
thickness of spacer as

Figure 5.4
𝑡
tan Ө ≅ Ө =
𝑙
𝜆 𝜆
If we know the fringe width ω then by using relation ω = = 𝑡 we get,
2µθ 2µ
𝑙

𝜆𝑙
t=
2µ.𝜔

Example 5.1: A white light is normally incident on a soap bobble film of thickness 0.40 µm
and refractive index 1.4. Which are the wavelengths may cause bright fringes.
Solution: For bright fringes, due to thin films, the condition is
𝜆
2µt Cos r = (2n+1) , where n= 0,1,2,3…..
2
4 µ𝑡 cos 𝑟
or λ=
(2𝑛+1)

Here r = 0, µ = 1.4 and t = 0.40 µm.

4×1.4×0.40×10−6 2.24 ×10−6
λ= = 𝑚
(2𝑛+1) (2𝑛+1)

For n = 0; 𝜆 = 2.24 × 10−6 𝑚

n = 1; 𝜆 = 0.74 × 10−6 𝑚
n = 2; 𝜆= 0.44 × 10−6 𝑚
Example 5.2: White light is incident on an oil film of thickness 0.01mm and reflected at an
angle 450 to vertical. The refractive index of oil is 1.4 and refracted light falls on the slit of a
spectrometer, calculate the number of dark bands seen between wavelengths 4000Å and
5000Å.
Solution: For the dark band, formed by interference, due to thin film

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2µt Cos r= nλ
In case of wave length λ1 =4000Å and µ=1.4 , t= 0.01 mm
2µ𝑡 𝐶𝑜𝑠 𝑟
n1 =
𝜆1

𝑆𝑖𝑛 𝑖 𝑆𝑖𝑛 𝑖
Now µ= = 𝑆𝑖𝑛 𝑟 =
𝑆𝑖𝑛 𝑟 µ

𝑆𝑖𝑛 2 𝑟 1
Cos r = √1 − 𝑆𝑖𝑛2 𝑟 = √1 − = √1 − 2×(1.4)2 = 0.86
µ2

2×1.4×0.001×.86
Thus n1 = = 60
4000× 10−8

Thus corresponding to λ1=4000Å wavelength light we observe 60th order band

Similarly corresponding to λ2 wavelength
2µ𝑡 𝐶𝑜𝑠 𝑟 2×1.4×.001×0.86
n2 = = = 48
𝜆2 5000×10−8

Thus corresponding to wavelength λ2 =5000Å light we observe 48th order band.

Thus the number of dark bands between λ2 and λ1 = n1 – n2 =60-48 =12.
Example 5.3: A parallel beam of light λ = 5890 Å is incident on a thin glass film and the
angle of refraction into the film is 600. Calculate the smallest thickness of the film which
appear dark on reflection.
Solution: The film appears dark if the destructive interference takes place in reflection.
Path difference in dark bands
∆ = 2µt Cos r=nλ
For smallest thickness n=01 then
𝜆 5890×10−10
t = 2µ𝐶𝑜𝑠 𝑟 = = 3927 × 10−10 𝑚 = 3927 Å
2×1.5×0.5

Example 5.4: A monochromatic light of wavelength 5890 Å is incident normally on glass

plates enclosing a wedge shaped air film. The two plates touch at one end and are separated at
15cm apart from that end by a wire of 0.05 mm diameter. Calculate the fringe width of bright
fringes.
Solution: In case of wedge shaped film the fringe width is given by

Figure 5.5

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𝜆
𝜔=
2µӨ

0.05 ×10−1
Given λ = 5890 Å, µ=1, Ө= tan Ө = = 3.3× 10−4
15

5890 ×10−10
𝜔= = 892.4 × 10−6 𝑚 = 0.89 mm
2×1.0×3.3×10−4

Example 5.5: Sodium light of wavelength λ = 5890Å is incident on a wedge shaped air film.
When viewed normally 10 fringes are observed in a distance of 1cm. Calculate the angle of
the wedge.
Solution: The fringe width 𝜔 for wedge shaped film is given us
𝜆
𝜔=
2µӨ

In this case, 10 fringes are observed in a distance of 1 cm. Therefore, fringe width
1
𝜔= = 0.1 𝑐𝑚
10

𝜆 5890 ×10−8
Now Ө= = = 2.94 × 10−4 𝑟𝑎𝑑𝑖𝑎𝑛𝑠
2µ𝜔 2×2×0.1
180 180×60
= 2.94 × 10−4 × degree =3.94 × 10−6 × minute
ᴫ ᴫ

= 1.01 minute
Example 5.6: A Wedge shaped film is form by using two glass plates of length 10cm touch
at one end and separate at another end by introducing a thin foil of thickness 0.02mm. If the
sodium light of wavelength 5890Å is indent normally on it. Find the separation between two
consecutive fringes.
Solution: The separation between two consecutive fringes is the same as the fringe width.
𝜆 𝑡 0.02
𝜔= where Ө = tan Ө = = = 2 × 10−4
2µӨ 𝑥 100

Given λ =5890Å, µ = 1 then

5890 ×10−8
𝜔= cm = 0.14 cm
2×1×2×10−4

5.5 NECESSITY OF EXTENDED SOURCE FOR

INTERFERENCE DUE TO THIN FILMS
If we use narrow source of light in case of interference due to thin film the light rays are
diverged as shown in figure 5.6 (a) and we can view a limited portion of interference pattern.
On the other hand, if we use an extended or broad source of light a large number of rays are
available for the production of interference pattern as shown in figure 5.6 (b). A large number
of rays are incident on film at different angles, and a large area of film can be viewed by our

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eye at the field of view. Therefore, extended source of light is beneficial to observe the good
interference pattern in thin film.

(a)

(b)
Figure 5.6

5.6 COLOURS OF THIN FILMS

When light coming from extended source is reflected by thin film of oil, mica, soap or
coating etc., different colours are shown due to interference of light. For interference, the
optical path difference is ∆ = 2µt Cos r = (2n+1) λ/2 for bright fringes. If thickness t is
constant then for different wavelengths, angle of refraction r should be different. Therefore
different colours are observed at different angle of incident. Sometime different colours are
over lopped on each other’s and a mixed colour may be observed.

5.7 CLASSIFICATION OF FRINGES

As we know, in case of thin films, the path difference ∆ is given as
2𝑛+1
2µt Cos r = ( )𝜆
2

For a monochromatic light, µ and λ remain constant. Now the path difference for constructive
interference arises due to variation in thickness t and angle of incident (inclination) r. On the
basis of t and r the fringes are two types.

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5.7.1 Fringes of Equal Thickness

If the thickness of film is varying and the light is coming at same angle of incident then
the fringes are formed due to variation in thickness. For example in case of wedge shaped
film where thickness is varying, the locus of points of constant thickness is a straight line
corresponding to which fringes are formed. Such fringes are called fringes of equal thickness.
Newton’s rings are example of such type of fringes.
5.7.2 Fringes of Equal Inclination
If the thickness of film is constant then path difference for constrictive interference is
only due to variation in angle of inclination r. In this case we consider a locus of points on
film at which the angle of inclination of light is equal. Corresponding to such points of equal
inclination we observed fringes which are called fringes of equal inclination. Since the light
rays of equal inclinations pass through the plate is a parallel beam of light, and hence meet at
infinity but by using telescope focused on such rays the fringes can be observed. In such case
fringes are called the fringes localized at infinity. Such fringes are also called Haidinger’s
fringes. The fringes formed in Michelson interferometer is an example of fringes of equal
inclination.

5.8 NEWTON’S RINGS

Newton’s rings in a special case of wedge shaped film in which an air film is formed
between a glass plate and a convex surface of lens. The thickness of air film is zero at the
center and increases gradually towards the outside.
When a plano-convex lens of large focal length is placed on a plane glass plate, a thin
air film is formed between the lower surface of plano-convex lens and upper surface of glass
plate. When a monochromatic light falls on this film the light reflected from upper and lower
surfaces of air film, and after interference of these rays, we get an inner dark spot surrounded
by alternate bright and dark rings called Newton’s rings. These rings are first observed by
Newton and hence called Newton’s rings.

5.8.1 Experimental Arrangement for Reflected Light

The experimental arrangement for Newton’s rings experiment is shown in Figure 5.7. A
beam of light from a monochromatic source S is made parallel by using a convex lens L. The
parallel beam of light falls on a partially polished glass plate inclined at an angle of 450. The
light falls on glass plate is partially reflected and partially transmitted. The reflected light
normally falls on the plano-convex lens placed on plane glass plate.

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Fig. 5.7

Fig 5.8
This light reflected from upper and lower surface of the air film form between plane
glass plate and plno-convex lens. These rays interfere and rings are observed in the field of
view. The figure 5.8 shows the reflection of light form upper and lower surfaces of air film
which are responsible for interference.
5.8.2 Formation of Bright and Dark Rings
As we know the interference occurs due to light reflected from upper and lower surface
of air film form between glass plate and plano-convex lens. The air film can be considered as
a special case of wedge shaped film. In this case, angle wedge is the angle made between the
plan glass plate and tangent from line of contact to curved surface of plano convex lens as
shown in figure. 5.8.
The path difference between two interfering rays reflected by air film
𝜆
∆ = 2µt Cos (r + Ө) − ....... (5.14)
2

where µ is the refractive index of the air film, t is the thickness of air film at the point of
reflection (say point P) r is angle of refraction and Ө is angle of wedge.
In this case the light normally falls on the plane convex lens for the angle of refraction r = 0.
Further, as we use a lens of large focal length the angle of wedge Ө is very small. So
Cos (r+Ө) = Cos Ө = Cos 00 =1 and thus the path difference
𝜆
∆ = 2µt - ....... (5.15)
2

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𝜆
At point of contact t = 0, therefore, ∆ =
2

Which is the condition of minima. Hence at centre or at point of contact there is a dark spot.
Condition of Bright Rings or Maxima
The condition for bright rings is path difference ∆ = n λ therefore
𝜆
∆ = 2µt - = n λ where n= 0, 1, 2, 3………
2
2𝑛+1
or 2µt = ( )𝜆
2
2𝑛−1
or 2µt = ( )𝜆 ....... (5.16)
2

Where n = 1, 2, 3………
Condition of Dark Ring or Minima
2𝑛−1
In case of dark rings, the path difference, ∆ = ( ) 𝜆
2

Where n= 1, 2, 3………
𝜆 2𝑛−1
Therefore ∆ = 2µt - =( )𝜆
2 2
or 2µt = nλ ....... (5.17)

Thus corresponding to n = 1, 2, 3….. we observe first, second third…..etc. bright or dark

rings. In Newton’s rings experiment the locus of points of constant thickness is a circle
therefore the fringes are circular rings.
5.8.3 Diameter of Bright and Dark Rings
In figure 5.9 the plano-convex lens BOPF is place on glass plate G and O is the point of
contact. Suppose, C is the centre of the sphere OBFP from which the plano-convex lens is
constructed. P is point on the air film at which the thickness of air film is t. At point P, the
light is incident and reflected form the upper and lower surface of air film, and rings are
formed. AP is the radius of ring passes through point P. According to property of circle

Figure 5.9

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AP ×AB = AO× AL
𝑟 2 = 𝑡 × (2𝑅 − 𝑡) ∵ AL=OL-OA
Where 𝑅 is the radius of curvature of lens.
𝑟 2 = 2𝑅𝑡 − 𝑡 2
Since 𝑅 is very large and t is very small, we can write
𝑟2
𝑟 2 = 2𝑅𝑡 or 𝑡=
2𝑅

Substituting this value of t in equation (5.16), we get,

𝑟2 2𝑛−1
2µ =( )𝜆
2𝑅 2
2𝑛−1 𝜆𝑅
or 𝑟2 = ( )µ
2

This expression contains n, i.e., r is a function of n. Thus it is better to use rn in place of r. If

Dn is the diameter of nth bright ring then we have r = rn = Dn / 2 and can write
2𝑛−1
𝐷𝑛 2 (
2
)𝜆𝑅
=
4 µ

2(2𝑛−1)𝜆𝑅
or 𝐷𝑛 2 = ....... (5.18)
µ

Where n= 1, 2, 3……. Similarly for dark rings

𝑟2 𝑛𝜆𝑅
2µ𝑡 = 𝑛𝜆 or 2µ = 𝑛𝜆 or 𝑟2 = .
2𝑅 µ

If 𝐷𝑛 is diameter of 𝑛𝑡ℎ dark ring then

𝐷𝑛 2 𝑛𝜆𝑅
=
4 µ

4𝑛𝜆𝑅
or 𝐷𝑛 2 = ....... (5.19)
µ

Where 𝑛 = 1, 2, 3……

Figure 5.10

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The alternate bright and dark rings are formed as shown in figure 5.10. The spacing between
two consecutive rings can be given as
2
𝑟𝑛+1 – 𝑟𝑛2 = (√𝑛 + 1 − √𝑛) 𝜆𝑅 (in case of air film µ =1)

Spacing between 1st and 2nd rings = (√2 − √1) 𝜆𝑅 = 0.4142 𝜆𝑅

Spacing between 2nd and 3rd rings = (√3 − √2) 𝜆𝑅 = 0.3178 𝜆𝑅

Spacing between 4th and 3rd rings = (√4 − √3) 𝜆𝑅 = 0.21 𝜆𝑅

Thus it is clear that the spacing between successive rings decreases with increase in order.
5.8.4 Determination of Wave Length of a Monochromatic Light Source
In Newton’s experiment if we use a light source of unknown wave length (say sodium
lamp) then we can determine the wavelength of light source by measuring the diameters of
Newton’s ring.
If Dn is diameter of nth dark ring formed due to air film then
𝐷𝑛2 = 4𝑛𝜆𝑅
Where n is any integer number.
Similarly if D(n+p) is the diameter of (n+p)th ring
2
𝐷𝑛+𝑝 = µ (𝑛 + 𝑝) 𝜆𝑅
Using this equation, we can write
2
𝐷𝑛+𝑝 – 𝐷𝑛2 = 4 (𝑛 + 𝑝) 𝜆𝑅- 4nλR= 4 P λR
2 − 𝐷2
𝐷𝑛+𝑝 𝑛
or 𝜆= ....... (5.20)
4𝑝𝑅

Where p is any integer number and R is radius of curvature of plano-convex lens.

5.8.5. Determination of Refractive Index of a Liquid by Newton’s Rings
Experiment
In Newton’s rings experiment the diameter of nth dark ring in case air film is
𝐷𝑛2 = 4𝑛𝜆𝑅 (∵ µ = 1)
The diameter of (n+p)th ring
D2n+p = 4(n+p)λR
If a liquid of refractive index µ is filled between the plane glass plate and convex lens then
4𝑛𝜆𝑅 4(𝑛+𝑝)𝜆𝑅
D2n = and D2n+p =
µ µ

Thus we can write

2 − 𝐷 2 ]𝑎𝑖𝑟
[𝐷𝑛+𝑝 𝑛 4𝑝 𝜆𝑅
2 − 𝐷 2 ]𝑙𝑖𝑞𝑢𝑖𝑑
[𝐷𝑛+𝑝
= 4𝑝 𝜆𝑅 =µ
𝑛
µ

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2 − 𝐷2 ]𝑎𝑖𝑟
[𝐷𝑛+𝑝 𝑛
or µ= 2 − 𝐷 2 ]𝑙𝑖𝑞𝑢𝑖𝑑 ....... (5.21)
[𝐷𝑛+𝑝 𝑛

Example 5.7: In Newton’s rings experiment if the radius of curvature of plano-convex lens
in 200 cm and wavelength of the light used is 5890 Å, calculate the diameter of 10th bright
ring.
Solution: The diameter of nth bright ring is given as (µ=1 for air film) is given by
Dn2= 2(2n-1) λ R
or 𝐷10 2 = 2 × (20-1) × 5890 × 10-8 × 200 cm2 = 6.69mm
The diameter of 10th bright ring is 6.69 mm.
Example 5.8: In a Newton’s ring experiment the diameter of 15th dark ring and 5th dark ring
are 0.59 cm and 0.33cm respectively. If the radius of curvature of the convex lens is 100cm
calculate the wave length of light used.
Solution: The wave length of unknown light source is Newton’s rings experiment is given as
2 − 𝐷2 ]
[𝐷𝑛+𝑝 𝑛
𝜆=
4𝑝𝑅

Here Dn+p = D15 = 0.59 cm, Dn = D5 = 0.33cm, p = 10, R = 100cm

(0.59)2 −(0.33)2
λ= = 5980 Å
4×10×100

Example 5.9: Newton’s rings are formed by using a monochromatic light of 6000Å. When a
liquid is introduced between the convex lens and plane glass plate the diameter of 6th bright
ring becomes 3.1mm. If the radius of curvature of lens is 1mt, calculate the refractive index
of liquid.
Solution: Given that, n= 6, Dn = 3.1mm = 3.1 ×10-3 m, λ = 6000Å = 6×10-7 m, R = 1m
2(2𝑛−1)𝜆𝑅 2×11×6×10−7 ×1
µ= 2 = = 1.37
𝐷𝑛 (3.1×10−3 )2

Example 5.10: In Newton’s ring experiment two light sources of wavelength 6000Å and
4500Å are used to form rings. It is observed that nth dark ring due to 6000Å light coinside
with (n+1)th dark ring due to 4500Å. If the radius of curvature of the plano convex lens is
100cm, calculate the diameter of nth dark ring due to λ1 and λ2.
Solution: For nth dark ring due to λ1, D2n =4n λ1R
Similarly for (n+1)th dark ring due to λ2, D2n+1 = 4 (n+1) λ2R
Since nth dark ring due to λ1 co-inside with (n+1)th dark ring due to λ2 therefore.
𝜆2
4nλ1R= 4(n+1) λ2R or nλ1= (n+1) λ2 or nλ1 - nλ2 = λ2 or n=
𝜆1 −𝜆2

Here λ1= 6000Å, λ2 =4500Å

4500
 n= =3
6000−4500

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Now the diameter of n=3rd dark ring due to λ1

D32 = 4nλ1R = 4×3×6000×10-10×1 m
Or D3 = 2.68 mm.
Similarly diameter of n=3rd dark ring due to λ2
D32= 4nλ2R = 4×3×4500 ×10-10×1 m
or D3 = 2.32 mm.
Same relation can also be obtained for bright rings.
5.8.6 Newton’s Rings in Case of Transmitted Light
The Newton’s rings can also be formed in case of interference due to transmitted light
as shown in figure 5.11. In this case the transmitted rays 1 and 2 interfere, and we can
observe the rings in the field of view. In this case the net path difference between the rays is
∆= 2µt. since we will not consider the path difference arises due to reflection from denser
medium. Therefore this is net path difference.

Figure 5.11
The condition for maxima (bright rings) is given by
2µt= nλ
𝑟2
And we know that in case of reflected light, t =
2𝑅
𝑟2
2µ = nλ
2𝑅
𝐷𝑛
Now if Dn is the diameter of nth bright ring then, = 𝑟 and thus
2
4𝑛𝜆𝑅
Dn2 =
µ

In case of air film, Dn2 = 4nλR

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Figure 5.12
Similarly in case of minima (dark ring) the diameter nth dark ring is given by
Dn2 = 2(2n-1) λR
We can see that, this is an opposite case of reflected light. In case at point of contact the path
difference is zero which is condition corresponding to bright fringe thus the centre point is
bright. The rings system in this case is shown in figure 5.12.

5.9 SUMMARY
1. A thin film is any transparent material in a shape of thin sheet of order 1µm to10 µm.
When a beam of light is incident on this sheet the interference may take place after
reflection or transmission of light. In case of interference due to reflected light, the path
difference
𝜆
∆=2µt Cos r –
2
The condition of bright fringes (maxima)
2𝑛+1
2µt Cos r = ( )𝜆 (where n= 0,1,2,3…..)
2
Similarly condition of dark fringes (minima) is
2µt Cos r = nλ (where n= 0,1,2,3….)
2. In case of interference due to transmitted light, the path difference become ∆= 2µt Cos r
The condition of bright fringe (maxima)
2µt Cos r = nλ
Similarly the condition of dark fringes (minima)
2𝑛+1
2µt Cos r = ( )𝜆
2
3. In case of wedge shaped film the net path difference is given as
𝜆
∆= 2µt Cos (r + Ө) -
2
where Ө is angle of wedge and other symbols have their usual meaning. For bright fringes
2𝑛−1
2µt Cos (r + Ө) = ( )𝜆 (where n=1, 2, 3…..)
2
For dark fringes
2µt Cos (r + Ө) = nλ (where n=1, 2, 3…..)
If xn is the distance of nth fringe from the edge then
𝑡
tan Ө =
𝑥𝑛

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(2𝑛−1)𝜆
xn =
4µ 𝐶𝑜𝑠 (𝑟+Ө) tan Ө
𝜆 3𝜆
x1 = , x2 = , …………
4µ 𝐶𝑜𝑠 (𝑟+Ө) tan Ө 4µ 𝐶𝑜𝑠 (𝑟+Ө) tan Ө

Fringe width ω= xn+1 - xn

For normal incident r = 00 and for small value of Ө (tan Ө ≈ Ө )
𝜆
ω=
2 µӨ

4. In case of interference due to thin film the extended source of light is more beneficial. In
extended source of light, a large number of rays are available for production of
interference pattern and larger area of the film can be seen by our eye in the field of view.
5. On the basis of variation in two parameters t and r, the fringes are two types ray fringes of
equal thickness and fringes of equal inclination.
In case of fringes of equal thickness, the thickness of film is varying and light coming at
same angle of incident then fringes are formed due to variation is thickness. The fringes
are formed on the locus of points of equal thickness. Examples are thin films and
Newton’s rings.
On the other hand, in case of inclination, the thickness becomes constant. Now the fringes
are formed at the locus of points of constant. Such fringes are called fringes of equal
inclinations. Examples are fringes formed in Michelson interferometer.
6. When a plano-convex lens of large focal length is place on a plane glass plate, an air film
is formed between the lens and glass plate. When a beam of light normally incident on
this film the interference takes place between the reflected rays and we observe alternate
dark and bright rings and called Newton’s rings.
7. In Newton’s rings the condition for bright rings is given by
2𝑛−1
2µt = ( )λ (where n=1, 2, 3….)
2
Similarly condition for dark rings
2µt = nλ (where n=1, 2, 3….)
th
The diameter of n bright ring is given by
2(2𝑛−1)𝜆 𝑅
D2n = ( )
µ
Similarly if Dn is diameter of nth dark ring then
4𝑛 𝜆𝑅
Dn2 =
µ
8. By using Newton’s rings experiment, the wave length of a unknown light source can be
determined as
2 − 𝐷2
𝐷𝑛+𝑝 𝑛
λ=
4𝑝𝑅
Where Dn+p is diameter of (n+p)th bright or dark ring and Dn is the diameter of nth bright
or dark ring.
9. Newton’s rings may also be observed in case of transmitted light. In this case if Dn is the
diameter of nth dark ring then it can be given as
2(2𝑛−1)𝜆 𝑅
Dn2 = = ( )
µ

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Similarly if Dn is diameter of nth bright ring then

4𝑛 𝜆𝑅
Dn2 =
µ

5.10 GLOSSARY
Thin film: A thin sheet of thickness of the order of 1-10 µm.
Wedge shaped film: A film of unequal thickness which gradually changes.
Newton’s rings: Circular bright and dark fringes formed in Newton’s experiment.
Narrow source: Point source
Extended source: A broader source

5.11. REFERENCE
1. N Subrahmanyam and Brijlal, A text Book of optics, S Chand and Sons, New Delhi.
2. Satya Prakash, Optics and Atomic physics, Ratan Prakashan Mandir, Agra
3. Ajoy Ghatak, Optics, Tata McGraw Hill Publishing company Limited, New Delhi.
4. https://en.wikipidia.org

5.12. SUGGESTED READING

1. Max Born and Emil wolf, Principles of optics Pergamon press, oxford, U.K.
2. Fundamentals of optics, Francis A. Jenkines and Harvey E. White, Tata MaGraw Hill, Publisher Limited N.
Delhi, India.

5.13. TERMINAL QUESTION

Short Answer Type Questions

1. Explain why different colours are exhibited by a thin film when illuminated in white
light.
2. With the help of diagram explain why an extended source of light is needed to observe
the interference in thin film.
3. Discuss the phase change in reflection of light from a denser medium.
4. Explain the interference in a thin film of uniform thickness.
5. Calculate the path difference between the light ray reflected from the upper and lower
surface of a thin film.
6. Find out the condition of maxima and minima in reflected light in case of thin film.
7. Why a thick film does not show colours when white light is incident on it.
8. What are Newton’s rings?
9. Obtain the path difference between the reflected rays in Newton’s rings experiment.
10. Find out the condition for bright and dark rings in Newton’s ring experiment.
11. Explain why Newton’s rings are circular?
12. Explain the difference in Newton’s rings formed in case of reflected and refracted light.

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Long Answer Type Question

1. Discuss the formation of bright and dark fringes formed by a thin film. Explain why
different colours are exhibited by thin film in white light.
2. Explain the formation of interference fringes in wedge shaped film. Obtain the condition
for bright and dark fringes, and fringe width.
3. What are Newton’s rings? Draw a ray diagram for Newton’s rings experiment. Find out
the diameter of bright and dark rings.
4. What are Newton’s rings? Derive the expression for diameter of bright and dark rings.
5. Give the theory of Newton’s rings and describe how the wave length of a unknown light
source can be determine with the help of these rings.
6. Describe the interference fringes observed when a thin wedge shaped film is observed by
reflected light. Calculate the separation between two consecutive bright and dark fringes.
7. Show that in Newton’s rings experiment, the diameter of dark rings are proportioned to
root of natural numbers.
8. Explain the formation of Newton’s ring. How the refractive index of a given liquid can be
determined with the help of Newton’s rings.
9. Describe the fringes of equal thickness and fringes of equal inclination.
10. What are Haidiger’s and Newton’s fringes?

Numerical Type Questions

1. A beam of monochromatic light of wavelength 5890Å is incident on a thin glass plate of

refractive index 1.50 with the angle of refraction in the glass plate is 600. Calculate the
smallest thickness of the plate which will make it appears dark by reflection.
2. Light of wave length 5000Å is incident on a soap film of refractive index 1.33 at an angle
600. When the reflected light is observed, a dark band is seen. If the thickness of the film is
1µm, calculate the order of the fringe dark band.
3. Calculate the thickness of a wedge shaped film at a point where the 4th bright fringe is
observed. The experiment is performed with a light source of wavelength 5890Å.
4. A wedge shaped film of angle 6×10-3 degree is illuminated normally with a
monochromatic light source. If the reparation between two consecutive fringes is 3.00mm,
find out the wave length of light source used.
5. In a Newton’s rings experiment the diameter of 5th and 12th dark rings are 0.42 cm and
0.726cm. The radius of curvature of plano convex lens is 2.00m. Calculate the wavelength of
light source.
6. In Newton’s ring experiment a light source of wavelength 5890Å is used. If the radius of
plano-convex lens is 2m and water is filled between the glass plate and plano convex lens,
calculate the diameter of 5th dark ring.
7. A wedge shaped film is formed with air between two glass plates, which touch each other
at one point and separated by a wire of diameter 0.05 mm at a distance of 15cm. If a light of
wave length 6000Å is used, calculate the fringe width.

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8. In Newton’s rings experiment the diameter of 4th bright ring is 2.52cm. If a liquid of
unknown refractive index is filled in place of air between lens and plane glass plate, the
diameter becomes 2.21cm. Find out the refractive index of liquid.
9. Show that in Newton’s rings experiment, the difference of square of diameters of two
consecutive rings remains constant.
10. Newton’s rings are formed with the help of a light source of wavelength 5890Å. If the
diameter of 10th dark ring is 0.5cm, calculate the radius of curvature of plano convex lens.
11. A thin equiconvex lens of focal length 4m and refractive index of 1.5 is place on a plane
glass plate. A light of wave length 5890Å falls normally on it. What will the diameter of 10 th
dark ring?

Objective Type Question

1. If the thickness of the parallel film increases, the path difference

(a) increases (b) decreases
(c) remains same (d) none of these
2. When a light wave is reflected from a surface of an optically denser medium, then the
phase difference involved is
(a) π/4 (b) π / 2 (c) π (d) 2π
3. When a light wave is reflected from a surface of an optically denser medium, then the path
difference involved is
(a) λ/4 (b) λ / 2 (c) λ (d) 2 λ
4. In case of the thin film, the condition for constructive interference in reflected light, the
path difference should be equal to
𝜆 𝜆 𝜆
(a) 2µt Cos r − (b) 2 (c) 2µt Cos r + (d) λ
2 2

5. In Newton’s rings experiment the diameter of nth bright ring is given by

2(2𝑛−1)𝜆𝑅 (2𝑛−1)𝜆𝑅
(a) 𝐷𝑛 2 = (a) 𝐷𝑛 2 =
µ µ

4𝜆𝑅 2𝜆𝑅
(c) 𝐷𝑛 2 = (d) 𝐷𝑛 2 =
µ µ

6. The lens used in Newton’s rings experiment, which is placed on a plane glass plate to trap
air film is
(a) concave (b) plano convex
(c) plano concave (d) none of these
7. In Newton’s rings experiment, the diameter of bright rings is proportional to
(a) odd natural numbers (b) natural numbers
(c) even natural numbers (d) square root of odd natural numbers

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Answer of Numerical Type Question

1. 0.39µm, 2. 4th, 3. 1.02 µm, 4. 6.28× 10-5cm, 5. 4.87×10-5cm 7. 0.9 mm, 8. 1.3, 10. 1.06 m.
11. Hint: The focal length is given as
1 1 1
= (µ − 1)(𝑅 − )
𝑓 1 𝑅2
Here, µ = 1.5 , 𝑅1 = 𝑅 𝑎𝑛𝑑 𝑅2 = −𝑅 ⟹ 𝑅 = 4 𝑚
D2n = 4nλR
Therefore, D10 = √4 × 10 × 5890 × 10−10 × 4 = 9.70 𝑚𝑚

Answer of objective Type Question

1. (a), 2. (c), 3. (b), 4. (a), 5. (a), 6. (b), 7. (a)

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UNIT 6: MULTIPLE BEAM INTERFEROMETRY

CONTENTS
6.1 Objective
6.2. Introduction
6.3 Interferometery
6.4 Fringes of Equal Inclination (Haidinger Fringes)
6.5 Michelson’s Interferometer
6.5.1. Construction
6.5.2 Working
6.5.3. Formation of Fringes
6.5.4. Determination the Difference between Two Neighboring Wavelengths
6.5.5. Determination of Refractive Index of a Material
6.5.6. Michelson- Morley Experiment and Its Result
6.6. Solved Examples
6.7 Summary
6.8. Glossary
6.9. References
6.10. Suggested Reading
6.11 Terminal Questions

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6.1 INTRODUCTION
In first unit of interference, we understood the basic principle of interference, condition
required for interference and experiment like Young double slit experiment and biprism
experiment which show interference. In second unit of interference, we understood different
types of thin films like wedge shaped or air films which cause interference under certain
conditions. Further, we understood the fringes of equal thickness and fringes of equal
inclinations.
Now in this unit of interference we are going to understand different types of
interferometers, especially Michelson’s interferometer. In interferometer we observe the
fringes occur due to equal inclination which are called Haidinger fringes. In an
interferometer, we study the different techniques of fringes formation and calculate the fringe
width with great accuracy. The interferometers like Michelson interferometer have a lot of
significant applications in the field of optics and other branches of physics.

6.2. OBJECTIVES
After reading this unit you will be able to understand
 Interferometry
 Haidinger fringes observed in interferometers
 Michelson interferometer
 Application and significance of Michelson’s interferometer

6.3. INTERFEROMETRY
Interferometry is a branch of science in which optical waves or any other
electromagnetic waves are superimposed on each other and interference phenomenon occurs.
Interferometry plays important role to study in the field of optics, astronomy, fiber optics,
spectroscopy, cosmology, remote sensing, particle physics plasma physics, velocity
measurements and bio-molecular interactions. In present unit we only discuss the optical
interferometry. Interferometers are devices use for different measurement of path difference,
fringe widths, refractive index and many other parameters with the help of interference
phenomenon.

6.4. FRINGES OF EQUAL INCLINATION (HAIDINGER

FRINGES)
Before going ahead, we should understand the fringe formation in a interferometer. As
we know the interference fringes are formed due to a path difference ∆ = 2µt Cos r between
the overlapping rays. Now for a particular wavelength, the path difference may occur due to
variation of thickness t and angle of inclination r.

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𝛿∆= 2𝜇∆𝑡 cos 𝑟 + 2𝜇𝑡 𝛿 (𝐶𝑜𝑠 𝑟) ....... (6.1)

In case of a film with constant thickness then variation in path difference occurs as
𝛿∆= 2𝜇𝑡 𝛿 (𝐶𝑜𝑠 𝑟) ....... (6.2)
Thus the path difference occurs with the variation in the angle of inclination r. If we use an
extended source of light, we have a large numbers of rays comes with equal angle of
inclination r, which produces a particular path difference and fringes are observed
corresponding to this path difference. Such fringes are called fringes of equal inclination. In
case of Michelson interferometer, the thickness of film remains constant then the fringes are
formed due to equal inclination and hence called fringes of equal inclination or Haidinger
fringes.

6.5 MICHELSON INTERFEROMETER

Michelson interferometer is a device used for the formation and study of interference
fringes by a monochromatic light. In this apparatus, a beam of light coming from an extended
source of light is divided into two parts, one is reflected part and another is refracted part
after passing through a partially polished glass plate. These two beams are brought together
after reflected from plane mirrors, and finally interference fringes are produced in the field of
view.
6.5.1 Construction
The apparatus is shown in Figure 6.1. The main part of the apparatus is a half silvered
glass plate P, on which a beam of monochromatic light is incident. The plate P inclined at an
angle 450 with incident light as shown in figur6.1, the incident light then divided into two
parts, one is reflected part and another is transmitted part. The transmitted light is then passes
through another glass plate Q which is of equal thickness as of P, and parallel to plate P, this
pate Q is called compensating plate. The transmitted and reflected parts of light are normally
incident on two mirrors M2 and M1 respectively. The mirror M1 and M2 are perpendicular to
each other as shown in figure. The mirror M1 is fixed in a carriage and can be moved to and
fro with help of a screw and micro scale. Therefore mirror M1 is movable and the mirror M2
is fixed. A telescope is also fixed as shown in figure. The light reflected from mirror M 1 and
M2 are superimposed and interference fringes are formed in the field of view.

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Figure 6.1
6.5.2. Working
S is a source of monochromatic light; the light coming from this source is rendered
parallel by mean of a convex lens L, and after passing through Less L the light falls on plate
P. Since plate P is partially polished, some part of light reflected back from P and going
toward direction AC and incident on mirror M1.
Similarly the light transmitted from plate P passing through compensating plate Q and
then incident on mirror M2. The compensating plate is used to compensate the optical path
travelled by transmitted light. The beam of light reflected by P, crosses plate P two times, for
transmitted light this optical path is compensated by using plate Q in which the transmitted
light crosses Q two time. Thus by using compensating plate Q, the reflected and transmitted
light travel equal optical path lengths.
Now the reflected light is incident on mirror M1 and reflected back towards the
telescope T. Similarly the transmitted light incident normally on mirror M2 and reflected back
towards plate P, and at P some part of this light again reflected toward the telescope. Now in
the direction of telescope we have two coherent beams of light reflected from mirror M1 and
M2, and interference takes place and we observed interference pattern/beam in the field of
view.
6.5.3 Formation of Fringes
Since the fringes are form by the light reflected from mirror M1 (movable) and M2
(fixed) and we can consider a virtual image of M2 called M2' in the field of view as shown in
figure 6.1. Further we can consider the interference fringes are now formed due to light
reflected from the surface of air film formed between mirror M1 and M2'. Now it is clear that
the shapes of fringes are depend upon the inclination of mirror M 1 and M2. Since M2 fixed
therefore the shape are depends upon the inclination of M1. Since OA = OB, therefore the
path difference between two rays is simply the path traveled in air film before reaching to
telescope. If t is the thickness of air film then path difference between light reflected from M 1
and M2 is 2t.
Condition for maxma ∆ = 2𝑡 = 𝑛𝜆
2t = nλ

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If the movable mirror M1 moved by a distance x and we observed fringes shift of N fringes
then
2(t + x) = (n + N) λ
or 2x = Nλ
2𝑥
or λ= ....... (6.3)
𝑁

It is clear that if M1 and M2 are exactly perpendicular to each other, then M1 and M2' are
parallel to each other and air film between M1 and M2' is of equal thickness in this case we
observed fringes of equal inclination or Haidinger’s fringes of circular shape. If however, the
two mirror M1 and M2 are not exactly perpendicular to each other then the shape of the air
film formed between mirror M1 and M2' is of wedge shaped and the fringes are now of
straight line parallel to the edge of wedge. This straight line fringes are because of the focus
of constant thickness in a wedge shape film is a straight line.
Thus the shapes of fringes are depends on the inclination. The fringes are in general
curved and convex toward the edge of wedge as shown in figure 6.2. These fringes are called
localized fringes.

Figure 6.2
6.5.4 Determination of Difference of Wavelengths between Two
Neighboring Wavelengths
Let us consider a source of light which emits two very close wavelengths. Sodium light
is an example of such case. In sodium light, there are two wavelength 𝐷1 𝑎𝑛𝑑 𝐷2 lines with
wavelength λ1 = 5890Å and λ2 = 5896Å. By using Michelson interferometer we can
determine the difference between these two wavelengths. In this case first we adjust the
aperture for circular fringes. We know that each wavelength produce its own ring spectrum.
Now the mirror M1 is moved in such a way that when the position of very bright fringes are
obtained. In this position the bright fringes due to λ1 coincident with the bright fringes due to
λ2 and we observe distinct fringes of order n.
Now the mirror M1 is further moved to a very small displacement, and the fringes are
disappeared. This case occurs when the maxima due to λ1 coincident on minima due to λ2.
This is the position of minimum intensity or uniform illumination with no clear fringes. In
this case we observed indistinct fringes of order (n+1). If we moved a distance x between
such two points of most bright and most indistinct fringes then
2 x = n λ1 = (n+1) λ2
λ2
or n=
λ1 – λ2

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λ1 λ2
or 2x =
λ1 – λ2

λ1 λ2
or λ1 – λ2 = ....... (6.4)
2x

If λ1 and λ2 are very close to each other then

λ1.λ2 = λ2
Where λ is the mean value of λ1 and λ2
λ1 + λ2
λ=
2
𝜆2
Then ∆λ = λ1 - λ2 = ....... (6.5)
2x

6.5.5 Determination of Refractive Index of a Material

In Michelson interferometer, the two interfering beam of light travel in different
directions, one is toward mirror M1 and second one is toward mirror M2. It is very easy to
introduce a thin transparent sheet of a material of refractive index is and thickness t, in the
path of one of the interfering beams of light. After introducing a sheet, the optical path of that
beam increases by µt. Now the net increase in the path is (µt – t). Since the beam crosses the
sheet twice, the net path difference becomes 2(µt-t).
If n is the number of fringes be which the fringe system is displaced, then
2(µt – t) = n λ
or 2(µ - 1)t = n λ ....... (6.6)
In experiment we first locate the central dark fringe by using while light. The cross wire
of telescope is adjusted in such a way that the cross wire of telescope is adjusted on central
dark fringes. Now the light is replaced by a monochromatic light of wavelength λ. Now a thin
sheet is introduced into the path of one beam. The position of movable mirror M1 is adjusted
in such a way that the dark fringe is again coincide with the cross wire of telescope. We note
the distance d through which the mirror is moved and count number of fringes displaced. By
using the relation given below we can determine the thickness of sheet.
t = n λ / 2(µ - 1) ....... (6.7)
Similarly if we know the thickness, we can determine the refractive index of material.
2(µ - 1) t = n λ
µ = (n λ / 2t) +1 ....... (6.8)
6.5.6 Michelson Morley Experiment and Its Result
In classical mechanics it was assumed that the preferred medium for light propagation is
ether which filled in all space uniformly. The ether is perfectly transparent medium of light
and material bodies may pass in this medium without any resistance. Ether remains fixed in
space and consider as absolute frame of reference. In the 19th century this ether drag
hypothesis of light was widely discuss.

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Michelson interferometer was originally designed to verify the existence of hypothetical

medium ether. The experiment performed to verify this hypothesis is called Michelson
Morley experiment. In this experiment, it was assumed that the Michelson interferometer is
moving along the earth direction of motion. Due to motion of apparatus with transmitted light
are not same. Mathematically the path difference between two ray (transmitted and reflected)
is lv2/c2 where l is distance between plate P and mirror M1 and v is velocity of ether
corresponding to this path difference there should be a fringe shift of n = 0.37. Thus if the
apparatus is at rest and starts motion, there should be a fringe shift of n = 0.37. But it is not
possible to make earth at rest. In this experiment we consider if the whole apparatus was
turned by 90o, the fringe shift should be observed.
The experiment was performed by many scientists, many times at different location on
earth but fringe shift was not observed. This is called negative result of Michelson Morley
experiment. The result shows the non existence of hypothetical medium of ether. After this
experiment, a foundation of modern though way lay down which led to Einstein theory of
relativity.

Self Assessment Questions

1. What is an interferometer?
2. What is the role of compensating plate in Michelson interferometer?
3. How the air film is formed in Michelson's interferometer?
4. How the path difference is calculated in Michelson's interferometer?
5. Why fringes are circular in Michelson's interferometer?
6. What is the meaning of localized fringes?
7. What happens when white light is used in Michelson's interferometer?
8. Determine the thickness of a thin transparent film with the help of Michelson's interferometer.
9. Determine the refractive index of a material with the help of Michelson's interferometer.
10. If the mirrors M1 and M2 of Michelson's interferometer are exactly perpendicular to each other,
how will be the shape of fringes?
11. How you will find the wavelength of a monochromatic light with Michelson's interferometer.
12. Give the application of Michelson's interferometer.

6.6 SOLVED EXAMPLES

6.1. In Michelson interferometer, when movable mirror M1 is shifted by a distance 0.030mm,
a fringe shift of 100 fringes is observed. Calculate the wavelength of light used.
Solution: In Michelson interferometer if the mirror is displaced by a distance x, the
corresponding fringe shift N is
2x = Nλ or λ = 2x/N = 2(0.030)/100 = 6000Å
6.2. The difference between two wavelengths of sodium light lines 𝐷1 𝑎𝑛𝑑 𝐷2 is determined
with the help of Michelson intereferometer. If the distance travelled by movable mirror for
two successive position of most distinct and most indistinct position is 0.2945 mm calculate

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the difference between two wavelengths 𝐷1 𝑎𝑛𝑑 𝐷2 , the mean wavelength of two lines is
5893Å
Solution: If the displacement between two position of mirror for two successive position of
most distinct and most indistinct position is x then
𝜆2
λ1 - λ2 = = (5893 × 5893)/(2 × 0.2945 × 107) = 6Å
2x

6.3. Reflective index of a glass plate is to be determined by the help of Michelson

interferometer. If is observed that when the glass plate is introduced, a fringe shift of 140 is
observed. If the length of glass plate is 20cm and the wavelength of light is 5460Å, calculate
the refractive index of material.
Solution: when a glass plate is introduce in one of the interfering ray of Michelson’s
interferometer then a fringe shift is observed as
2(µ - 1) t = nλ or µ = (n λ /2t) +1= [(140×5460×10-10) ÷ (2×20×10-8)] + 1 = 1.0029
6.4. In Michelson interferometer 790 fringes cross the field of view when the movable mirror
is displaced through a distance 0.233mm. Calculate the wavelength of light used.
Solution: In Michelson interferometer if movable mirror is displaced through a distance x,
the corresponding fringe shift n is given as
2x = nλ or λ = 2x/ n = 2× 0.233/790 mm = 5896Å

6.7 SUMMARY
1. Interferometer is a device used for measurement of path difference, fringe width,
refractive index, wavelength of a monochromatic light source and many other parameters
with the help of interference phenomenon.
2. In Michelson’s interferometer, an air film is formed with the help of two perpendicular
mirrors. The light reflected from two mirrors M1 and M2 is equivalent to light reflected
from the upper and lower surface of air film formed between mirror M1 and M2'.
3. The condition for bright fringes is given as 2x = Nλ
Where, x = displacement of mirror M1, N = number of fringe shifts on displacement of x,
λ = wavelength of light used.
4. In Michelson interferometer if M1 and M2 mirror are exactly perpendicular to each other,
the shape of fringes are circular which are called fringes of equal inclination or Haidinger
fringes. If however, two mirrors are not perpendicular to each other, the shape of film
formed between M1 and M2' is of wedge shape and the fringes are straight line or
localised.
5. The difference between two neighboring wavelength of a source is given as
𝜆2
∆λ = λ1 - λ2 =
2x
6. The refractive µ index of a medium can be determine by
2(µ - 1) t = nλ or µ = (n λ/2t) + 1
7. The thickness t can be determine by, t = n λ/2 (µ- 1)

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6.8 GLOSSARY
Interferometer: A device used for measurement of path difference, fringe width, wavelength
of light, refractive index etc. with the help of interference phenomenon.
Inclination: Degree of sloping, slope
Haidinger fringes: The fringes of equal of inclination.
Compensating plate: A plate used in Michelson interferometer for compensating the path
difference in transmitted light raised due to glass plate.

6.9 REFERENCE
1. N Shubramanyan and Brijlal, A text of optics, S. Chand and company, New Delhi.
2. C.L. Arora and P.S. Hemene, Physics for Degree Students, S. Chand and Company, New Delhi.
3. http://wikipedia.org
4. http://nptel.ac.in
5. http://books.goole.com

6.10 SUGGESTED READING

1 Prank S.J. Pedrotte, Introduction to Optics, Pentice Hall India limited
2. Ajay Ghtak, Optics, McGraw Hill Company, NEW Delhi.

6.11 TERMINAL QUESTIONS

Short Answer Type Questions
1. Describe the construction of Michelson interferometer.
2. Describe the working of Michelson interferometer.
3. How Michelson's interferometer may be used to obtain circular and streight line fringes.
4. Explain why circular fringes shift in the field of view when we move the mirror M1.
5. Outline the theory of Michelson's interferometer.
6. With the help of Michelson interferometer how the D1 and D2 lines of sodium light can be
distinguished. Find out the difference between D1 and D2 lines of sodium light.
7. How the refractive index of a medium can be determined with the help of Michelson
interferometer.
8. Explain the method of determine the thickness of sheet/fill with the help of Michelson
interferometer.
9. Explain the role of compensation plate in Michelson's interferometer.
10. What are localised fringes in Michelson's interferometer?
11.

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Long Answer Type Questions

1. 1 With the help of neat diagrams, describe the construction and working of Michelson's
interferometer.
2. Explain the working of Michelson's interferometer. How the interferometer produces
straight line and circular fringes.
3. Give the applications of Michelson’s interferometer in detail.
4. Explain how circular fringes are produced in Michelson's interferometer. Show that the
radii of circular fringes obtained by the Michelson's interferometer are proportional to the
square root of natural number.
Numerical Type Questions
1. 1. Calculate the displacement between two successive positions of movable mirror giving
the best fringes in case of sodium light. [Answer: 0.029cm]
2. 2. In Michelson's interferometer when movable mirror is displaced through a distance
0.589mm, a fringe shift of 200 is observed across the cross wire in the field of view.
Calculate the wavelength of light used. [Answer: 5890A]
3. Determine the difference between the wavelengths of two D1 and D2 lines in sodium
light. The wavelengths of D1 and D2 lines are 5896 Å and 5890 Å respectively. The scale
reading of two successive distinct and indistinct points are 0.6939mm and 0.9884mm.
[Answer: 6Å]
4. In Michelson's Interferometer when movable mirror is displaced through a distance
0.844mm a fringe shift of 300 is observed. Calculated the wave length of light used.
[Answer: 562Å]
5. 3. Determine the difference between the wavelengths of two D1 and D2 lines in sodium
light. The wave length of D1 and D2 are 5896 Å and 5890 Å respectively. The scale
readings of two successive distinct and indistinct points are 0.6939mm and 0.9884mm.
[Answer: 6 Å]
5. In Michelson's Interferometer when movable mirror is displaced through a distance
0.844mm a ping shift of 300 is observed. Calculated the wave length of light used.
[Answer: 562 Å]
Objective Type Questions
1. In Michelson interferometer, when mirror M1 and M2 are perpendicular to each other,
then the shape of the fringes are
(a) Straight line (b) Circular
(c) elliptical (d) inclined
2. The use of compensating plate in the Michelson Interferometer is
(a) To make path difference equal between light beams reflected from mirror M1 and M2
(b) To make frequency equal between light beams reflected from mirror M1 and M2
𝜆
(c) To make path difference 2 between light beams reflected from mirror M1 and M2

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(d) To make path difference λ between light beams reflected from mirror M1 and M2

[ Answers 1(b), 2(a)]

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UNIT-7: DIFFRACTION OF LIGHT WAVES AND

FRESNEL DIFFRACTION
CONTANTS
7.1 Introduction

7.2 Objectives

7.3 Diffraction of Light

7.4 Difference between Interference and Diffraction

7.5 Fresnel and Fraunhofer Classes of Diffraction

7.6 Fresnel’s Half Period Zones

7.6.1 Construction of Zones

7.6.2 Radii And Area of Zones

7.6.3. Resultant Amplitude at Point P

7.7. Rectilinear Propagation of Light

7.8 Zone Plate

7.8.1 Construction and Theory of Zone Plate

7.8.2 Action of a Zone Plate

7.8.3 Multiple Foci of Zone Plate

7.8.4 Comparison of Zone plate and Lens

7.9 Diffraction at a Straight Edge

7.9.1 Theoretical Analysis

7.9.2 Positions of Maximum and Minimum Intensities

7.9.3 Intensities at various positions

7.10 Summary

7.11Glossary

7.12 Terminal Questions

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7.13 Objective Type Questions

7.14 Answers/Hints

7.14.1 Self Assessment Questions

7.14.2 Terminal Questions

7.14.3 Objective Type Questions

7.15 References

7.15 Suggested Readings

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7.1 INTRODUCTION
In the preceding units we have read, that the interference phenomenon arises when two
or more coherent light beams, obtained either by division of wavefront or by division of
amplitude, meet each other. In this unit we shall discuss the interference effect of secondary
wavelets originating from the same wavefront or from single aperture. This is called
diffraction. The wave nature of light was further confirmed by the phenomenon of
diffraction.

Diffraction refers to various phenomena which occur when a wave encounters an

obstacle or a slit (or aperture). Since at the atomic level, physical objects have wave-like
properties, they can also exhibit diffraction effects. The diffraction of light was first observed
and characterized by an italian mathematician Francesco Maria Grimaldi. The word
diffraction originated from Latin word ‘diffractus’ which means ‘to break into pieces’. Thus
he referred this phenomenon as breaking up of light into different directions. Isaac Newton
attributed them to inflexion of light rays. James Gregory used a bird feather and observed the
diffraction patterns. This was effectively the first diffraction grating to be discovered.
Augustin-Jean Fresnel did more studies and calculations of diffraction and thereby gave great
support to the wave theory of light that had been advanced by Christiaan Huygens.

The effects of diffraction are often seen in everyday life. For example, the closely
spaced tracks on a CD or DVD act as a diffraction grating for incident light and form a
rainbow like pattern when seen at it. The hologram on a credit card is another example.
Almost the same colourful pattern is formed due to the diffraction of light. A bright ring
around a bright light source like the sun or the moon is because of the diffraction in the
atmosphere by small particles.

7.2 OBJECTIVES
Upon completion of this unit you will be able to

 State the diffraction of light and the necessary conditions for producing this effect
 Differentiate the phenomena interference and diffraction
 Describe the Fresnel and Fraunhofer classes of diffraction
 Define the construction of half period zones and compute their radii and area
 Find the resultant amplitude at a point on the screen due to a number of zones
 Prove that the light propagate along a rectilinear path
 Describe a zone plate, its construction, its action and theory.
 List the similarities and dissimilarities between a zone plate and a lens
 To understand ‘what kind of diffraction effect is produced by a sharp straight edge at
various points on the screen’
 Find the expressions for the positions of maxima and minima, and the intensity
distribution due to diffraction effect produced by a sharp edge

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7.3 DIFFRACTION OF LIGHT

As per the rules of geometric optics, the light should caste a well defined and distinct
shadow of an object placed in its path. If the direction of incidence of light is perpendicular to
the length of obstacle then due to its rectilinear propagation, the size of the image should be
equal to the size of the object (fig. 7.1). No light should reach into the regions of shadow. The
same thing happens with aperture. Light enters from the open region of aperture and reaches
to the screen (fig.7.2). When the direction of incidence is not normal to length of obstacle (or
aperture), the size of image (or shadow) will be different from that of obstacle or aperture
(fig.7.3 and 7.4).

Figure 7.1 Figure 7.2

Figure 7.3 Figure 7.4

A very close and careful observation of light distribution reveals that there are dark and
bright fringes near the edges. As the size of the aperture is decreased the fringes become
more and more distinct. When the size of aperture becomes comparable to the wavelength of
incident light the fringes become broad and practically cover the entire shadow region, so
instead of a sharp shadow we obtain bright and dark fringes on the screen.

Figure 7.5
In simple language we can say that ‘when the size of the opaque obstacle (or aperture)
is small enough and is comparable to the wavelength of incident light, the light bends round
the corners’. If the opening is much larger than the light's wavelength, the bending will be
almost unnoticeable. The phenomenon of bending of light round the corner or edge and
spreading into the geometrical shadow region of the obstacle (or aperture), placed in its path,

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is known as diffraction. The bending of light for a small slit is shown in figure 7.5. The
formation of alternate bright and dark fringes, by the redistribution of light intensity, is called
the diffraction pattern. The amount of bending depends on the relative size of the wavelength
of light to the size of the opening.

Figure 7.6
Dominique Arago placed a small circular disc in between a point light source and
screen and obtained almost a regular pattern of alternate dark and bright rings. There was a
bright circular spot at the centre of this pattern. The formation of this kind of diffraction
pattern could not be explained on the basis of rectilinear propagation of light. Thus wave
theory of light was used to explain the bending of light into the regions of geometrical
shadow. One such pattern is depicted in figure 7.6.

Self Assessment Question (SAQ) 1: What do you understand by the term diffraction? What
is the condition of obtaining observable diffraction pattern?

7.4 DIFFERENCE BETWEEN INTERFERENCE AND

DIFFRACTION
(i) The interference occurs between two separate wavefronts originating from two coherent
sources while in the phenomenon of diffraction the interference occurs between the
secondary wavelets originating from different points of the exposed part of same wavefront.

(ii) In the interference pattern all the maxima are of the same intensity but in diffraction
pattern the intensity of central maximum is maximum and goes on decreasing as we move
away.

(iii) The interference fringes are usually equally spaced while the diffraction fringes are never
equally spaced.

(iv) In interference the minima are perfectly dark but it is not so in diffraction pattern.

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7.5 FRESNEL AND FRAUNHOFER CLASSES OF

DIFFRACTION
The diffraction phenomenon is usually divided into two classes; the Fresnel diffraction
and Fraunhofer diffraction. Following are the main differences between these two types of
diffractions.

(i) In Fresnel diffraction either the source of light or the screen or both are in general at
finite distance from the diffracting element (obstacle or aperture) whereas in Fraunhofer
diffraction both the source of light and the screen are at infinite distance from diffracting
element.
(ii) In Fresnel diffraction no lenses are used for rendering the rays parallel or convergent
therefore the incident wavefront is divergent either spherical or cylindrical. In Fraunhofer
class of diffraction generally two convergent lenses are used; one to make the incoming light
parallel and other to focus the parallel diffracted rays on the screen. The incident wavefront
is, therefore, plane.
(iii) In Fresnel diffraction the phase of secondary wavelets is not the same at all points in the
plane of aperture while converse is true for Fraunhofer diffraction.
(iv) Depending on the number of Fresnel’s zones formed, the centre of the diffraction pattern
may be either dark or bright in Fresnel diffraction but in Fraunhofer diffraction it is always
bright for all paths parallel to the axis of lens.
(v) In Fresnel class of diffraction the lateral distances are important while in Fraunhofer
diffraction the angular inclination plays important role in the formation of diffraction pattern.
(vi) In Fresnel diffraction the diffraction pattern formed is a projection of diffracting element
modified by the diffracting effects and the geometry of the source and in Fraunhofer
diffraction the diffraction pattern is the image of the source modified by the diffraction at
diffracting element.

SAQ 2: How will you differentiate the interference and diffraction phenomenon?
SAQ 3: Write any four differences between Fresnel and Fraunhofer class of diffraction.

7.6 FRESNEL’S HALF PERIOD ZONES

According to Huygens principle each point on a wavefront acts as a source of secondary
disturbance. When a wavefront is made to incident on a slit, most of it is obstructed by the
slit. The small portion of the wavefront passed through the slit is, thus, equivalent to a string
of coherent point sources. The intensity at any point on the screen may be obtained by
suitably summing the intensities of wavelets originating from those point sources at the slit
and superposing at that point of screen. Thus diffraction pattern is formed at screen due to the
interference of secondary wavelets.
Since the coherent sources are located at different distances from any point on the
screen, the waves reach that point with differing phases. Their superposition produces
interference pattern with maxima and minima formation. Therefore, the diffraction of light is

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due to the superposition of waves from coherent sources of the same wavefront after the
wavefront is obstructed by obstacle or aperture.
7.6.1. Construction of Zones
For the qualitative understanding of the diffraction pattern, Fresnel introduced the idea
of half period zones. The wave-front originated from the source and striking the obstacle or
aperture is divided into a number of the circular and the concentric zones. Zone is the small
area on the plane wave-front with reference to the point of the observation such that all the
waves from the area reach the point without any path difference. The paths of light rays from
the successive zones differ by λ/2. Since path difference of λ/2 corresponds to half time
period, these zones are known as half period zones.

Figure 7.7
In order to understand the construction of half period zones taking a plane wavefront
AAʹ and droping a perpendicular PO on the wavefront from an external point P. If the distance
PO is b then taking P as a centre draw spheres of radii b+λ/2, b+2(λ/2), b+3(λ/2) etc. The
spheres will cut the wavefront AAʹ in circles of radii OM1, OM2, OM3 etc as shown in figure
7.7. The annular regions between two consecutive circles are called half period zones, e.g.,
the annular region between (n-1)th circle and nth circle is called the nth half period zone.

7.6.2. Radii and Area of Zones

From simple geometry the radius of nth such circle, OMn, can be written as
1/2
𝜆 2
𝑂𝑀𝑛 = 𝑟𝑛 = [(𝑏 + 𝑛 ) − (𝑏 2 )]
2
𝑛𝜆 1/2
= √𝑛𝜆𝑏 [1 + 4𝑏 ] = √𝑛𝜆𝑏 ....... (7.1)

Here we have assumed b>> λ, which is true in most of the experiments using visible light.
We have also assumed here that n is not a very large number. From expression given by
equation (7.1), it is clear that the radii of half period zones are proportional to the square roots
of natural numbers. Therefore, the radii of first, second, third etc. half period zones are √𝜆𝑏,
√2𝜆𝑏, √3𝜆𝑏 etc
With the help of equation (7.1), the area of nth half period zone is given by

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2
𝐴𝑛 = 𝜋𝑟𝑛2 − 𝜋𝑟𝑛−1 = 𝜋[𝑛𝜆𝑏 − (𝑛 − 1)𝜆𝑏] = 𝜋𝜆𝑏 ....... (7.2)
Thus for b>> λ and n not very large, the areas of half period zones are independent of n and
are approximately equal for fixed value of λ and b. The area of the zone may be varied by
varying the wavelength of light used and the distance of the point from the wavefront.

Example 7.1. A screen is placed at a distance of 100 cm from a circular hole illuminated by a
parallel beam of light of wavelength 6400 Å. Compute the radius of fourth half period zone.
Solution: If b is the distance of the point of consideration from the pole on the wavefront
then the radii of the spheres whose sections cut by by the wavefront from the half period
𝜆 2𝜆 3𝜆
zones are 𝑏 + 2 , 𝑏 + ,𝑏 + etc. Hence the radius of fourth half period zone is given by
2 2

4𝜆 2
𝑟4 = √(𝑏 + ) − 𝑏 2 = √(4𝜆2 + 4𝑏𝜆) ≅ √4𝑏𝜆. Because 4𝑏𝜆 ≫ 𝜆2
2

It is given that, b = 100 cm and 𝜆 = 6400 Å = 6400 × 10−8 𝑐𝑚

∴ 𝑟4 = √4 × 100 × 6400 × 10−8 = 0.16 𝑐𝑚

Example 7.2: A plane wavefront of light of wavelength 1000 Å is allowed to pass through an
aperture and a diffraction pattern is obtained on the screen placed at a distance of 1m from
aperture find the radius and area of 1000th half period zone.
Solution: Given that λ = 1000x10-10 m = 10-7 m, b = 1m and n =1000
We know that the radius of nth zone is given by, rn = √𝑛𝑏𝜆

∴ 𝑟1000 = √1000 × 1 × 10−7 = 10−2 𝑚 = 1.0 𝑐𝑚

The area of zone = bλ = 3.14x1x10-7 = 3.14x10-7 m2
Example 7.3: A light of wavelength 5x10-7 m is made to incident on a hole. Calculate the
number of half period zones lying within the hole with respect to a point at a distance of 1.0
m from the hole if the radius of hole is (i) 10-3 m and (ii) 10-2 m.
Solution: It is given that λ = 5x10-7 m, b = 1 m. If An is the area of hole of radius rn
containing n-half period zones each of area bλ then, we have, An = rn2 = n. bλ
(i) For rn = 10-3 m,
Substituting in the above equation, we get,
 x (10-3)2 = n x  x 1 x 5 x 10-7
10−6
∴ 𝑛= =2
5 × 10−7
(ii) For rn = 10-2 m
 x (10-2)2 = n x  x 1 x 5 x 10-7

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10−4
∴ 𝑛= = 200
5 × 10−7

SAQ 4: The radius and area of nth zone are 1.0 cm and 3.14x10-7 m2. Find the value of n.
SAQ 5: Light of 5000 Å is passed through a hole and two half period zones are formed with
respect to a point at a distance of 1.0 m from the hole. Calculate the diameter of the hole.

7.6.3. Resultant Amplitude at Point P

According to Fresnel the resultant amplitude at any point due to whole of the wavefront
will be the combined effect of all the zones, while the amplitude produced by a particular
zone is proportional to the area of the zone and inversely proportional to the distance of the
zone from the point of consideration, P. This amplitude also varies with obliquity factor
1
(1 + 𝑐𝑜𝑠𝜃). Where 𝜃 is the angle between the normal PO to the wavefront and the line QP.
2
Thus if un represents the amplitude produced by the secondary wavelets emanating from the
nth zone then we can write
𝐴 (1+𝑐𝑜𝑠𝜃𝑛 )
𝑢𝑛 = (𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡) × 𝑄 𝑛𝑃 × ....... (7.3)
𝑛 2

Figure 7.8
th
Where 𝜃𝑛 is the value of 𝜃 for n zone. If we take infinitesimal areas around point Qn in
the n half period zone and around a corresponding similar point Qn-1 in (n-1)th half period
th

zone as shown in the figure 7.8 such that

Qn P - Qn-1 P = λ/2 ....... (7.4)
This path difference of λ/2 corresponds to a phase difference of 𝜋. Although the areas of the
zones are almost the same but the distance of the zone from point P and the value of 𝜃
increases as we move from lower to higher n. The amplitudes u1, u2, u3 etc. of 1st, 2nd, 3rd etc.
zones at point P will be, therefore, in gradually decreasing order as shown in figure 7.9. The
opposite directions of alternate amplitudes correspond to the phase change of 𝜋 between
consecutive zones.
Thus the resultant amplitude at P can be written as

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up = u1 - u2 + u3 - u4 + ………+ (-1)n+1un ....... (7.5)

The positive and negative signs on the right hand side between alternate terms of this
equation may be ascribed to the fact that the disturbances produced by two consecutive zones
at P will be out of phase by 𝜋 radians.
As the disturbances at P due to various zones are of gradually decreasing magnitudes, the
amplitude due to any zone may be taken approximately equal to the average of the
amplitudes due to the preceding zone and the succeeding zone. That is, we can take
𝑢1 +𝑢3 𝑢3 +𝑢5
𝑢2 = , 𝑢4 = etc. ....... (7.6)
2 2

Figure 7.9

In equation (7.5), the last term on right hand side will be positive if n is odd and negative if it
is even. We can rewrite equation (7.5) as
𝑢1 𝑢 𝑢3 𝑢 𝑢5
𝑢𝑝 = + ( 21 − 𝑢2 + ) + ( 23 − 𝑢4 + )+ … ....... (7.7)
2 2 2

𝑢1 𝑢 𝑢3 𝑢𝑛−2 𝑢𝑛 𝑢𝑛
Thus if n is odd we have, 𝑢𝑝 = + ( 21 − 𝑢2 + ) + … …+ ( − 𝑢𝑛−1 + )+
2 2 2 2 2

𝑢1 𝑢𝑛
Using equation (7.6), we get, 𝑢𝑝 = + ....... (7.8)
2 2

And if n is even then,

𝑢1 𝑢 𝑢3 𝑢𝑛−3 𝑢𝑛−1 𝑢𝑛−1
𝑢𝑝 = + ( 21 − 𝑢2 + ) + … …+ ( − 𝑢𝑛−2 + )+ − 𝑢𝑛
2 2 2 2 2

𝑢1 𝑢𝑛−1
Using equation (7.6), we have, 𝑢𝑝 = + − 𝑢𝑛 ....... (7.9)
2 2

If the number of half period zones formed is large enough then due to gradually decreasing
amplitudes of zones, the values of un and un-1 may be neglected as compared to u1, and
therefore we can write
𝑢1
𝑢𝑝 ≅ ....... (7.10)
2

And the intensity at point P, therefore, may be given by

𝑢12
𝐼𝑝 ≅ 𝑢𝑝2 = ....... (7.11)
4

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Thus the resultant amplitude produced by whole of the wavefront is equal to one half of that
produced by the first zone and the intensity due to the entire wavefront is the one fourth of
that by the first zone.

Example 7.4: A plane wavefront of light of wavelength 5x10-5 cm falls on a circular hole and
is received at a point 200 cm away from that hole. Calculate the radius of the hole so that the
amplitude of light on the screen is two times the amplitude in the absence of hole.

Solution: It is given that λ = 5x10-5cm = 5x10-7 m and b = 200 cm = 2.0 m

We know that the amplitude due to the whole wavefront is only half to that due to first half
period zone, therefore

Radius of hole = Radius of first half period zone = √𝑏𝜆 = √(2.0 × 5 × 10−7 ) = 10−3 𝑚 =
1.0 𝑚𝑚

SAQ 6: The radius of an opening is 4.47x10-2 cm. The light of wavelength λ is passed
through that opening and collected at a distance of 40 cm from opening. Calculate the
wavelength of light so that the intensity of light on the screen is four times the intensity in the
absence of the opening.

7.7. RECTILINEAR PROPAGATION OF LIGHT

With the help of the theory discussed so far we can explain the rectilinear propagation
of light. Suppose a plane wavefront of monochromatic light is made to incident on a screen
with square aperture ABCD and whole of the wavefront except ABCD portion is blocked by
the screen as shown in the figure 7.10. Let P be a point at which the intensity of the light is
required and its pole O with respect to the aperture ABCD is well inside from the edges.
Taking O as centre if we draw the half period zones in the incident wavefront then the
number of the wavefronts will be quite large before they intersect the edges AB, BC, CD, and
DA. Thus practically all the effective zones are exposed and the resultant amplitude at P due
to aperture ABCD is given by equation (7.10). This amplitude is equal to the one half that due
to the first zone and since the areas of these zones are extremely small, we can consider the
light to be travelling along a straight line along OP. This condition is the same as if the screen
with square aperture ABCD was removed.

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Figure 7.10

The poles O1 and O2 of the points like P1 and P2 on the screen lie very close to edges of
the aperture ABCD. If we draw the half period zones around these poles then some of the
zones are obstructed and some are exposed. Thus there will be neither uniform illumination
nor complete darkness at points P1 and P2. For the points near the edges the light, therefore,
enters into the geometrical shadow region. The point P3 is well inside the geometrical shadow
region and its pole is O3. Since the amplitude at a point due to a zone decreases on increasing
its order, almost all the effective zones around O3 are cut off. The amplitude reaching at P3 is
nearly zero and there is a complete darkness. This is possible only when light travels along a
straight line.

From the above mentioned facts this may be concluded that there is almost uniform
illumination at the points whose poles lie well inside the edges of the aperture and complete
darkness at the points whose poles lie well outside the edges. This strongly supports the
rectilinear propagation of light. There is a slight deviation from the rectilinear path for the
points whose poles lie very close to the edges. However due to very small value of the
wavelength of light this region is very small as compared to whole of the aperture. Thus as a
whole the propagation of the light may be considered along a rectilinear path.

7.8. ZONE PLATE

A zone plate is a device used to focus light; however zone plates use diffraction instead
of refraction or reflection as in case of lenses and curved mirrors. It is a specially designed
diffraction screen consisting of a large number of half period zones. In the honor of Augustin-
Jean Fresnel they are sometimes called Fresnel zone plates. It is constructed in such a way
that every alternate zone blocks the light incident on it. In other words we can say that it
consists of alternate opaque and transparent set of radially symmetric rings (zones).

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Figure 7.11 Figure 7.12

The zones can be spaced so that the diffracted light constructively interferes at the
desired focus. The light may be cut off either by even numbered zones or by odd numbered
zones. When the light is obstructed by even numbered zones the plate is known as positive
zone plate and when obstructed by odd numbered zones it is called negative zone plate. These
two kinds of zone plates are shown in figures 7.11 and 7.12.

7.8.1. Construction and Theory of Zone Plate

From equation (7.1) of section 7.6.2, it is evident that the radii of half period zones are
proportional to square roots of natural numbers. Thus to construct a zone plate, we draw the
concentric circles of the radii proportional to square roots of natural numbers on a white
paper. The alternate regions between the circles are painted black. If the odd numbered zones
are painted black then drawings appears like figure 7.12 and if even numbered zones are
covered with black ink then the drawing looks like figure 7.11. Suppose the drawing
resembles with figure 7.11. If we take a reduced photograph of it then the developed negative
resembles with figure 7.12. This negative is then used as a zone plate.

Figure 7.13

If a beam of light is made to incident on such a zone plate normally and a screen is
placed on the other side of this plate to get an image then the maximum brightness is obtained
at a particular point of the screen. Suppose this point is P at a distance of b units from the
zone plate as shown in figure 7.13. Only upper half portion of the zone plate is shown in this

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figure. If λ is the wavelength of light used then radius of the first zone (OM1=r1), second zone
(OM2=r2) etc are given by 𝑟1 = √𝑏𝜆 and 𝑟2 = √2𝑏𝜆 etc.

The general expression for radius may be written as

𝑟𝑛2
𝑟𝑛 = √𝑛𝑏𝜆 or 𝑏 = ....... (7.12)
𝑛𝜆

Since the wavelength of light has a small value, the sizes of the zones are usually very
small as compared to the distance of the light source from the zone plate. Hence OM1, OM2,
OM3 etc are extremely small as compared to distance a (source S to zone plate AB
separation). But to make the points M1, M2, M3 etc distinct and to show the complete figure
the distances are not taken in this ratio in figure 7.14. Because of this reason the incident
wavefront may be taken as a plane wavefront.

Figure 7.14
Now suppose even numbered zones are opaque to incident light then from equation
(7.5), the resultant amplitude reaching at P may be written as (n is odd)

up = u1 + u3 + u5 +……… + un ....... (7.13)

In this case if all the zones are transparent to light then from equation (7.5), the resultant
amplitude at P is given by

up = u1 - u2 + u3 - u4 + ………+ un ....... (7.14)

For large value of n, from equation (7.10), we have,

𝑢1
up = ....... (7.15)
2

If we compare the values of the resultant amplitudes from equations (7.13) and (7.15),
we find that, when the even numbered zones are opaque the intensity at point P is much
greater than that when all the zones are transparent to incident light. Again from the above
discussion we can state that a zone plate behaves like a converging lens. The focal length of
the zone plate may be given by

𝑛 𝑟2
𝑓𝑛 = 𝑏 = 𝑛𝜆 ....... (7.16)

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Therefore, the focal length of a zone plate varies with the wavelength of incident light
that is why it is called a multi foci zone plate. For this reason if white light is made to
incident on a zone plate different colours come to focus on screen at different points and it
shows chromatic aberration.

7.8.2. Action of a Zone Plate

Refer to figure 7.14; AB is the section of zone plate perpendicular to the plane of paper,
S is the point light source at a distance a from zone plate and point P is on the screen placed
at a distance b from the zone plate. As compared to the radii of zones, the distance of source
from the zone plate is extremely large and therefore we can take approximation as SO ≈
SM1 ≈ SM2…… = a. The position of the screen is chosen such that the light rays reaching at
P from successive zones have a path difference of 𝜆/2. We can write

SO + OP = a + b ....... (7.17)

SM1 + M1P ≈ SO + (OP + 𝜆/2) = a + b + 𝜆/2 ....... (7.18)

Similarly, SM2 + M2P = a + b + 2 𝜆/2 ....... (7.19)

Now from right angle triangle ∆SOM1, we have,

𝑟1 2 1/2
(SM1)2 = (SO)2 + (M1O)2 or SM1 = (a2 + r12)1/2 = a(1+ )
𝑎2

Since a>>r1, expanding above and neglecting higher order terms, we get,

𝑟1 2 𝑟1 2
SM1 = a(1+ )=(a+ ) ....... (7.20)
2𝑎2 2𝑎

Proceeding in a similar way we can obtain,

𝑟1 2
M1P = (b+ ) ....... (7.21)
2𝑏

Substituting values of SM1 and M1P from equations (7.20) and (7.21) in the left hand side of
equation (7.18), we get,

𝑟 2
1 1 𝑟 2
(a+ 2𝑎 )+ (b+ 2𝑏 ) = a + b + 𝜆/2

1 1
or 𝑟12 ( + ) = 𝜆
𝑎 𝑏

1 1
From equation (7.19), we have, 𝑟22 ( + ) = 2𝜆
𝑎 𝑏

Proceeding similarly for higher order zones, we obtain

1 1
𝑟𝑛2 ( + ) = 𝑛𝜆 ....... (7.22)
𝑎 𝑏

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Now comparing the zone plate with converging device like convex lens and using
similar sign convention for the distances of the object and image from the lens, the equation
(7.22) may be modified as
1 1 𝑛𝜆
( − )= ....... (7.23)
𝑏 𝑎 𝑟𝑛2

1 1 1
This equation is similar to the lens equation ( − ) = . Thus a zone plate behaves like a
𝑣 𝑢 𝑓
𝑟𝑛2
converging lens of focal length, fn = . Thus the focal length of zone plate depends on the
𝑛𝜆
number of zones and the wavelength of light used.

7.8.3. Multiple Foci of Zone Plate

A zone plate has a multiple foci. In order to prove this, taking an object at infinity, i.e.
at a = ∞ in equation (7.23), we get, 𝑟𝑛2 = 𝑏𝑛𝜆 and therefore, the area of nth zone is given by
2
𝐴𝑛 = 𝜋𝑟𝑛2 − 𝜋𝑟𝑛−1 = 𝜋[𝑛𝜆𝑏 − (𝑛 − 1)𝜆𝑏] = 𝜋𝜆𝑏 ....... (7.24)

Since the object is at infinity, the light rays will be parallel to principal axis and the image
𝑟𝑛2
will be formed at the principal focus at a distance 𝑏 = from the zone plate.
𝑛𝜆

If we take a point P3 at a distance b/3 from the zone plate somewhere in between O and
P then the area of each half period zone with respect to P3 will now becomes 𝜋𝜆(b/3), that is,
one third to the previous case. Thus each zone, in this case, can be assumed to contain three
half period elements corresponding to P3. If the amplitude due to these elements are
represented by m1, m2, m3 etc. then the first zone (amplitude u1) will consist of the first three
elements (amplitudes m1, m2 and m3), second zone (amplitude u2) will consist of the next
three elements (amplitudes m4, m5 and m6) etc. Again similar to half period zones there will
be a phase difference of 𝜋 between the successive elements. Thus while adding the
amplitudes; the m1 will be taken positive, m2 as negative etc. Substituting the values of u1, u2,
u3 etc. with m1, m2, m3 etc., equation (7.13) changes to

𝑢𝑝3 = (𝑚1 − 𝑚2 + 𝑚3 ) + (𝑚7 − 𝑚8 + 𝑚9 ) + (𝑚13 − 𝑚14 + 𝑚15 ) + … … …

𝑚1 +𝑚3 𝑚7 +𝑚9 𝑚13 +𝑚15

= (𝑚1 − 2
+ 𝑚3 ) + (𝑚7 − 2
+ 𝑚9 ) + (𝑚13 − 2
+ 𝑚15 ) + ……

1
= 2 (𝑚1 + 𝑚3 + 𝑚7 + 𝑚9 + 𝑚13 + 𝑚15 + … … ) ....... (7.25)

Here it should be noted that each of the amplitudes m1, m2, m3 etc is one third of u1, u2, u3 etc.

If we compare the equations (7.13) and (7.25), we find that the intensity reaching at P3 is
sufficiently large but is less than that reaching at P. Thus the image of S is also formed at P3
and therefore, it may be taken as the second focal point. The second focal length is given by

𝑟𝑛2
f3 = ....... (7.26)
3𝑛𝜆

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Similarly the images of S can be formed on points P5, P7, P9 etc. but with decreasing
𝑟𝑛2 𝑟𝑛2 𝑟𝑛2
intensity. The distance of these points from the zone plate are , , etc. Thus a zone
5𝑛𝜆 7𝑛𝜆 9𝑛𝜆
𝑟𝑛2 𝑟𝑛2 𝑓 𝑟𝑛2 𝑓1
plate has multiple foci given by f1= , f3 = = 1 , f5 = = etc.
𝑛𝜆 3𝑛𝜆 3 5𝑛𝜆 5

7.8.4. Comparison of Zone Plate and Lens

Some of the features of zone plate are similar to a lens and in some it has dissimilarity.
The following are the resemblance and differences between the two.

(i) Similar to a lens, a zone plate forms an image of an object placed on its axis. The same
sign convention is used while representing the distance of the object and image in both
the cases.
(ii) The focal length formula in terms of distance of object and image for zone plate is
1 1 1 1 1 1
(𝑏 − 𝑎) = 𝑓 and for the convex lens is (𝑣 − 𝑢) = 𝑓, which are identical.
(iii)The image due to a convex lens is more intense as compared to that due to a zone plate.
1 1 1
(iv) The convex lens has a focal length given by = (𝜇 − 1) (𝑅 − 𝑅 ) which depends on
𝑓 1 2
wavelength (refractive index varies with wavelength) and the focal length of zone plate fn
𝑟𝑛2
= also varies with wavelength. Hence both exhibit chromatic aberration.The focal
𝑛𝜆
length of a zone plate is inversely proportional to the wavelength hence red rays come to
focus at a smaller distance from the zone plate than violet rays. The reverse is true for
convex lens. Thus fv > fr in zone plate while fr > fv in lens. The order of colours in
chromatic aberration is therefore opposite in the two cases.
(v) A convex lens has one focal length for a fixed wavelength while a zone plate has a
number of foci at which the images of diminishing intensities are formed.

Example 7.5: Calculate the focal length of the zone plate and the radius of the first zone
when a point source of light of wavelength 6x10-7 m is placed at a distance of 100 cm from a
zone plate. Its image is formed at a distance of 200 cm on the other side.
1 1 𝑛𝜆 1
Solution: For a zone plate we have. (𝑎 + 𝑏) = = 𝑓 . Given that, a = 1 m, b = 2 m and 𝜆 =
𝑟𝑛2
1 1 1 3 2
6x10-7 m. Thus 𝑓 = 1 + 2 = 2 or 𝑓 = 3 𝑚.

𝑟 2 2
For first zone, = 1×1 𝜆 , thus 𝑟12 = 𝑓 × 𝜆 = 3 × 6x10−7 or r1 = 6.32x10-4 m.

Example 7.6: A plane wavefront of light of wavelength 5x10-5 cm fall on a zone plate. The
radius of the first half period zone is 0.5 mm. Where should a screen be placed so that the
light is focused at the brightest spot?

Solution: We know that the brightest spot is formed at the first focus of the plate, i.e. at f1.
Given that r1 = 0.5 mm = 5x10-2 cm and 𝜆 = 5 × 10−5 𝑐𝑚

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2
𝑟2𝑛 𝑟1 2 (5×10−2 )
𝑓𝑛 = , Therefore, 𝑓1 = = = 50 𝑐𝑚
𝑛𝜆 𝜆 5×10−5

Example 7.7: Calculate the radius of 10th zone in a zone plate of focal length 0.2 m for light
of wavelength 5x10-7 m.
𝑟𝑛2 10 𝑟 2
Solution: From 𝑓𝑛 = , we have, 0.2 = 10×5×10−7
or r10 = 0.01 m =1.0 cm
𝑛𝜆

Example 7.8: Calculate the radii of first three clear elements of a zone plate which is
designed to bring a parallel light of wavelength 6000 Å to its first focus at a distance of two
meters.

Solution: It is Given that, f = b =2.0 m, 𝜆 = 6000Å = 6 × 10−7 𝑚.

If odd number half period zones are clear (transparent) then taking n=1, 3, 5 in the expression
𝑟𝑛 = √𝑛𝑏𝜆 , we get 𝑟1 = √𝑓𝜆 = √6 × 10−7 × 2 = 10.95 × 10−4 m.

𝑟3 = √3𝑓𝜆 = √3 × 6 × 10−7 × 2 = 1.9 × 10−3 m.

𝑟5 = √5𝑓𝜆 = √5 × 6 × 10−7 × 2 = 2.45 × 10−3 m.

SAQ 7: What is the radius of first zone in a zone plate of primary focal length 20 cm for a
light of wavelength 5000 Å.

SAQ 8: If the focal length of zone plate is 1 m for light of wavelength 6.0x10-7 m. What will
be its focal length for the wavelength 5x10-7m.

7.9. DIFFRACTION AT A STRAIGHT EDGE

To show the diffraction effect of a straight edge, the light from a monochromatic light
source S is passed through a narrow slit AB and a sharp edge of an opaque obstacle like blade
is placed in its path as shown in figure 7.15. The slit, opaque obstacle and screen P’P’’ are
parallel to each other and perpendicular to the plane of the paper. The sharp edge is placed in
such a way that the line joining the slit to edge O when reproduced meet the screen at P and
OP is normal to screen.

In the absence of diffraction of light due to sharp straight edge there should be a
uniform illumination above point P and complete darkness below it. As we move towards P’,
unequally spaced bright and dark bands are obtained near P. On further moving towards P’,
i.e. with increasing value of x the intensity reaches a steady value Io resulting a uniform

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illumination. Because of the diffraction effect, the light enters to a certain distance below P
(towards P’’) in the geometrical shadow region.

Figure 7.15

In this region the intensity of light decreases to zero very rapidly without forming
maxima and minima in a small but finite distance as shown in intensity distribution curve of
figure 7.16. If the average intensity is Io then at point P on the screen (corresponding to the
edge) it reduces to Io/4. This all is due to the diffraction of light produced by sharp straight
edge.

Figure 7.16
7.9.1. Theoretical Analysis
Refer to figure 7.15. Suppose we want to find the resultant at any point, say P’, on the
screen. The pole of the wavefront YY’ with respect to point P’ will be O’. With P’ as centre if
we draw the circles of radii O’P’+𝜆/2, O’P’+2𝜆/2, O’P’+3𝜆/2 etc, the wavefront is divided
into half period strips. Thus for point P’, the wavefront is divided in two similar parts; one
above point O’ another below it. The light from entire upper half portion of the wavefront
reaches to P’. The resultant due to this will be equivalent to one half to that due to first half
period strip, i.e. m1/2. Now the number of half period strips within the lower half portion of
the wavefront, i.e. O'O will depend on the position of the point P’ on the screen. Suppose the
lower half portion contains only one half period strip then the amplitude due to it at P’ will be
only m1 and therefore, the total amplitude at P’ by whole of the exposed wavefront is given
𝑚1
by +𝑚1. This is the position of first maximum.
2

If O’O contains two, three, four etc half period strips then the resultant amplitude at p’
𝑚 𝑚 𝑚
is given by 21 + 𝑚1 − 𝑚2 , 21 + 𝑚1 − 𝑚2 + 𝑚3 , 21 + 𝑚1 − 𝑚2 + 𝑚3 − 𝑚4 etc. and the

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position of P’ gives the position of first minimum, position of second maximum and the
position of second minimum respectively. Thus at point P’, a maximum or a minimum is
formed according as O’O contains odd or even number of half period strips.

As we move away from P towards P’ alternate maxima and minima are obtained. From
the previous discussion we see that the amplitude or intensity of these maxima and minima
are comparable, hence the bands have a poor contrast. If the point of consideration is at a
sufficiently large distance from P then entire upper half and a large number of half period
strips of the lower half are exposed. The diffraction bands merge together to produce uniform
illumination. The resultant amplitude at the point of consideration, in this case, is therefore,
𝑚1 𝑚1
+ = 𝑚1 and the intensity is 𝑚1 2 .
2 2

7.9.2. Positions of Maximum and Minimum Intensities

In figure 7.15, the path difference between the rays O’P’ and OP’ is given by

∆ = OP’- O’P’= (𝑂𝑃2 + 𝑃𝑃′2 )1/2 − (𝑆𝑃′ − 𝑆𝑂′)=(𝑂𝑃2 + 𝑃𝑃′2 )1/2 − [{𝑆𝑃2 + 𝑃𝑃′2 }1/2 −
𝑆𝑂′]

= (𝑏 2 + 𝑥 2 )1/2 − [{(𝑎 + 𝑏)2 + 𝑥 2 }1/2 − 𝑎]

∵ YY’ is the spherical wavefront of the point light source S with S as a centre, thus SO’ = SO
= a, is the radius of the sphere.

In actual experimental set up we have, x<<b. Thus taking b out (common) from the first
term and (a+b) out from the second term on the right hand side of the above equation,
expanding the series and neglecting higher order terms, we obtain
𝑥2 𝑥2 𝑥2 𝑎
∆ = 𝑏 {1 + 2𝑏2 } − (𝑎 + 𝑏) {1 + 2(𝑎+𝑏)2 } + 𝑎 = . 𝑏(𝑎+𝑏) ....... (7.27)
2

Now if O’O contains an odd number of half period strips then a maximum will be formed at
point P’ and the path difference ∆, in this case, will be an odd number of half-wavelengths,
and vice-versa. Thus for maxima we have,
𝜆
∆ = (2𝑛 − 1) 2 ....... (7.28)

𝜆
For minima we have, ∆ = 2𝑛. 2 ....... (7.29)

On comparing equations (7.27) and (7.28), we get the position of nth maximum as

(2𝑛−1)(𝑎+𝑏)𝑏𝜆
𝑥𝑛 = √ 𝑎
= 𝐾√2𝑛 − 1 ....... (7.30)

(𝑎+𝑏)𝑏𝜆
Where, = √ , is a constant.
𝑎

Similarly the comparison of equations (7.27) and (7.29) gives the position of nth minimum as

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2𝑛(𝑎+𝑏)𝑏𝜆
𝑥𝑛 = √ 𝑎
= 𝐾√2𝑛 ....... (7.31)

From equation (7.30), we have, 𝑥1 = 𝐾, 𝑥2 = 𝐾√3, 𝑥3 = 𝐾√5 etc. Thus the separations
between successive maxima are 𝑥2 − 𝑥1 = 0.732𝐾, 𝑥3 − 𝑥2 = 0.504𝐾, 𝑥4 − 𝑥3 = 0.409 etc.
We see that with increasing order of maxima the separation between consecutive maxima
decreases and the fringes come closer. The same is true for minima.

7.9.3. Intensities at Various Positions

The intensity variation curve is shown in figure 7.16. Now we will find out the value of
intensity at some specific points.

(i) Intensity at the Edge of Geometrical Shadow

In figure 7.16 the edge of geometrical shadow is represented by P. The pole of this edge
at wavefront is point O, which is nothing but the edge of sharp obstacle. Thus with respect to
the edge of geometrical shadow region (point P), the incident wavefront can be divided in
two parts; one above point O (OY) and other below point O (OY’). The light from the entire
upper half portion of the wavefront reaches to point P while the light from the lower half
portion of the wavefront is completely cut off by sharp edge obstacle. The resultant amplitude
at P, in this case, is mP = m1- m2+ m3+ m4- ……… , which is m1/2. Thus the resultant
intensity at P is m12/4=Io/4. Where Io is the value of intensity at P in the absence of obstacle.

(ii) Intensity at a Point Inside the Geometrical Shadow

If the point of consideration is inside the geometrical shadow region then the pole of the
point will be below point O, i.e. in the wavefront region OY’. Suppose we take a point P’’
then its pole will be O’’. In this case the complete lower half portion and most of the upper
half portion of the wavefront is obstructed by the obstacle. Only a small part of the upper half
portion of the wavefront (OY) is exposed. As we move down gradually from point P inside
geometrical shadow, the first, the first two, the first three etc. half period strips of the upper
half of the wavefront are obstructed and the amplitudes are thus m2/2, m3/2, m4/2 etc.
respectively. The intensities, therefore, will be (m2/2)2, (m3/2)2, (m4/2)2 etc. respectively.
Since the amplitudes m1, m2, m3 etc. are in decreasing order of magnitude, the intensity
of light decreases rapidly as we move inside the geometrical shadow. This is because of the
fact that most of the effective half period strips of the upper half portion of wavefront are cut
off.

Example 7.9: A narrow slit illuminated by light of wavelength 4900 Å is placed at a distance
of 3m from a straight edge. If the distance between the straight edge and screen is 6 m,
calculate the distance between the first and fourth band.

2𝑛(𝑎+𝑏)𝑏𝜆 (𝑎+𝑏)𝑏𝜆
Solution: For minima we have, 𝑥𝑛 = √ 𝑎
= 𝐾√2𝑛, where 𝐾 = √
𝑎

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It is given that b = 6 m, a = 3 m, 𝜆 = 4.9x10-7 m.

(3+6)×6×4.9x10−7
Therefore, K =√ =2.97x10-3
3

For first minimum, x1 = 𝐾√2 = 2.97 × 10−3 × √2 = 4.20 × 10−3 𝑚

For fourth minimum, x4 = 𝐾√8 = 2.97 × 10−3 × √8 = 8.40 × 10−3

Separation between the two, x4 – x1 = (8.40 – 4.20) x 10-3= 4.20 × 10−3 𝑚

SAQ 9: In an experiment with straight edge diffraction, the slit to edge distance is 1.0 meter
and the edge to screen distance is 2.0 m. If 𝜆 = 6000 Å, calculate the position of the first
three maxima and their separation.

7.10. SUMMARY
In this unit you have studied that Huygens’s principal is the basic principle to explain
the diffraction phenomenon. Diffraction is mainly due to interference of the secondary
wavelets. Diffraction pattern is formed whenever a wave encounters an object or aperture, the
size of which is comparable to wavelength of light. To make the concept more clear the
difference between interference and diffraction, construction and theory of half period zones
and zone plate are explained. It is stated that for b>>λ the radii of half period zones are
proportional to square root of natural numbers and the zones have the same areas. The
expressions for radius and area are given by √𝑛𝜆𝑏 and 𝜋𝜆𝑏. If the incident wavefront contains
a large number of half period zones and all zones are exposed then the resultant amplitude at
a point on the screen will be equal to half of that due to first zone, i.e. u1/2. With the help of
zone theory it is proved that the light propagates along a rectilinear path. The zone plate may
1
be used as a focusing device and the focal length of it is given by the expression =
𝑓𝑛
1 1 𝑛𝜆 𝑟𝑛2 𝑟𝑛2 𝑟𝑛2
( − )= 2 . It is a multiple foci device having focal lengths , , etc. In some of
𝑏 𝑎 𝑟𝑛 𝑛𝜆 3𝑛𝜆 5𝑛𝜆
the features, the zone plate, resembles with a lens and has some dissimilarity.

The formation of diffraction pattern is explained by taking the obstacle in the form of a
sharp and straight edge. If almost all the wavefront is exposed, the amplitude produced at a
point on the screen is m1 and the intensity is m12. The maxima and minima formed are not
2𝑛(𝑎+𝑏)𝑏𝜆
equally spaced. Their position of maxima is given by 𝑥𝑛 = √ 𝑎
= 𝐾√2𝑛 and that of
(2𝑛−1)(𝑎+𝑏)𝑏𝜆
minima is given by 𝑥𝑛 = √ 𝑎
= 𝐾√2𝑛 − 1. If Io is the value of intensity at a point on
the screen in the absence of obstacle then Io/4 will be the intensity at the edge of geometrical

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shadow. If we move inside the geometrical shadow region the intensity decreases and
diminishes to zero rapidly.

7.11 GLOSSARY
Annular – ring-shaped, forming a ring.

Aperture – an opening, a gap or a space through which light passes in an optical or

photographic instrument.

Ascribe – attribute or impute, regard as belonging.

Attribute – ascribe to or regard as the effect of (a stated cause).

Convention – general agreement, esp. on social behaviour etc. by implicit consent of the
majority, a custom or customary practice esp. an artificial or formal one.

Converse – opposite, contrary, reverse.

Depict – to describe.

Distinct – not identical, separate, individual, different in kind or quality, unlike.

Emanate – issue, originate (from a source), proceed.

Evident – plain or obvious (visually or intellectually), manifest.

Illumination – an act to light up or to make bright.

Inflexion – the act or condition of inflecting or being inflected, an instance of this.

Lateral – of, at, towards, or from the side or sides, in direct line.

Monochromatic – light or other radiation of single wavelength, containing only one colour.

Obstruct – block up, make hard or impossible to pass along or through.

Opaque – not transmitting light, impenetrable to light.

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Rectilinear – bounded or characterized by straight lines, in or forming a straight line.

Render – cause to be or become, make.

Respectively – in the order mentioned, for each separately or in turn.

Reveal – display or show, allow to appear, disclose, divulge, betray.

Vary – undergo change (become or be different).

7.12 TERMINAL QUESTIONS

1. Calculate the radii and areas of the first two half period zones for a plane wavefront. The
point of observation is at a distance of 1.0 m from theb wavefront and wavelength of light is
4900 Å.

2. The diameter of the first ring of a zone plate is 1.1 mm. If plane waves (6000 Å) fall on the
plate, where should the screen be placed so that light is focused to a brightest spot?

3. A light of wavelength 5000 Å is allowed to fall on a zone plate for which the radius of the
first zone is 3x10-2 cm. Find the first three focal lengths for this zone plate.

4. Light of wavelength 5896 Å is made to incident on a zone plate placed at a distance of 150
cm from it. The image of the point source is obtained at a distance of 3 m on the other side.
What will be the power of equivalent lens which may replace the zone plate withought
disturbing the set up? Also calculate the radius of the first zone of the plate.

5. For axial point source for a zone plate, a series of images is obtained. If the sharpest image
is obtained at 30 cm and the next sharpest at 6 cm on the other side of the source, calculate
the distance of the source from the zone plate.

6. For a light of wavelength 4000Å, the brightest image is formed by a zone plate at a
distance of 20 cm for an object placed at a distance of 20 cm from it. Calculate the number of
Fresnel’s zones in a radius of 1 cm of that plate.

7. A point source of 𝜆 = 5.5 × 10−7 𝑚 is placed 2 meters away along the axis of a circular
aperture of radius 2 mm. On the other side a screen is moved along the axis from infinity to
closer distances. Calculate the first three positions where minima are observed.

8. A parallel beam of wavelength 6x10-7 m falls normally on a narrow circular aperture of

radius 0.9 mm. At what distance along the axis will the first maximum intensity be observed?

9. A straight edge is placed at a distance of 50 cm from a slit illuminated by monochromatic

light of wavelength 5000 Å. If the distance of the screen from the edge is 1.50 m, calculate
the positions of first, second, third and tenth bright fringe from the edge of the geometrical
shadow. Also find the separation between first-second and second-third bright fringes.

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7.13 OBJECTIVE TYPE QUESTIONS

Q1. The bending of light rays round the corners of an obstacle is called

(a) interference (b) polarization

(c) dispersion (d) diffraction

Q2. For obtaining the diffraction pattern the size of the obstacle should be

(a) 10 mm (b) 10-1 mm

(c) 10-4 mm (d) 0.1 cm

Q3. The phenomenon of diffraction was discovered by

(a) Francesco Maria Grimaldi (b) Isaac Newton

(c) Fraunhofer (d) Huygen

Q4. The tip of a needle does not give a sharp image on the screen because of the following

(a) reflection (b) diffraction

(c) polarization (d) refraction

Q5. Fresnel half period zones differ from each other by a phase difference of

(a) 2 (b) 

(c) /2 (d) /4

Q6. For a light of wavelength 5x10-7m, a zone plate of focal length 0.5 m is to be constructed.
The radius of first zone will be

(a) 0.25 cm (b) 2.5x10-2 cm

(c) 0.5 cm (d) 5x10-2 cm

Q7. The constant area of half period zone is given by

(a) 𝜋𝑏𝜆 (b) 𝜋𝑏/𝜆

(c) 𝜆/𝜋𝑏 (d) 2𝜆/𝜋𝑏

Q8. The first (principal) focal length of a zone plate has least value for the following colour

(a) red colour (b) green colour

(c) violet colour (d) yellow colour

Q9. The focal length of a zone plate is given by the expression

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𝑟 𝑟𝑛2
(a) 𝑛𝜆𝑛 (b) 𝜆
𝑛

𝑟2
𝑛 𝑟𝑛2
(c) 𝑛𝜆 (d) 𝑛
𝜆

Q10. A zone plate behaves like

(a) concave lens (b) convex lens

(c) plane mirror (d) glass plate

7.14 ANSWERS/HINTS
7.14.1 Self Assessment Questions
1. Refer article 7.3, 2. Refer article 7.4, 3. Refer article 7.5,

4. It is given that the radius of nth zone is given by rn = √𝑛𝑏𝜆 = 1.0 𝑐𝑚 = 10−2 𝑚 and the
area of zone, An = bλ = 3.14x10-7 m2
2
𝑟𝑛2 𝑛𝑏𝜆 𝑛 𝑟𝑛2 (10−2 )
Thus, = = or 𝑛 =  = 3.14 × 3.14×10−7 = 1000
𝐴𝑛 bλ  𝐴𝑛

5: It is given that λ = 5x10-7 m, b = 1 m and n = 2. If An is the area of hole of radius rn

containing n-half period zones each of area bλ then, we have, An = rn2 = n. bλ
Substituting the given values in the above equation, we get,
rn2 = 2 x  x 1 x 5 x 10-7 or rn = 10-3 m, thus diameter, dn = 2x10-3 m.
6: The intensity due to whole wavefront is only one fourth to that due to first half period
zone, therefore, Radius of opening = Radius of first half period zone = √𝑏𝜆 =
4.47x10−2 cm, b = 40 cm (given).
2 2
(4.47x10−2 ) (4.47x10−2 )
Thus, λ = = = 5 × 10−5 𝑐𝑚
𝑏 40

𝑟𝑛2
7: Hint: 𝑓𝑛 = , ∴ 𝑟1 = √𝑓𝜆 = 3.16 × 10−4 𝑚.
𝑛𝜆

8: Hint: If f and f’ are the focal lengths for the wavelengths 𝜆 and 𝜆′ then we have

𝑟𝑛2 𝑟𝑛2 𝜆 6×10−7

𝑓= and ′ = . Dividing we get, 𝑓 ′ = 𝑓 𝜆′ = 1 × 5×10−7 = 1.2 𝑚
𝑛𝜆 𝑛𝜆′

(2𝑛−1)(𝑎+𝑏)𝑏𝜆 (𝑎+𝑏)𝑏𝜆
9: For maxima we have, 𝑥𝑛 = √ = 𝐾√2𝑛 − 1, where 𝐾 = √
𝑎 𝑎

It is given that b = 2 m, a = 1 m, 𝜆 = 6000 Å = 6x10-7 m.

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(1+2)×2×6x10−7
Therefore, K =√ =1.897x10-3m
1

For first maximum, x1 = 𝐾√1 = 1.897 × 10−3 × √1 = 1.897 × 10−3 𝑚

For second maximum, x2 = 𝐾√3 = 1.897 × 10−3 × √3 = 3.286 × 10−3 𝑚

For third maximum, x3 = 𝐾√5 = 1.897 × 10−3 × √5 = 4.243 × 10−3 𝑚

Separation between the two, x4 – x1 = (8.40 – 4.20) x 10-3= 4.20 × 10−3 𝑚

7.14.2 Terminal Questions

1. Radii are 7x10-4m and 9.9x10-4m respectively, and area of each is 1.54x10-6m2, 2. 50 cm,
1
3. 18 cm, 6 cm, 3.6 cm, 4. 1.0 dioptre, 0.0768 cm, (Hint: Power, P = 𝑓 dioptre where

1 1 1 1 1 𝑛𝜆 1 1 𝑛𝜆
= 𝑎 + 𝑏 and 𝑟𝑛 = √𝑓𝑛𝜆, 5. a=30 cm (Hint: (𝑎 + 𝑏) = , Thus (𝑎 + 30) = and
𝑓 𝑟𝑛2 𝑟𝑛2
1 1 3𝑛𝜆 𝑟2
𝑛 1 1 1
(𝑎 + 6) = ), 6. 2500 (Hint: 𝑛 = 𝑓𝜆 where f can be calculated by (𝑎 + 𝑏) = 𝑓), 7. 19.98 m,
𝑟𝑛2
𝑟2 1 1 7.273
3.076 m, 1.664 m (Hint: 𝑛 = (𝑎 + 𝑏), ∴ 𝑛 = 3.636 + , For first three positions of
𝜆 𝑏
minima, n=4, 6, 8), 8. 1.35 m (Hint: For parallel beam, a=∞, and for first maximum n=1), 9.
x1 = 0.173 cm, x2 = 0.300 cm, x3 = 0.66 cm, x10 = 0.533 cm, x2-x1 = 𝛽12= 0.127 cm, 𝛽23=
0.066 cm,

7.14.3 Objective Type Questions

1. (b), 2. (c), 3. (a), 4. (b), 5. (b), 6. (d), 7. (a), 8. (a), 9. (c), 10. (b)

7.15 REFERENCES
1. Ajoy Ghatak (2012), OPTICS, Tata Mc Graw Hill, New Delhi.

2. R. Fowels Grant, Introduction to Modern Optics, Dover Publications Inc., New York

3. Max Born and Emil Wolf, Principles of Optics (sixth Edition), Pergamon Press, New York

4. Frank L. Pedrotti, S.J., Leno S. Pedrotti, Leno M. Pedrotti, Introduction to Optics (Third Edition), Pearson
Education, India

5. Germain Chartier, Introduction to Optics, Springer, New York

7.16 SUGGESTED READINGS

1. Avadhanulu, M.N., Kshirsagar, P.G., Engineering Physics, S. Chand, New Delhi

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2. http://ocw.mit.edu

3. https://en.m.wikipedia.org

4. https://cds.cern.ch

5. H. Webb Robert, Elementary Wave Optics, Dover Publicatios Inc, New York

6. Arora, C.L. and Hemne, P.S. (2012), Physics For Degree Students, S. Chand and Company
Ltd., New Delhi

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UNIT 8: FRAUNHOFER DIFFRACTION

CONTANTS
8.1 Introduction
8.2 Objectives
8.3 Classes of Diffraction
8.4 Fraunhofer Diffraction Due to a Single Slit
8.5 Fraunhofer Diffraction Due to Double Slit
8.5.1 Missing Orders
8.6 Fraunhofer Diffraction at Circular Aperture
8.7 Plane Diffraction Grating
8.7.1 Missing Orders
8.7.2 Maximum Number of Order Available in a Grating
8.8 Solved Examples
8.9 Summary
8.10 Glossary
8.11 References
8.12 Suggested Readings
8.13 Terminal Questions
8.14 Answers

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8.1 INTRODUCTION
If an opaque obstacle is placed between a source of light and a screen then light bends
around the corner of the obstacle into the geometrical shadow. This bending of light is called
diffraction. The phenomenon of diffraction depends on the size of the obstacle and the
wavelength of the light beam.
Diffraction is one particular type of wave interference, caused by the partial obstruction
or lateral restriction of a wave. Not all interferences are diffraction; for example, sound waves
emitted by two stereo speakers will interfere with each other if they are of the same frequency
and have a definite phase relationship, but this is not diffraction. Diffraction will not occur if
the wave is not coherent, and diffraction effects become weaker (and ultimately undetectable)
as the size of obstruction is made larger and larger compared to the wavelength. In well-
defined cases, a diffraction pattern may be observed. It is necessary to mention here that
diffraction is not the same as refraction, although both are phenomena in which a wave does
not propagate in a single direction.

8.2 OBJECTIVES
After studying this unit, you will be able to
 have the basic idea of diffraction and its various classes.
 know the diffraction output at various structure like single, double and multiple slit.
 introduce the plane diffraction grating.
 determine the missing orders for diffraction spectra.

8.3 CLASSES OF DIFFRACTION

Based on the distance between source, aperture and screen, and also on the shape of
wavefront, diffraction pattern is classified into two classes
1. Fresnel Diffraction-If the source of light and the screen are at finite distances from the
diffracting aperture, then the wavefront falling on the aperture will not be plane (spherical
or cylindrical). The diffraction obtained under this type of arrangement is called Fresnel
Diffraction. This type of diffraction is also called near-field diffraction. No lenses are
used to make the rays parallel or convergent.
Fresnel Diffraction is obtained when light suffers diffraction at a straight edge, a thin
wire, a narrow slit etc. Both the size and shape of the pattern depends on the distance
between the diffracting aperture and the screen.
2. Fraunhofer Diffraction-If both the source of light and the screen are effectively far
enough from the aperture so that the wavefronts reaching the aperture and the screen can
be considered plane. Then the source and the screen are said to be at infinite distances
from the aperture. This kind of diffraction is called Fraunhofer Diffraction. This is also
called far-field diffraction.

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Fraunhofer Diffraction is encountered in the case of gratings that contain number of

slits. When the screen is moved, the size of the diffraction pattern changes uniformly
while the shape of the pattern does not change.

8.4 FRAUNHOFER DIFFRACTION DUE TO A SINGLE SLIT

Let AB is a slit of width b, the diffracted beam through the slit is tilted at an angle θ
with respect to straight direction.

Figure 8.1
Path difference between two rays diffracted from two extreme points of slit
= BK = AB sinθ = b sinθ

Phase difference = 2 x path difference = 2 ( b sin )

 
Let the width AB of the slit be divide into n equal parts. The amplitude of vibration at P due
to the waves from each part will be same, say a. The phase difference between the waves
from any two consecutive parts is
1  2  , say
 b sin    2 
n  

Then the resultant amplitude at P is given by

 b sin 
a sin 
a sin( nd 2 )   
R 
sin( d 2 )  b sin 
sin 
 n 

 
Let us put  b sin    
  

a sin a sin na sin 

Then R   ....... (8.1)
sin(  n )  n 

When n   , a  0 , but the product na remains finite.

Let na = A
The resultant intensity at P, being proportional to the square of the amplitude, is

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 sin 
2

I R A 
2 2
 ....... (8.2)
  

Condition for Maxima

A sin A  3  5 7 
R       ......
  3! 5! 7! 

A sin  2 4 6 
R  A1     ...... ....... (8.3)
  3! 5! 7! 

For  0, R  A
This is the intensity of central maximum
 
  b sin   0 or sin  0
 

Condition for Minima

sin
 0 or sin  0 , but   0

   m , Where m has an integral value 1, 2, 3 except zero
 
So  b sin     m  b sin    m ....... (8.4)
 

This equation gives the position of first, second, third etc. minima for m = 1, 2, 3 etc
Secondary Maxima
dI
0
d

d  2  sin  2 
or A    0
d     

 2 sin    cos  sin 

or A2   0
   2
 cos  sin 
0
2
 cos   sin   0
  tan   y ( say)
y   and y  tan 

The maxima will occur when

3 5 7
 , ,
2 2 2

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
or   ( 2n  1 ) n  1,2,3..... ....... (8.5)
2
These are points of secondary maxima

 sin 
2

I  I0   ....... (8.6)
  
3 5 7
Put   , , etc.
2 2 2

4 4 4
I1  I0 , I 2  I0 , I 3  I 0 etc
9 2 25 2 49 2

Figure 8.2

8.5 FRAUNHOFER DIFFRACTION DUE TO DOUBLE SLIT

Let a parallel beam of monochromatic light of wavelength λ be incident normally upon
two parallel slits AB and CD, each of width b and their separation as d. The distance between
the corresponding points of two slits will be (b+d).

Figure 8.3
Suppose each slit diffracts the beam in a direction making an angle θ with the direction of
incident beam. From the theory of diffraction at single slit, the resultant amplitude will be
A Sin


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bSin
Where 

Now consider the two slits equivalent to two coherent sources, placed at the middle points S1
A Sin
and S2of the slits and each sending a wavelet of amplitude .

Therefore, the resultant amplitude at point P on the screen will be the result of the
interference between two waves of same amplitude A Sin and having a phase difference δ.


∴ Path difference between the wavelets coming from S1 and S2in direction θ is given by
S2K = (b+d) sinθ
2 2
Phase difference = x path difference = ( b  d ) sin ) = 2β
 
Resultant amplitude R at point P can be obtained by vector addition method as
Sin 2 Cos 2 
I  R 2  4 A2 ....... (8.7)
2

Here Sin 2 gives the diffraction pattern due to each individual slit and Cos 2  gives the
2


interference pattern due to double slit. Sin 2 gives a central maximum in the direction β= 0,
2


having alternate minima and secondary maxima of decreasing intensity on either side.
The minima are obtained in the directions given by
Sinα = 0 or α = ± mπ
b Sin
∵ 

∴ b Sin   m ....... (8.8)

Where m = 1, 2, 3…. (except zero).

The term Cos 2  in the intensity pattern gives a set of equidistant dark and bright fringes.

Cos 2   1

∴ β=± nπ

( b  d ) sin   n

( b  d ) sin   n ....... (8.9)

Where n = 0, 1, 2, 3…., correspond to zero-, first-, second- etc. order Maxima.

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8.5.1 Missing Orders

In the output intensity pattern of a double slit, for certain values of d, few interference
maxima become absent.
As, the directions of interference maxima are given by
(b + d) sin θ = nλ ....... (8.10)
The directions of diffraction minima are given by
b sin θ =mλ ....... (8.11)
If the values of b and d are such that both the equations are satisfied for the same value of a,
then a certain interference maximum will overlap the diffraction minimum and hence the
spectrum order will be missing (absent).
Dividing equation (8.10) by equation (8.11), we get,
bd n
 ....... (8.12)
b m
If b=d
n
 2 or n  2m. If m  1, 2, 3....... etc., then n  2, 4, 6.......etc
m
This means that the 2, 4, 6 etc. orders of interference maxima will be missing in the
diffraction pattern. Thus the central diffraction maxima will have three interference maxima
(the zero order and two first-orders).
If d=2b
b  2b n
 or n  3m. If m  1, 2, 3 ....... etc., n  3, 6, 9....... etc
b m
This means that 3rd, 6th, 9th etc, orders of interference maxima will be missing in the
diffraction pattern. On both sides of the central maximum, the number of interference
maximum is 2 and hence there will be five interference maxima in the central diffraction
maximum.

8.6 FRAUNHOFER DIFFRACTION AT CIRCULAR

APERTURE
The problem of diffraction at a circular aperture was first solved by Airy in 1835. The
amplitude distribution for diffraction due to a circular aperture forms an intensity pattern with
a bright central band surrounded by concentric circular bands of rapidly decreasing intensity
(Airy pattern). The 1st maximum is roughly 1.75% of the central intensity. 84% of the light
arrives within the central peak called the airy disk

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Let us consider a circular aperture of diameter d is shown as AB in figure below. A

plane wave front WW’ is incident normally on this aperture. Every point on the plane wave
front in the aperture acts as a source of secondary wavelets. The secondary wavelets spread
out in all directions as diffracted rays in the aperture. These diffracted secondary wavelets are
converged on the screen SS′ by keeping a convex lens (L) between the aperture and the
screen. The screen is at the focal plane of the convex lens. Those diffracted rays traveling
normal to the plane of aperture [i.e., along CP0] are get converged at P0.

Figure 8.4

Figure 8.5
All these waves travel some distance to reach P0 and there is no path difference between these
rays. Hence a bright spot is formed at P0 known as Airy’s disc. P0 corresponds to the central
maximum.
Next consider the secondary waves traveling at an angle θ with respect to the direction
of CP0. All these secondary waves travel in the form of a cone and hence, they form a
diffracted ring on the screen. The radius of that ring is x and its center is at P 0. Now consider
a point P1 on the ring, the intensity of light at P1 depends on the path difference between the
waves at A and B to reach P1. The path difference is BD = AB sin θ = d sin θ. The diffraction
due to a circular aperture is similar to the diffraction due to a single slit. Hence, the intensity
at P1 depends on the path difference d sin θ. If the path difference is an integral multiple of λ
then intensity at P1 is minimum. On the other hand, if the path difference is in odd multiples
of images, then the intensity is maximum.
i.e., d sin θ = nλ, for minima ....... (8.13)

and d sin θ = (2n−1) , for maxima ....... (8.14)
2

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Where n = 1, 2, 3… etc. n = 0 corresponds to central maximum.

The Airy disc is surrounded by alternate bright and dark concentric rings, called the
Airy’s rings. The intensity of the dark ring is zero and the intensity of the bright ring
decreases as we go radially from P0 on the screen. If the collecting lens (L) is very near to the
circular aperture or the screen is at a large distance from the lens, then
x
Sin    ....... (8.15)
f

Where, f is the focal length of the lens.

Also from the condition for first secondary minimum [using equation (8.13)]

Sin    ....... (8.16)
d
Equations (8.15) and (8.16) are equal
x  f
 or x  ....... (8.17)
f d d

But according to Airy, the exact value of x is

1.22 f
x ....... (8.18)
d
Using equation (8.18) the radius of Airy’s disc can be obtained. Also from this equation we
know that the radius of Airy’s disc is inversely proportional to the diameter of the aperture.
Hence by decreasing the diameter of aperture, the size of Airy’s disc increases.

8.7 DIFFRACTION DUE TO A PLANE DIFFRACTION

GRATING OF N PARALLEL SLITS

Figure 8.6
Here, S1, S2, S3….. SNare N narrow slits, in between points A and B. Let b= width of slit, d=
width of opaque part between two slits.
The amplitude from each slit in the direction θ is

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A sin
R0 

b
Where   sin (As derived, in case of Single slit Fraunhofer diffraction)

The path difference between the wavelets from S1 and S2 in the direction θ is
S 2 K 1  ( b  d ) sin 
Hence the phase difference between them
2
( b  d ) sin  2  , say

If N be the total number of slits in the grating, the resultant amplitude in the direction of θ
will be
sin N  A sin   sin N
R  R0   ....... (8.19)
sin     sin 

Thus, the resultant intensity at point P is

2
 sin    sin N 
2

I  R 2  A2     ....... (8.20)
    sin  

The factor A2  sin  gives the intensity distribution due to single slit, while  sin N
2 2


    sin  
gives the distribution of intensity in the diffraction pattern due to the interference in the
waves due to N slits.
Principal Maxima
2
 sin    sin N 
2

IR A    
2 2

    sin  

The intensity will be maximum when

sin   0     n
Where, n  0,1, 2, 3.....

This result in
sin N 0 (Indeterminate)

sin  0

Applying L’ Hospital rule

d
(sin N )
sin N d
Lim  Lim
   n sin     n d
(sin  )
d

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N cos N  N
Lim
   n cos 

This result in

 sin 
2

I  A2   N
2

  

The condition for principal maxima is

sin   0 or    n

( b  d )Sin   n

(b  d ) Sin  n ....... (8.21)

For n = 0, we get θ = 0 and this gives the direction of zero order principal maxima. The value
of n = 1, 2, 3 etc. gives the direction of first, second, third etc. order principal maxima.

Minima
2
 sin    sin N 
2

I  R 2  A2    
    sin  

The intensity will be minimum when

sin N  0 but sin   0

Therefore, N    m ....... (8.22)

8.7.1 Missing Orders

As the resultant intensity due to N-parallel slits (plane diffraction grating) is given by
2
 sin    sin N 
2

I  R 2  A2    
    sin  

b
Where,  sin


And  ( b  d ) sin

Now the direction of principal maxima in grating spectrum is given as
(b  d ) Sin   n ....... (8.23)

Further the direction of minima of a single slit pattern is

b Sin   m ....... (8.24)

Where m= 1, 2, 3……

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If both the conditions are simultaneously satisfied, a particular maximum of order n will be
absent in the grating spectrum, these are known as absent spectra (or missing order
spectrum).
Dividing equation (8.23) by equation (8.24), we get
bd n
 ....... (8.25)
b m

If b = d, then 2nd, 4th, 6th etc. orders maxima will be missing in the grating diffraction pattern.
If d = 2b, then 3rd, 6th, 9th etc. orders maxima will be missing in the grating diffraction pattern.

8.7.2 Maximum Number of Order Available in a Grating

The grating equation is (b  d ) Sin   n
(b  d ) Sin
or n ....... (8.26)

Maximum possible value of θ is 900.
Therefore, Maximum possible order will be
( b  d ) Sin 90 (b d )
n max   ....... (8.27)
 

8.8 SOLVED EXAMPLES

Example 8.1: A single slit is illuminated by two wavelengths λ1 and λ2. One observes that
due to Fraunhofer diffraction the first minimum for λ1 coincides with the second diffraction
minimum for λ2. What is the relation between λ1 and λ2.
Solution: In a single slit diffraction pattern, the direction of minimum intensities are given as
a sin    mn , where m= 1, 2, 3 …..

Hence for m = 1, we have, a sin  1

and for m = 2, we have, a sin  22

Equating above two equations, we get, λ1= λ2
Example 8.2: In a double slit Fraunhofer diffraction pattern, the screen is placed 170 cm
away from the slits. The width of the slit is 0.08 mm and slits are 0.4 mm apart. Calculate of
the wavelength of light, if the fringe width is 0.25 cm. Also find the missing order.
Solution: In a double slit Fraunhofer diffraction pattern, the fringe width is given by-
D
w
2d
Here D=170 cm=1.7 m, W= 0.25 cm= 2.5 x 10-3m, a=0.08 mm = 8 x 10-5 m and b = 0.4 mm=
4 x 10-4 m, 2d = b = 4 x 10-4 m

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2 dW
∴  = 0.5882 x 10-6 = 5882 Å
D
The condition for missing order is-
ab n ab  8  10 5  4  10 4 
 or n   m   m  6m
a m  a   8  10 5 

n=6m
Hence the missing orders are 6, 12, 18, 24, 30…..

8.9 SUMMARY
The basics of the diffraction phenomena along with various classes of diffraction have
been discussed. The Fraunhofer diffraction for single slit, double slit, circular aperture and N
slits (grating) have been discussed in the details. The calculation for the intensity of the
principal maxima, secondary maxima and minima has been derived. Their relative
comparison in terms of their intensities has also been made. Determination of missing orders
in case of double slit and N slits (grating) diffraction pattern has also been made.

8.10 GLOSSARY
Fraunhofer Diffraction- Far field diffraction
Grating- Fine and equidistant slits in large number
Missing Order- Absent maxima

8.11 REFERENCES
1. Optics by Ajoy Ghatak.
2. Optics and Atomic Physics by D. P. Khandelwal, Himalaya Publishing House, New Delhi, 2015

8.12 SUGGESTED READINGS

1. OPTICS- Principles and Applications, K. K. Sharma Academic Press, Burlington, MA, USA, 2006.
2. Introduction to Optics- Frank S. J. Pedrotti, Prentice Hall, 1993

8.13 TERMINAL QUESTIONS

Objective Type
1. Grating element is equal to

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A. nλ/sinθ B. nλ C. sinθ D. cosθ

2. In Fraunhofer’s diffraction, incident light waves have________ type of wavefront.
A. Circular B. Spherical C. Cylindrical D. Plane
3. In single-slit experiment, if the red color is replaced by blue then_____________.
A. The diffraction pattern becomes narrower and crowded together
B. The diffraction bands become wider
C. The diffraction pattern does not change
D. The diffraction pattern disappears.
4. On increasing the width of a single slit, the width of the central maximum
A. increases B. remains constant C. decreases D. becomes zero
5. Maximum number of orders possible with a grating is
A. Independent of grating element
B. Inversely proportional to grating element
C. Directly proportional to grating element
D. Directly proportional to wavelength.
6. When white light is incident on a diffraction grating, the light diffracted more will be
A. Blue B. Yellow C. Violet D. Red
7. Diffraction phenomena are usually divided into __________ classes.
A. One B. Two C. Three D. Four.
8. Light of Wavelength 5000 Å is incident on a single slit of width 0.1 mm. The screen is at a
distance of 2 m from the slit. The width of the central bright fringe on the screen will be
A. 18 mm B. 36 mm C. 20 mm D. 6 mm
9. Light of Wavelength 6000 Å is incident normally on a single slit of width 24 x 10 -5 cm.
The angular position of the second minimum from the central minimum from the central
maximum will be -
A. 300 B. 600 C. 900 D. 450
10. In a diffraction grating, the condition for principal maxima is
A.𝑏 sin 𝜃 = 𝑛𝜆 B. (𝑏 + 𝑑) sin 𝜃 = 𝑛𝜆
C.𝑑 sin 𝜃 = 𝑛𝜆 D. sin 𝜃 = 𝑛𝜆.

Long Answer Type

1. Define diffraction phenomena. What do you mean by the Fresnel class and Fraunhofer
class of diffraction?

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2. Describe Fraunhofer diffraction due to single slit for central maxima and prove that the
relative intensities of the successive maximum are nearly 1:1/22:1/61…
3. What are missing orders in double slit Faunhofer diffraction? Further in a grating, if the
widths of transparencies and opacities are equal.
4. Give an account of the diffraction effects produced by a slit. Explain what happens when
the slit width is gradually increased and also when the screen is gradually moved away
from the slit.
5. Discuss Fraunhofer diffraction at a circular slit; describe the formation of Airy’s disc.
6. Give the theory of a plain transmission grating. What particular spectra would be absent if
the widths of transparencies and opacities of the grating are equal.
Numerical Questions
1. A circular aperture of 1.2 mm diameter is illuminated by a plane wave of monochromatic
light. The diffracted light is received on a distant screen which is gradually moved
towards the aperture. The center of the circular path of the light first becomes dark when
the screen is 30 cm from the aperture. Calculate he wavelength of light.
2. Light of wavelength 5500 Å falls normally on a slit of width 22 × 10-5 cm. Calculate the
angular position of the first two minima on either side of the central maximum.
3. Plane wave of wavelength 6 × 10-5 cm fall normally on a slit of width 0.2 mm. Calculate
(i) the total angular width of the central maximum (ii) the linear width of the central
maximum on a screen placed 2 m away.
4. Calculate the angle at which the first dark band and the next bright band are formed in the
Fraunhofer diffraction pattern of a slit 0.3 mm wide (λ = 5890 Å).
5. In a single slit diffraction pattern the distance between the first minimum on the right and
first minimum on the left is 5.2 mm. The screen on which the pattern is displayed is 80
cm from the slit and the wavelength is 5460 Å. Calculate the slit width.
6. Calculate the wavelength of light whose first diffraction maximum in the diffraction
pattern due to a single slit falls at θ = 300 and coincides with the first minimum for the red
light of wavelength 6500 Å.
7. Light of wavelength 600 nm is incident normally on a diffraction grating. Two adjacent
maxima occur at angles given by sin θ= 0.2 and sin θ = 0.3. The fourth-order maxima are
missing. (a) What is the separation between adjacent slits? (b) What is the smallest slit
width this grating can have? For that slit width, what are the (c) largest, (d) second
largest, and (e) third largest values of the order number m of the maxima produced by the
grating?
8. A diffraction grating is made up of slits of width 300 nm with separation 900 nm. The
grating is illuminated by monochromatic plane waves of wavelength λ= 600 nm at normal
incidence. How many maxima are there in the full diffraction pattern?

8.14 ANSWERS
Objective Type
1 (A), 2 (D), 3 (A), 4 (C), 5 (C), 6 (C), 7 (C), 8(C), 9(A), 10(B)

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Numerical Questions
1. 6000 Å
2. 140 29’ & 30o
3. 6 × 10-3 radians & 1.2 cm
4. 0.1120& 0.1680
5. 1.68 × 10-4 cm
6. 4333.3 Å
a. 6μm (b) 1.5 μm (c) 9 (d) 7 (e) 6
7. 3

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UNIT 9: RESOLUTION AND RESOLVING POWER

CONTANTS
9.1 Introduction
9.2 Objectives
9.3 Rayleigh Criterion of Resolution
9.4 Resolving Power of Transmission Grating
9.5 Resolving Power of Prism
9.6 Resolving Power of Telescope
9.7 Resolving Power of Microscope.
9.8 Solved Examples
9.9 Summary
9.10 Glossary
9.11 References
9.12 Suggested Readings
9.13 Terminal Questions
9.14 Answers

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9.1 INTRODUCTION
When the two objects are very near to each other or they are at very large distance
from our eye, the eye may not be able to see them as separate. If we want to see them
separate, optical instruments such as telescope, microscope etc. (for close objects) and prism
and grating etc. (for spectral lines) are employed. Even if we assume that the instruments
employed are completely free from all optical defects, the image of a point object or line is
not simply a point or line but it is a diffraction pattern with a bright central maximum and
other secondary maxima, having minima in between of rapidly decreasing intensity. Thus an
optical instrument is said to be able to resolve two point objects if the corresponding
diffraction patterns are distinguishable from each other.
The ability of an optical instrument to resolve (i.e. view separately) the images of two close
point source is known as resolving power.
Limit of Resolution: The minimum separation between two objects that can be resolved
by an optical instrument is called the limit of resolution (or just resolution).

9.2 OBJECTIVE
After studying this unit, you will be able to –
 have the basic idea of resolution.
 know the Rayleigh criterion of resolution.
 calculate the resolving power of various instruments/ accessories like grating, prism,
telescope and microscope.

9.3 RAYLEIGH CRITERION OF RESOLUTION

According to Rayleigh, two close point objects are said to be just resolved if the
principal maxima of one coincides with the first minima of the other and vice-versa.

Figure 9.1

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9.4 RESOLVING POWER OF TRANSMISSION GRATING

Let λ andλ+dλ are two closely spaced spectral lines (wavelengths).The resolving power of the
grating is defined as the ratio of wavelength (λ) to the difference dλ of the wavelengths to be
resolved

R .P. 
d
The direction of nth principal maxima for wavelength λ is given by
( b  d ) sin  n

The direction of nth principal maxima for wavelength λ+dλ is given by

(b  d ) sin (  d )  n (  d ) ....... (9.1)

Figure 9.2
The minima in the direction θ is given by (due to wavelength λ+dλ)
N (b  d ) sin (  d )  m (  d )

Here m can have all integral values except 0, N, 2N, 3N……

(Because for these values of m the condition of maxima is satisfied)
The first minimum adjacent to nth principal maxima in the direction (θ+dθ) can be obtained
by putting m as (nN+1) (due to wavelength λ)
N (b  d ) sin (  d )  (nN  1)( )

( nN  1 )
( b  d ) sin (   d )  ....... (9.2)
N
Comparing equation (1) and (2)
( nN  1 )
n(   d ) 
N

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
or n  nd  n 
N
 
nd    nN ....... (9.3)
N d
Since resolving power is directly proportional to N, it means that larger will be the number of
lines per cm of a grating greater will be the resolving power.

9.5 RESOLVING POWER OF PRISM

An example of the use of Rayleigh criterion for the resolving power of a rectangular
aperture is found in the prism spectroscope, if we assume that the face of the prism limits the
refracted beam to a rectangular section. The resolving power of a prism is defined as its
capacity to form separate spectral lines of two wavelengths which are very near to each other.
It is measured by λ/dλ, where λ is the wavelength of either of them or mean wavelength and
dλ is the difference in their wavelengths.
Expression for Resolving Power
Let ABC be the section of prism as shown in Fig. 3. A parallel beam of light consisting
of wavelengths λ and λ + dλ is incident on the prism placed in the minimum position for
these two wavelengths (this is possible because the wavelength difference dλ is very small).
BD represents the incident plane wavefront. As the two wavelengths are refracted by
different amounts in passing through the prism, therefore CE and CF represent the
corresponding emergent wavefronts. Let dθbe thedifference in deviation for the light rays of
wavelengths λ and (λ + dλ) respectively. The telescope objective of thespectrometer focuses
the wavefront CE at I1 and CF at I2. Thus I1 corresponds to the principal maximum for
wavelength λ and λ + dλ.

Figure 9.3
The face AC ofthe prism acts like a rectangular aperture. Hence the Rayleigh criterion can be
applied here. According to Rayleigh's criterion, the two wavelengths can be resolved by the
prism if the principal maximum of one falls on the first minimum of the other in the same
direction.
Let μ and (μ – dμ) be the values of refractive indices of the prism, for wavelengths λ and
λ+dλ respectively.

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Equating the path covered by ray 1 and 2

DA+ AE = μ(BC)= μ t (for λ) ....... (9.4)
DA+ AF = (μ -dμ) BC= (μ - dμ)t (for λ + d λ) ....... (9.5)
Equations (9.4) and (9.5) are obtained by applying the Fermat’s principle which states that for
any wavelength all the actual optical paths between the incident and the emergent wavefronts
must be equal. Subtracting equation (9.5) from equation (9.4), we get,
AE – AF=dμ.(AC) = dμ.t
From the geometry of the figure
AE - AF=AE - AG = dμ.t (Since AF = AG, approximately)
or GE = dμ.t
If GE = λ, then according to the theory of Fraunhofer diffraction, Rayleigh criterion of
resolution is satisfied and spectral lines of wavelengths λ and λ + dλ, will be just resolved.
Thus λ = t.dμ
Dividing both sides by dλ, we obtain the expression for the resolving power of prism will be
  d 
t   ....... (9.6)
d  d 
From equation (3), it is evident that the resolving power of a prism varies directly as
(i) t, the width of the base of the prism, and
(ii) dμ/dλ, rate of change of refractive index with wavelength.

9.6 RESOLVING POWER OF TELESCOPE

A telescope is used to see the distinct objects. The details which it gives depend on the
angle subtended at its objective by two point objects and not on the linear separation between
them. The resolving power of a telescope is defined as the reciprocal of the smallest angle
subtended at the objective by the two distinct object points which can be just seen as separate
ones through the telescope.
Expression for the resolving power: Let d isthe diameter of the objective of the
telescope (Fig. 4). Consider the incident rays of light from two neighboring points(say two
stars lying very close to each other, not shown in the figure). Suppose dθ is the angle
subtended by the two distant objects at the objective of the telescope. The ring supporting the
telescope objective and the lens itself serveas a circular aperture and produce Fraunhofer
diffraction patterns in the focal plane of the objective.
Let P1 and P2 be the positions of the central maxima of the two images. The pattern will be
very close to each other with a large amount of overlapping. If the overlapping is too much,
the telescope may not be able to distinguish them as separate. According to Rayleigh's

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criterion, the patterns will be just resolved if the central maxima of one just falls on the first
minima of the other.
Now the secondary waves travelling in direction AP2 and BP2 meet at P2 and have a
path difference equal to (BP2-AP2) = BC = d.dθ

Figure 9.4

BC=AB sin dθ = AB.dθ (for small angles)

If this path difference d.dθ=λ, the position of P2 corresponds to the first minimum of first
image and we have,

d .d   or d  ....... (9.7)
d
The above idea may be understood in the following way:
If we consider that the whole wavefront AB isdivided into two halves AO and OB, then the
path difference between the secondary waves from the corresponding points in the two halves
is λ/2. All the secondary waves from the two halves interfere destructively with one another
and hence P2 corresponds to the first minimum of the first image.
The condition (9.7) holds good for rectangular aperture. According to Airy this condition in
case of a circular aperture can be expressed as
1.22
d  ....... (9.8)
d
Here dθ represents the minimum resolvable angle between the two distant point objects or
this gives the limit of resolution of the telescope. The reciprocal of dθ measures the resolving
power of the telescope. Hence
1 d
 ....... (9.9)
d 1.22
Thus a telescope with large diameter of objectivehas a higher resolving power.

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9.7 RESOLVING POWER OF MICROSCOPE

The function of a microscope is to magnify an object and give its finer details which
cannot be observed by naked eye. The ability of a microscope to form distinctly separate
images of two neighboring small objects is known as its resolving power. It is measured by
the smallest linear separation between two point objects whose images are just resolved by
the objective of the microscope. The smaller is the linear separation which can be resolved,
the higher is said to be the resolving power.
Expression for Resolving Power: In Figure 9.5, AB is the aperture of the objective of
the microscope; O1 and O2 are the self-luminous point objects very close to each other and
separated at a distance d. The periphery of the objective acts as a circular aperture and as a
result the images of O1 and O2 are Fraunhofer diffraction patterns. The patterns consisting of
a central bright disc surrounded by a series of alternate dark and bright rings.P 1 represents the
central maximum of the diffraction pattern of the point object O1. Similarly P2 represents the
central maximum of the diffraction pattern of the other point object O2.
According to the Rayleigh's criterion the two objects may be resolved if the central
maximum of one pattern falls on the first minimum of the other. In this case the two objects
may be resolved if P1 islocated at the first minima of the diffraction pattern centered at P2.

Figure 9.5
According to the Rayleigh's criterion the two objects may be resolved if the central
maximum of one pattern falls on the first minimum of the other. In this case the two objects
may be resolved if P1 islocated at the first minima of the diffraction pattern centered at P2.
Thus we have to find out condition under which the first minima of the diffraction pattern due
to O2 lies at the central maxima of diffraction pattern due to O1. This will happen when the
path difference between the extreme rays O2BP1 and O2AP2 is equal to λ. To consider this
path difference, the magnified view of O1O2 and the rays starting from them are shown in
Fig. 6. The path difference is given by

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Figure 9.6
(O2B+BP1)- (O2A + AP1) = O2B- O2A (Since BP1=AP1)
In Figure 9.6,O1C is perpendicular to CA and O1D is perpendicular to O2B.
O2B- O2A = (O2D + DB)- (EA-EO2) =O2D + EO2(As DB= O1B= O1A=EA)
Therefore, path difference = O2D + EO2 = 2d sin α
If the path difference 2d sin α = 1.22λ, then P1 corresponds to the first minimum of the image
P2 and the two images appear just resolved.
2d sin α = 1.22λ
or d = 1.22λ/ 2sin α ....... (9.10)
The result is derived on the assumptions that the objects viewed with microscope are self-
luminous and emitting light of wavelength λ.
In case of objects illuminated by some external source of light of wavelength λ, Abbe showed
that the factor 1.22 may be omitted and we can write
d = λ/ 2sin α ....... (9.11)
The high resolution power microscopes are generally oil immersion types in which the space
between the object and objective is filled with an oil of refractive index μ. In this case as the
path difference will then be multiplied with the factor μ
d = λ/ 2μsin α ....... (9.12)
Here, the factor μsin α is known as the numerical aperture of the microscope.

∴ Resolving power of the microscope = 1  2  sin  ....... (9.13)

d 1.22
Thus, using small wavelengths (UV) and using quartz lenses, the resolving power of the
microscope can be increased. Such microscopes are known as the ultra-microscope.

9.8 SOLVED EXAMPLES

Example 9.1: Calculate the minimum number of lines in a grating which will just resolve the
sodium lines in the first order spectrum. The wavelengths are 5890 and 5896 Å.

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Solution: We know that resolving power λ/dλ = nN

Here n=1, λ1 = 5890 Å = 5890×10-8 cm
∴ dλ = 5896 × 10-8 - 5890 × 10-8 = 6 × 10-8 cm.
Now, N=(1/n) x (λ/dλ) = (5890 x 10-8)/(1 x 6 x 10-8) =982 approximately
Example 9.2: A grating has 15 cm of the surface ruled with 16000 lines per cm. What is the
resolving power of the grating in the first order?
Solution: The resolving power of a grating is given by-
λ/dλ = nN, here n = 1, N = 15 x 6000 = 90000
λ/dλ = 1 x 90000 =90000
Example 9.3: A prism spectrometer uses a prism of base 5 cm and material whose dispersion
d is 200 in the range λ = 5000Å. What is the smallest difference of the wavelength in this
d
range which this spectrometer may resolve?
Solution: The expression for the resolving power of prism is
  d  ,
 t 
d  d 

Here, t= 5 cm, λ = 5000Å = 5 x 0-5 cm, d = 200

d

Putting the values, we get, dλ =5 x 10-8 cm =5 Å

Example 9.4: Two pin holes 1.5 mm apart are placed in front of a source of light of
wavelength 5.5 x 10-5 cm and seen through a telescope with objective diameter of 0.4 cm.
Find the minimum distance from the telescope at which the pin holes can be resolved.
1.22 x
Solution: We know that d  and also d 
d a
1.22 x xd 0.15  0.4
∴  or a  = 894.2 cm
d a 1.22 1.22  5.5  10 5
Example 9.5: The smallest object detail that can be resolved with a certail microscope with
light of wavelength 6000 Å is 3.5 x 10-5 cm. Find (i) The numerical aperture of the objective
when used dry, and (ii) The numerical aperture obtained if an immersion oil of refractive
index 1.5 is used.
Solution: The resolving power of microscope is –
 
d  , where NA is numerical aperture
2 Sin 2 NA

6000  10 8
(i) NA = NA    0.86 approx.
2 d 2  3.5  10  5
(ii) Oil immersion numerical aperture= μ x dry aperture = 1.5 x 0.86 =1.44

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9.9 SUMMARY
The resolving power of an optical instrument is defined as its ability to just resolve the
images of two close point sources or small objects. The Rayleigh Criterion gives a
quantitative account of the phenomena of resolution. The definitions and physical meanings
for the resolving powers of diffraction grating, prism, telescope and microscope were
discussed. Their mathematical expressions have also been derived in the present chapter.

9.10 GLOSSARY
Resolving Power: Ability of an optical instrument to see the close objects separately
Limit of resolution: Minimum resolvable distance
Principal maxima: Central maxima

9.11 REFERENCES
3. Optics by AjoyGhatak.
4. Optics and Atomic Physics by D. P. Khandelwal, Himalaya Publishing House, New Delhi, 2015

9.12 SUGGESTED READINGS

3. OPTICS- Principles and Applications, K. K. Sharma Academic Press, Burlington, MA, USA, 2006.
4. Introduction to Optics- Frank S. J. Pedrotti, Prentice Hall, 1993.

9.13 TERMINAL QUESTIONS

Objective Type
1.The maximum resolving power of a microscope can be obtained with
(A) Violet light (B) Yellow Light (C) Red Light (D) Green Light
2.What will be limit of resolution of a microscope if its numerical aperture is 0.5 and the
wavelength of light used 5000 Å
(A) 6100 mm (B) 6100 cm (C) 6100 m (D) 6100 Å
3.Two stars distant eight light years are just resolved by a telescope. The diameter of the
telescope lens is 26 cm. If the wavelength of the light used is 5000 Å , the minimum distance
between the stars will be
(A) 1.95 x 1012 M (B) 1.95 x 1011 M (C) 1.95 x 1010 M (D) 1.95 x 109 M
4.The resolving power of a telescope can be increased by having a

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(A) Large focal length of eyepiece (B) Small focal length of eyepiece (C)Large aperture
of objective lens (D) Small aperture of objective lens
5.The Resolving power of a grating having N slits in nth order will be
(A) (n+N) (B) (n-N) (C) nN (D) n/N
6.The resolving power of a prism is
(A) Directly proportional to the rate of change of refractive index with wavelength
(B) Inversely proportional to rate of change of refractive index with wavelength
(C) Inversely proportional to the thickness of the prism
(D) Independent of thickness of prism
Long Answer Type
1. Discuss Rayleigh criterion for resolution. What is limit of resolution? Determine an
expression for the resolving power of a grating.
2. Explain clearly, what is meant by the resolving power of an optical instrument and
deduce an expression for the resolving power of a prism.
3. Explain what do you understand by the limit of resolution of a telescope and obtain an
expression for it. What is the effect of the size of the image of a star as aperture of the
objective increases?
4. On the basis of diffraction theory, explain the need of large apertures for telescopes used
for astronomical purposes.
5. Define the resolving power of a microscope. Deduce an expression for it and discuss it.
Numerical Questions
1. Find the separation of the two points that can be resolved by a 500 cm telescope. The
distance of the moon is 3.8 x 105 KM. The eye is highly sensitive to light of wavelength
of 5500 Å.
2. Show that for a transmission grating with 1 inch ruled space, the resolving power cannot
exceed 5 x 104 at normal dence for λ = 5080 Å.
3. A microscope objective gathers light over a cone of semi-angle 300 and uses visible light
of 5500 Å. Estimate its resolving limit.
4. Calculate the minimum thick ness of the base of a prism which will just resolve the D1
and D2 lines of sodium. Given μ for wavelength 6563 Å = 1.6545 and for wavelength
5270 Å = 1.6635.
d
5. Calculate the resolving power of a prism which has a dispersion = 600 per cm and a
d
base of 3 cm. Will this be adequate to resolve two spectral lines (i) 5890 Å (ii) 5230 Å
6. A diffraction grating with a width of 2.0 cm contains 1000 lines/cm across that width. For
an incident wavelength of 600 nm, what is the smallest wavelength difference this grating
can resolve in the second order?
7. How many rulings must a 4.00-cm-wide diffraction grating have to resolve the
wavelengths 415.496 and 415.487 nm in the second order? (b) At what angle are the
second-order maxima found?

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9.14 ANSWERS
Objective Type
1.(A), 2. (D), 3. (B), 4. (C), 5. (C), 6. (A)
Numerical Questions
1.61 m, 3. 6.1 x 10-5 cm, 4. 1.41,5.(i) Prism will resolve this line (ii) Prism will not
resolve this line, 6.Δλ= 0.15 nm, 7. (a) 23100, (b) 28.7°

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UNIT 10: POLARIZATION

CONTANTS
10.1 Introduction
10.2 Objectives
10.3 Polarization
10.3.1 Two Slit Analogy of Polarized Light
10.3.2 A Comparison of Unpolarized Light and Polarized Light
10.4 Types of Polarization
10.5 Concept of Plane Polarized Light, Circularly and Elliptically Polarized Light
10.6 Pictorial Representation of Plane Polarized Light
10.7 Plane of Vibration and Plane of Polarization of Plane Polarized Light
10.8 Methods of Production of Plane Polarized Light
10.9 Plane Polarized Light by Reflection
10.9.1 Biot’s Polariscope
10.10 Brewster’s Law
10.11 Plane Polarized Light by Refraction (Piles of Plates Method)
10.12 Malus Law
10.13 Summary
10.14 Glossary
10.15 Reference Books
10.16 Suggested Readings
10.17 Terminal Questions
10.17.1 Short Answer Type Questions
10.17.2 Long Answer Type Questions
10.17.3 Numerical Questions
10.17.4 Objective Questions

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10.1 INTRODUCTION
Waves are basically of two types; transverse waves and longitudinal waves. A wave in
which particles of the medium oscillate to and fro, in the form of compression and
rarefaction, along the direction of wave propagation is called a longitudinal wave, e.g., waves
produced on string and sound waves. On the other hand waves produced in ripples on water
waves and waves on a rope, in which every particle of the medium oscillates up and down, in
the form of trough and crest, at right angles to the direction of wave propagation is called a
transverse wave. Interference, diffraction and polarization are three major phenomenon
exhibited by waves, out of these three interference and diffraction are shown by any type of
wave whether longitudinal or transverse but polarization is shown by only transverse wave.
As light shows all three phenomenon it is clear that light is a transverse wave.

10.2 OBJECTIVE
After the study of this unit the student will be able
 To understand the concept of polarization
 To explain different types of polarization
 To differentiate between plane, circular and elliptically polarized light
 To describe different methods of plane polarized light
 To explain Brewster’s law
 To explain Malus law

10.3 POLARIZATION
The phenomenon of interference and diffraction has proved the wave nature of light but
it doesn’t tell us regarding the character (whether longitudinal or transverse?) and nature of
vibration of light (whether linear, circular, elliptical or torsional). It is the phenomenon of
polarization that shows that light wave is definitely transverse in nature. Light wave is a
transverse electromagnetic wave made up of mutually perpendicular, fluctuating electric and
magnetic fields vibrating perpendicular to each other as well as direction of propagation too.
In general, natural light is unpolarized in nature, i.e., it consists of a very large number
of wavelength with electric vector vibrates in all possible planes with equal probability when
ordinary light is allowed to pass through tourmaline crystal, the vibration of electric field are
confined only to one direction in a plane perpendicular to the direction of propagation of
light. This light which has acquired the property of one sidedness and whose electric field
vibration lacks in symmetry called polarized light. This polarization of light means departure
from complete symmetry about the direction of propagation. Let us explain the concept of
polarization using two slit analogy and their optical equivalent.

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10.3.1 Two Slit Analogy of Polarized Light

Let S1 and S2 be the two slit adjacent to each other (figure 10.1). A string AB passing through
the slit and attached to a fixed point at B. Now if we move the end A of the string up and
down perpendicular to AB, the string vibrates and a transverse wave propagates along CD. If
slit S2 is placed parallel to S1, vibration passing through S1 and S2 reach the end B without
any change in amplitude (figure 10.1(a)). Now let us rotate slit S2 as S2 becomes
perpendicular to S1, the vibration pass through the slit S1 undisturbed as before but are not
able to pass through S2 and therefore the string does not vibrate between S2 and B (figure
10.1(b)).

Figure 10.1
In the intermediate positions of the slit S2, the vibration are partially transmitted and
partly stopped, reaching the end B with diminished amplitude. The variation of amplitude as
the rotating slit S2 is only because of transverse vibration in the string. On the other hand the
end A is moved to and fro parallel to the length of the string instead of up and down setting
longitudinal vibration, we see that the rotation of any of the slide about AB as axis does not
affect the passage of vibrations and hence vibration reach at B with undiminished amplitude.
Therefore we can say that variation in amplitude of vibration passing through S2 on rotation
of S2 signifies the transverse vibration of the string.
Now if we replace the slit with tourmaline crystal and string with a source of light,
exactly similar phenomenon is observed (Figure 10.2). When light from source S falls in a
tourmaline crystal A cut parallel to its crystal axis the emergent light is slightly coloured.
Now if we place a similar cut crystal B in the path of beam partially to the axis of crystal A
we observed that emerging light is still coloured and the intensity is maximum. If now
keeping crystal ‘A’ fixed we rotate the crystal B about the axis the intensity decreases and

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becomes zero when B is perpendicular to A. By further rotation light reappears and becomes
maximum again when ‘A’ and ‘B’ again becomes parallel.
This variation in amplitude proves that light is transverse in nature. Whereas, if it is
longitudinal, there shouldn’t be any variation by rotating the crystals discussed in two slit

Figure 10.2
analogy. It also shows that light vibration after passing through rotating slit are not
symmetrical about the direction of propagation.

10.3.2 Comparison of Unpolarized Light and Polarized Light

S.N. Unpolarized Light Polarized Light

1 Unpolarized light consists of waves with Polarized wave consists of waves with light
planes of vibration equally distributed in all vector vibrating in a single plane
directions about the direction of perpendicular to the direction of
propagation. propagation.
2 Unpolarized light is symmetrical about the Polarized light is asymmetrical about the
ray direction. ray direction.

3 Unpolarized light is produced by Polarized light is generally obtained from

conventional light sources unpolarized light with the help of polarizer.

10.4 TYPES OF POLARIZATION

The polarization of a light wave describes the shape and locus of the tip of the E vector
at a given point in space as a function of time. Depending upon the locus of the tip of the E
vector light may be exhibit three different states of polarization. They are
1. Plane polarized light

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2. Circularly polarized light

3. Elliptically polarized light
Apart from these the light may also be partially polarized.

10.5 CONCEPT OF PLANE POLARIZED LIGHT, CIRCULARLY

POLARIZED LIGHT AND ELLIPTICALLY POLARIZED LIGHT
As we know that light is an electromagnetic wave consist of mutually perpendicular
electric and magnetic field vector both are vibrating perpendicular to the direction of
propagation of light wave. Also electric vector is dominating and is responsible for optical
effects of wave hence the electric vector is also called light vector.

Figure 10.3
As mentioned earlier, unpolarized light have vibrations along all possible straight lines
perpendicular to the direction propagation the light. Light which has acquired property of one
sidedness is called polarized light. Therefore plane polarized light is not symmetrical about
the direction of propagation but the vibrations of light vector (electric vector) are confined to
a single direction i.e. along a line of course perpendicular to direction of propagation also
known as linearly polarized light. We can also say that in plane or linearly polarized light the
magnitude of light vector changes but its orientation remains unchanged. Usually light is a
mixture of plane polarized light and unpolarized light, known as partially plane polarized.
On the other hand a light wave is circularly polarized if the magnitude of light vector
remains constant but its orientation rotates at a constant rate about the direction of
propagation so that the tip of the light vector traces a circle. It completes one evolution within
one wavelength. Circularly polarized wave may be considered as the result of superposition
of two mutually perpendicular plane polarized waves having equal amplitude but a phase

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difference of 900. If rotation of tip of light vector E is seen clockwise it is called right
circularly polarized light if it rotates anticlockwise the wave is said to be left circularly
polarized light.
Similarly a light wave is called elliptically polarized if the magnitude of light vector as
well as its orientation changes about the direction of propagation so that the tip of the light
vector traces an ellipse. Elliptically polarized wave may be considered as the result of
superposition of two mutually perpendicular plane polarized waves having different
amplitude and not in same phase.
Like circularly polarized light if rotation of tip of light vector E is seen clockwise it is
called right elliptically polarized light if it rotates anticlockwise the wave is said to be left
elliptically polarized light.

10.6 PICTORIAL REPRESENTATION OF PLANE POLARIZED

LIGHT
Ordinary or unpolarized light obtained from any source consists of vibration of electric
field vector in al possible plane perpendicular to the beam direction i.e. the electric field
vibrations are symmetrical. Unpolarized light can be considered as consisting of two sets of
vibrations-one set vibrating in one plane and other perpendicular to it. It may also be
represented respectively by arrows and dots. Hence unpolarized lights pictorially represented
end view would be as shown in Figure 10.4 (a).
In a plane polarized light the vibrations of electric vector are along a single straight line
thus having departure of complete symmetry. When electric field vector or light vector of
plane polarized light has vibration in the plane of the paper they are represented by arrows as
shown in Figure 10.4 (b).When the vibrations of light vector are in a direction perpendicular
to the plane of the paper they are represented by dots as shown in Figure 10.4 (c).

Figure 10.4

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10.7 PLANE OF VIBRATION AND PLANE OF POLARIZATION

OF PLANE POLARIZED LIGHT
As discussed earlier plane polarized light may be defined as the light in which the
electric vector or light vector vibrates along a fixed straight line in a plane perpendicular to
the direction of propagation. However to define the properties of plane polarized light
completely we have to define two planes, one containing the vibrations and other
perpendicular to it, as the properties of plane polarized light differ with respect to these two
planes. The plane containing the direction of vibration and direction of propagation or the
plane in which the vibration takes place is called the plane of vibrations and a plane
perpendicular to plane of vibration is called plane of polarization. We can also define plane of
polarization as the plane passing through the direction of propagation and containing no
vibrations.

Figure 10. 5
As shown in Figure 10.5 the plane ABCD is the plane of vibration and the plane
EFGH is the plane of polarization.

10.8 METHODS OF PRODUCTION OF PLANE POLARIZED

LIGHT
Plane polarized light may be produced from unpolarized light using one of the five
optical phenomena listed as below:
1. Polarization by reflection (example - Biot’s polariscope)
2. Polarization by refraction (example - piles of plates method)
3. Polarization by double refraction (example-Nicol prism)
4. Polarization by selective absorption or dichroism (example-Polaroids)
5. Polarization by scattering (example-light from a blue sky)

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10.9 PLANE POLARIZED LIGHT BY REFLECTION

Producing plane polarized light by reflection is the simplest way. In 1808 E.L. Malus
noticed that when natural or unpolarized light is incident on a transparent medium like glass
or water the reflected light is partially plane polarized. The degree of polarization depends
upon the incident angle on the surface and upon the material of the surface. At a certain angle
of the incidence depending upon the nature of the reflecting surface the reflected light is
completely plane polarized. This angle of incidence is called angle of polarization or
polarizing angle.

Figure 10.6
Here it must be noted that light reflected from the metallic surface contained a variety
of vibration directions; i.e. reflected light from metallic surface is unpolarized. But if light is
reflected from dielectric surface such as glass, water etc. is linearly polarized. If the extent of
linear polarization is large enough as glare from field of snow on bright sunny day, its glare
from the surface may be almost blinding to human eye.
When light wave is incident on a boundary between two dielectric materials, part of it is
reflected and part of it is transmitted. Let a beam of unpolarized light incident along AB on a
glass surface YY’ and reflected as BC. To show that reflected light is plane polarized a
tourmaline crystal is placed in the path of reflected ray BC and rotated about BC as axis. It is
observed that the transmitted light shows variation in intensity. It proves partial plane
polarized nature of reflected ray. At polarizing angle of incidence, reflected ray from crystal
is almost completely extinguished shows that maximum percentage of plane polarized light.
Further rotation about BC as axis, the intensity of reflected beam through crystal is twice
maximum and minimum in one complete rotation depending upon whether axis of crystal is
perpendicular or parallel to plane of incidence respectively. It indicates that the light vibration
in reflected beam is perpendicular to plane of incidence.
However this particular method of polarizing light is not very advantageous as only a
small portion of incident beam is reflected therefore the intensity of the reflected beam is
very small.
10.9.1 Biot’s Polariscope
It is a simple instrument for producing and detecting plane polarized light by reflection.
In place of tourmaline crystal Biot’s polariscope consists of two glass plates P 1 and P2. To

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avoid internal reflection and absorb refracted light both the plates are painted back on their
back surfaces. A monochromatic light ray AB falls on plate P1 at polarizing angle and
reflected vertically upwards along BC and incident on other plate P2, which is held parallel to
P1, also at polarizing angle and is reflected along CD. When the plate P2 is gradually rotated
about BC as axis the angle of incidence on plate P 2 is still same. Intensity of reflected ray CD
decreases and becomes zero for a rotation of 900 of plate P2. On further rotation of P2
intensity of CD goes maximum at 1800 and then decreases till 2700 where it again becomes
zero and becomes again maximum at 3600.

Figure 10.7
As the beam AB is incident on P1 at polarizing angle, the reflected beam BC is
completely plane polarized with its vibration perpendicular to the plane of incidence. When
P2 is parallel to P1 (when P2 is rotated through 1800 or 3600) the vibrations of BC are
perpendicular to the plane of incidence therefore ray BC is completely reflected as Ray CD
and intensity is maximum but when P2 is perpendicular to P1 (when P2 is rotated through 900
or 1800) vibrations of BC are parallel to the plane of incidence with respect to plate P 2
therefore no light is reflected.
As the plate P1 causes the beam BC to be polarized and P2 for analyzing the polarized
light, plate P1 is known as polarizer and P2 as analyzer.

10.10 BREWSTER’S LAW

In 1811 A series of experiments on polarization of light by reflection from different
reflecting surface was performed by Sir David Brewster. He observed that degree of
polarization of ordinary light varies with angle of incidence and at a particular angle of
incidence on the surface of a given transparent medium the reflected light is completely plane
polarized. This particular angle of incidence is called polarizing angle or Brewster angle.
However refracted light is still partially plane polarized. Further in case of reflected ray of
light the plane of vibration is perpendicular to plane of incidence while it is parallel to plane
of vibration in case of partially polarized transmitted light.

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Brewster found that polarizing angle depends upon the refractive index of the medium.
As a result of experiments he proved that the tangent of angle of polarization is numerically
equal to the refractive index of the reflecting medium. If ip is angle of polarization and µ is
the refractive index of the medium then according to Brewster’s law
µ = tan 𝑖𝑝 ....... (10.1)
It must be noted here as refractive index depends upon the wavelength of incident light,
angle of polarization varies with the wavelength hence complete polarization is possible with

Figure 10.8
monochromatic light and not with white light. A direct consequence of Brewster’s law is that
when light is incident at the angle of polarization ip, the reflected ray is at right angle to the
refracted ray. This can be easily proved as follows
Let unpolarized beam AB is incident on a smooth glass surface at the polarizing angle
ip. It is reflected along BC and refracted ray along BD as shown in figure 10.8.
𝑠𝑖𝑛 𝑖𝑝
Then from Brewster’s law 𝜇 = tan 𝑖𝑝 = cos 𝑖
𝑝

𝑠𝑖𝑛 𝑖𝑝
Also from Snell’s law 𝜇=
sin 𝑟
𝑠𝑖𝑛 𝑖𝑝 𝑠𝑖𝑛 𝑖𝑝
∴ =
cos 𝑖𝑝 sin 𝑟

𝑠𝑖𝑛 𝑖𝑝 𝑠𝑖𝑛 𝑖𝑝
or 𝜋 =
sin ( −𝑖𝑝 ) sin 𝑟
2
𝜋
or − 𝑖𝑝 = 𝑟
2
𝜋
or 𝑖𝑝 + 𝑟 = 2

As ∠𝑁𝐵𝑁 ′ = 𝜋,
𝑖𝑝 + ∠𝐶𝐵𝐷 + 𝑟 = 𝜋
𝜋 𝜋
or ∠𝐶𝐵𝐷 = 𝜋 − ( 𝑖𝑝 + 𝑟) = 𝜋 − =
2 2

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That is, at polarizing angle of incidence, the reflected ray is perpendicular to refracted ray.
Another important consequence of the law is that if unpolarized light is incident on the
upper surface of a smooth parallel sided glass slab at the angle of polarization, the refracted
ray also meets the lower surface of the slab at angle of polarization. As shown in the figure
10.8, the angle of incidence and refraction at the lower surface are 𝑟 and 𝑖𝑝 respectively.
Hence at lower surface boundary between glass and air the refractive index of second
medium (air) with respect to the first (glass)
1 sin 𝑟
=
𝜇 𝑠𝑖𝑛 𝑖𝑝

sin 𝑟 sin 𝑟 sin 𝑟 1

Also tan 𝑟 = cos 𝑟 = 𝜋 = =
cos ( −𝑖𝑝 )
2
𝑠𝑖𝑛 𝑖𝑝 𝜇

𝜋 𝜋 𝜋
As 𝑖𝑝 + 𝑟 = or 𝑟 = − 𝑖𝑝 or cos 𝑟 = cos ( − 𝑖𝑝 ) = sin 𝑖𝑝
2 2 2
1
= tan 𝑟 ....... (10.2)
𝜇

Therefore, r is the angle of polarization for the lower surface of the slab. Hence the light
reflected from lower surface of glass slab is completely plane polarized while that refracted
into the air is partially plane polarized.

Example 10.1: A ray of light is incident on the surface of a glass plate at the polarizing
angle. Calculate the angle of incidence and angle of refraction.(µ for glass plate=1.732)
Solution: According to Brewster’s law 𝜇 = tan 𝑖𝑝

Given µ for glass plate = 1.732, hence 1.732 = tan 𝑖𝑝

𝜋
or 𝑖𝑝 = tan−1 1.732 = tan−1 √3 =
3
𝜋
Now if 𝑟 is the angle of refraction we know that 𝑖𝑝 + 𝑟 = 2
𝜋 𝜋 𝜋 𝜋
or 𝑟 = − 𝑖𝑝 = − = = 300
2 2 3 6

Example 10.2: The polarizing angle of a piece of glass for green light is 600. Calculate the
angle of minimum deviation for a prism made of the same glass? (given angle of prism = 600)
Solution: According to Brewster’s law 𝜇 = tan 𝑖𝑝
Given 𝑖𝑝 = 600 hence from Brewster’s law, refractive index of glass is
𝜇 = tan 600 = 1.732
𝐴+𝛿𝑚
sin
2
Now we know that 𝜇= 𝐴
sin
2

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600 +𝛿𝑚 600 +𝛿𝑚

sin sin
2 2
0
Given angle of prism A = 60 , hence 1.732 = 600
=
sin 𝑜.5
2

600 +𝛿𝑚
or sin = 1.732 ⨯ 0.5 = 0.8660
2

600 +𝛿𝑚
or = sin−1 0.8660 =600
2

or 𝛿𝑚 = 120 − 60 = 60

10.11 PLANE POLARIZED LIGHT BY REFRACTION (PILES

OF PLATES METHOD)
As stated before when light wave is incident on a boundary between two a dielectric
material, part of it is reflected and part of it is transmitted. When polarized light is incident at
Brewster angle on a smooth glass surface, the reflected light is totally polarized while the
refracted light is partially polarized. If unpolarized light is transmitted through a single plate,
the transmitted beam is only partially polarized contains vibrations parallel as well as
perpendicular to plane of incidence.

Figure 10.9
By increasing the number of plates more and more vibration perpendicular to the plane
of incidence are reflected from successive surfaces resulting the filtering of from transmitted
ray (figure 10.9). Consequently the refracted or transmitted beam gets richer and richer in the
percentage of and ultimately the transmitted light is free from perpendicular vibration and
consists of plane polarized vibrations parallel to plane of incidence only. The piles of plates
consists of a number of thin glass plates supported in a tube of suitable size and inclined at an
angle of about 330 to the axis of the tube. A beam of monochromatic light is incident at
polarizing angle. The reflected as well as the transmitted beams of light are plane polarized
parallel to the plane of incidence.

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Figure 10.10
The graph shows the increase in intensity of the reflected light with increased number
of plates (figure 10.10). However it serves no purpose to increase the number of plates after
nearly 90% of the incident light is reflected.
If Ip and Id denote the intensities of components with the vibrations parallel and perpendicular
to the plane of incidence respectively in the transmitted light, then degree of polarization or
proportion of polarization is given by
𝐼𝑝 −𝐼
𝑃 = 𝐼 +𝐼𝑑
𝑝 𝑑

𝐼𝑝 −𝐼𝑑
and % polarization is given by 𝑋 100
𝐼𝑝 +𝐼𝑑

For plane polarized light 𝐼𝑑 = 0 hence P=1 and % polarization is 100%. For unpolarized light
𝐼𝑝 = 𝐼𝑑 hence P = 0 and % polarization is 0%. As worked out by Provastaye and Desains
𝑛
𝑃= 2
2𝜇
𝑛+( )
1−𝜇2

Where n is number of plates and 𝜇 is the refractive index of the material.

However the concept of the degree of polarization cannot be applied to elliptically and
circularly polarized light.

Example 10.3: A beam of light is passing through a pile of plate consisting of 12 plates of
glass. Refractive index for the glass is 1.54. Find out the percentage of polarization.

Solution: According to Provastaye and Desains,

𝑛
𝑃= 2
2𝜇
𝑛+( )
1−𝜇2

12
Given n = 12 and 𝜇 = 1.54, hence 𝑃= 2⨯1.54
2 = .7041
12+( 2 )
1−(1.54)

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or % 𝑃 = 70.41%

10.12 MALUS LAW

It is observed that whenever a plane polarized light falls on a rotating analyzer the intensity of
light coming out from analyzer changes. Malus explains it by stating that “when plane
polarized light falls on an analyzer the intensity of light coming out from the analyzer is
proportional to the square of the cosine of the angle between the directions of transmission of
the analyzer and the direction of vibration of electric vector in incident light. Mathematically
Malus law can be expressed as
I=I0 Cos2θ

Fig 10.11
Where I0 is the intensity of incident polarized light, θ is the angle between the direction of
transmission of analyzer and polarizer.
Proof: Let A be the amplitude of the plane polarized light incident on analyzer and θ be angle
between the plane of polarizer and analyzer. The amplitude A may be resolved into two
components A cosθ & A sinθ which are parallel and perpendicular to the plane of
transmission of analyzer respectively (Fig.11.12). As A cos θ is parallel to the plane of
transmission of analyzer it will be transmitted while component A sin θ will be blocked.
Therefore intensity of transmitted beam I= (A cos θ)2 = A2cos2θ = I0 cos2 θ Where I0 = A2 is
the intensity of incident plane polarized light. This proves the Malus law.
However Malus law doesn’t hold good for unpolarized light as angle θ made by electric field
vector with the plane of transmission is not constant for unpolarized light. As in unpolarized
light the light vector vibrates in all possible direction in a plane perpendicular to direction by
propagation. We have to put average value of cos2θ over all possible values of θ. Thus the
intensity of the beam transmitted from the polarizing sheet is given by
I = I0 cos2 θ = ½ I0

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Fig.10.12
As the average value of cos2θ over all possible values of θ is ½ hence only 50% of incident
unpolarized light is transmitted as plane polarized light by ideal polarizer.
When planes of polarizer and analyzer are parallel to each other the transmitted light is
maximum. As in this case θ = 00 hence I = I0 cos2 θ = I0 (cos 0)2 = I0
Also when planes of polarizer and analyzer are perpendicular to each other the transmitted
light is zero. As in this case θ=900 hence I= I0 cos2 θ = I0 (cos 90)2 = 0

Example 10.4: If the intensity of the transmitted light by analyzer is 25% of the incident
polarized light. What will be the angle between plane of polarizer and analyzer?
Solution: According to malus law I = I0 cos2 θ
Given I = 25% of I0 i.e. I0/4
Hence I0/4= I0 cos2 θ
or cos2 θ = 1/4
or cos θ = 1/2 or θ = 600
Example 10.5: What is the intensity of the resultant beam if a) unpolarized light of intensity
I0 is incident on a polarizer? b) Plane polarized light of intensity I0 is incident on a polarizer
with its electric field makes an angle 600 with the axis of the polarizer?.
Solution: a) If unpolarized light of intensity I0 is incident on a polarizer then the intensity of
resultant beam is I0/2.
b) When Plane polarized light of intensity I0 is incident on a polarizer then according to
Malus law I = I0 cos2 θ. Given θ = 600, hence I = I0 cos260 = Io/4

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10.13 SUMMARY
Polarization is the property which shows that light is a transverse wave. It not only tells
the character of light wave but also tells whether the vibrations are linear, circular or
elliptical. Generally light coming from a common light source is upolarized however can be
transformed into different types of polarization. Unpolarized light is symmetrical while in
polarized light electric field vibration deviates from completely symmetry and vibrates in
single direction. Superposition of two mutually perpendicular plane polarized wave of equal
amplitude and phase difference of 900 gives rise to circularly polarized light while different
amplitude and any phase difference give rise to elliptically polarized light.
Plane polarized light can be obtained by many different methods like reflection,
refraction, double refraction, dichroism and scattering. Brewster showed that reflected light is
completely polarized at particular angle of incidence is called angle of polarization. He also
showed that tangent of angle of polarization is equal to the refractive index of the medium.
On the other hand Malus explains changes in intensity of light coming out from analyzer by
stating that intensity of light coming out from the analyzer is proportional to the square of the
cosine of the angle between the directions of transmission of the analyzer and the direction of
vibration of electric vector in incident light.

10.14 GLOSSARY
Longitudinal Wave: A wave in which particles of the medium oscillate to and fro, in the
form of compression and rarefaction, along the direction of wave propagation.
Transverse Wave: A wave in which every particle of the medium oscillates up and down, in
the form of trough and crest, at right angles to the direction of wave propagation.
Unpolarized Light: The light in which the light vector (Electric vector) vibrates in all
possible direction i.e. symmetrical about direction of propagation.
Polarized Light: The light, which acquired the property of one sidedness i.e. departure from
complete symmetry.
Plane Polarized Light: The light in which the light vector (Electric vector) vibrates along a
fixed straight line in a plane perpendicular to the direction of propagation.
Plane of Vibration: The plane containing the direction of vibration and direction of
propagation or the plane in which the vibration takes place.
Plane of Polarization: A plane perpendicular to plane of vibration. It can also be defined as
the plane passing through the direction of propagation and containing no vibrations.
Angle of Polarization: It is the angle of incidence on the surface of a given transparent
medium for which the reflected light is completely plane polarized.
Polarizer: An optical device used to convert unpolarized light into polarized light.
Analyzer: Optical device which is used to analyze polarized light is called analyzer.
.

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10.15 REFERENCE BOOK

1. Optics by Ajoy Ghatak
2. A textbook of Optics by Brij Lal and Dr. N. Subrahmnyam
3. Optics by Dr. S.P. Singh and Dr. J.P. Agarwal

10.16 SUGGESTED READINGS

1. Fundamental of Optics by F. A. Jenkins and H. E. White.
2. The Feynman Lectures on Physics by Richard Feynman
3. Optics by Eugene Hecht

10.17 TERMINAL QUESTIONS

10.17.1 Short Answer Type Questions
1. What is polarization? Why sound waves can’t be polarized?
2. Explain planes of polarization and plane of vibration?
3. State Brewster and Malus law.
4. Distinguish between polarized and unpolarized light.
5. Discuss some of the application of Brewster’s law.
10.17.2 Long Answer Type Questions
1. Explain the concept of polarization of mechanical waves in a string by a two slit analogy.
Also explain it with equivalent optical experiment.
2. Describe how to produce polarized light by reflection. Explain construction and working of
Biot’s polariscope.
3. State and explain Brewster’s law? Prove that at polarizing angle of incidence the reflected
and refracted rays are at right angles.
4. Describe how one can produce plane polarized light by refraction through piles of plate
method.
5. State and prove law of Malus.
10.17.3 Numerical Questions
1. If a beam of light is passing through a pile consists of 8 plates of glass; what will be the
percentage of polarization. (given µglass=1.5) (Ans. 58.1%)
2. A glass plate is to be used as a polarizer .find the angle of polarizer for it. Also find the
angle of refraction. Given µ for glass=1.5 (Ans. 50.31)

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3. The critical angle of incidence for total refraction in some case is 450. Find out the
refractive index, polarization angle and angle of refraction corresponding to the polarizing
angle. (Hint if c is critical angle than µ=1/Sin C)
(Ans. refractive index = 0.707, polarization angle = 35.30 and angle of refraction = 54.70)
4. Find the polarizing angle for light incident from (i)air to glass (ii)glass to air (iii)water to
glass (iv)glass to water (v)air to water (vi)water to air. (Given Refractive index of glass =1.5
and for water=1.33). (Ans. (i) 570 (ii) 330 (iii) 49012’ (iv) 40049’ (v) 5304’ (vi) 36057’)
5. What will be the angle between plane of polarizer and analyzer if the intensity of the
transmitted light by analyzer is 50% of the incident polarized light? (Ans. 450)

10.17.4 Objective Questions

1. Transverse phenomenon of light wave is proved by the phenomenon of
a. Polarization b. Interference
c. Diffraction d. Reflection
Ans. Option ‘a’
2. Plane of polarization and plane of vibration are
a. Inclined with 450 with each other b. Parallel to each other
c. Perpendicular to each other d. None of above
Ans. Option ‘c’
3. Sound wave in air can be polarized. This statement is:
a. True as they are transverse b. False as they are transverse.
c. True as they are longitudinal d. False as they are longitudinal
Ans. Option ‘d’
4. Malus law is
a. μ = tan ip b. I = I0 Cos2θ
c. μ= μ0 Cos2θ d. I = I0 Cosθ
Ans. Option ‘b’

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UNIT 11: DOUBLE REFRACTION

CONTANTS
11.1 Objectives
11.2 Double Refraction or Birefringence
11.3 Geometry of Calcite Crystal
11.4 Optic Axis, Principal Section and Principal Plane
11.4.1 Optic Axis
11.4.2 Principal Section and Principal Plane
11.5 Huygen’s Explanation of Double Refraction in Uniaxial Crystal
11.5.1 Positive and Negative Crystals
11.6 Nicol Prism
11.6.1 Construction
11.6.2 Action of Nicol Prism
11.6.3 Nicol Prism as a Polarizer and Analyzer
11.6.4 Limitation of Nicol Prism
11.7 Action of Polarizer on Light of Different Type of Polarizations
11.8 Huygen’s Construction for Double Refraction in Uniaxial Crystal
11.8.1 When Optic Axis is Inclined to the Refracting Edge of Calcite Crystal
11.8.2 (a) When Optic Axis is in the Plane of Incidence and Parallel to the Refracting
Edge of Calcite Crystal
11.8.2 (b) When Optic Axis is Perpendicular to the Plane of Incidence and Parallel to
the Refracting Edge of Calcite Crystal
11.8.3 When Optic Axis is Perpendicular to the Refracting Edge and Lying in the
Plane of Incidence of Calcite Crystal
11.9 Double Images Polarizing Prisms
11.9.1 Rochon Prism
11.9.2 Wollaston Prism
11.10 Dichroism or Selective Absorption
11.11 Polaroids
11.11.1 Uses of Polaroids
11.12 Polarization by Scattering
11.13 Summary
11.14 Glossary
11.15 Reference Books
11.16 Suggested Readings
11.17 Terminal Questions

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11.17.1 Short Answer Type Questions

11.17.2 Long Answer Type Questions
11.17.3 Numerical Questions
11.17.4 Objective Questions

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11.1 INTRODUCTION
Splitting of a beam of unpolarised light into two refracted beam is called double
refraction or birefringence. This phenomenon was first discovered by a Dutch philosopher E.
Bartholinius in the year 1669.
Explanation: As shown in figure 11.1(a), if an image of ink dot on a sheet of paper is
viewed through a calcite crystal we observe two images instead of usual one and if crystal is
rotated slowly one of the two images rotates around the other stationary image.

a b
Figure 11.1
The refracted ray which produce stationary image is known as ordinary ray (O-ray) and
the image as ordinary image. The refracted ray which produces the rotating image is called
extraordinary ray (E-ray) and the image as extraordinary image. O-ray obeys the ordinary
laws of refraction while E-ray doesn’t.
Now let us explain this. As shown in Figure.11.1(b), if a narrow beam of light AB is
incident on calcite crystal with angle of incidence i, it is split up into two rays (instead of one
as usual) inside the crystal. The two images are obtained on the screen corresponding to O-
ray and E-ray. O-ray travels along the side BC with angle of refraction r1 while the E- ray
travels along BD make angle of refraction r2 and both the rays emerges out along CO and DE
which are parallel to each other and also to incident beam.
𝑆𝑖𝑛 𝑖 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑣𝑎𝑐𝑐𝑢𝑚𝑒 𝑉𝑣𝑎𝑐𝑐𝑢𝑚𝑒
By Snell’s law 𝜇= = =
𝑆𝑖𝑛 𝑟 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 𝑉𝑚𝑒𝑑𝑖𝑢𝑚

Therefore, refractive index for O- ray and E- ray are respectively

𝑆𝑖𝑛 𝑖 𝑆𝑖𝑛 𝑖
𝜇𝑜 = and 𝜇𝑒 = in case of crystal having 𝑟1 < 𝑟2 (e.g. calcite) 𝜇0 > 𝜇𝑒
𝑆𝑖𝑛 𝑟1 𝑆𝑖𝑛 𝑟2

Hence 𝑣𝑜 < 𝑣𝑒 i.e inside the calcite like crystal the velocity of E-ray is greater than O-ray or
we can say that Inside the calcite crystal E-ray travels faster in comparison to O-ray.
It is also observed that for O-ray the refractive index is constant for all angle of
incidence as it obeys ordinary law of refraction while for E-ray the refractive index is not
constant but varies with angle of incidence. Therefore it can be easily concluded by this
observation that the O-ray travels with the same speed in all the directions within the crystal
while the E-ray travels with the different speeds in the different directions. Also both E-ray

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and O-ray shows variations in intensity when passes through rotating tourmaline crystal
prove that both ordinary and extraordinary ray obtained by double refraction are plane
polarized. It is also observed that their plane of polarization are at right angles to each other
with O-ray vibrations perpendicular to the plane of paper while vibrations of E-ray is parallel
to the plane of paper.

11.2 OBJECTIVES
After study of this unit the student will be able
 To understand the concept of double refraction or birefringence.
 To explain different terms e.g. optic axis, principal section and principal plane, positive
and negative crystals.
 To understand Huygen’s explanation of double refraction in uniaxial crystal.
 To describe construction and working of Nicol prism, double image prisms.
 To explain Huygen’s construction for double refraction in uniaxial crystal.
 To explain polaroids and their uses.

11.3 GEOMETRY OF CALCITE CRYSTAL

Calcite crystal also known as Iceland spar (as its fine specimen are generally found in
Iceland) is a colourless transparent crystal belongs to hexagonal system. Chemically it is
hydrated calcium carbonate (CaCO3). A piece of Iceland spar can be reduced to simple
rhombohedron (as shown in figure 11.2) by cleavage or breakage. It is bounded by six faces
each of which is a parallelogram with angles roughly equal to 1020 and 780 (more accurately
these are equal to 101055’and 7805’). The rhombohedron has only two diametrically opposite
corners C and F where all the face angles are obtuse 1020 (101055’). These two corners
termed as the blunt corners of the crystal. Out of the rest six corners, two angles are acute and
the remaining one is obtuse

Figure 11.2

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11.4 OPTIC AXIS, PRINCIPAL SECTION AND PRINCIPAL

PLANE
Let us discuss about optic axis, principal section and principal plane one by one
11.4.1 Optic Axis
It is a direction along which or parallel to it a ray does not exhibit double refraction i.e.
E-ray and O-ray travel with the same speed and in the same direction along the optic axis.
Generally a line passing through any of the blunt corner and equally inclined with the three
faces which meets at the corner, gives the direction of the optic axis of the crystal. We have
to keep in our mind that optic axis is a direction not a particular line. Any line parallel to
optic axis is also an optic axis. Crystal which has only one optic axis is known as uniaxial
crystal e.g. calcite, quartz, tourmaline While in Borax, mica, aragonite & selenite there are
two optic axes. Such crystals are known as biaxial crystal. However in case of biaxial crystal
both the refracted rays are extraordinary.
Special cases: It has been observed that
a) A ray of light incident along the optic axis or in a direction parallel to optic axis the ray is
not split up into ordinary and extraordinary components as E-ray and O-ray travel with the
same speed and in the same direction along the optic axis.
b) A ray of light incident perpendicular to the optic axis the ray is not split up into ordinary
and extraordinary components but in this case E-ray and O-ray travel with the same direction
but with different speed.
11.4.2 Principal Section and Principal Plane
A plane containing the optic axis and perpendicular to the two opposite faces of crystal
is called the principal section of the crystal for that pair of plane. For every point inside the
crystal there are three principal sections, one for each pair of opposite crystal faces. For the
calcite crystal principal section is a parallelogram having angles of 710 and 1090.Since the
opposite faces of a calcite crystals are always parallel the O-ray and E-ray emerges out
parallel to each other and also parallel to incident ray.
To understand the directions of vibrations for the E-ray and O-ray we also have to
define principal plane or O-ray and E-ray respectively. The principal plane of the ordinary
ray is as the plane containing optic axis and O-ray and the plane containing optic axis and E-
ray is called principal plane for E-ray. Except the case plane of incidence is the principal
section of the crystal these two principal planes generally don’t coincide.

11.5 HUYGEN’S EXPLANATION OF DOUBLE REFRACTION

IN UNIAXIAL CRYSTAL
In order to explain the double refraction exhibited by uniaxial crystal Huygen’s
extended his wave theory of secondary wavelets. Huygen’s postulated that

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1. When light is incident on a doubly refracting crystal, every point of it becomes source of
secondary wavelets and excites two separate wavelets within the crystals; one spherical
wavelet associated with O-ray and other elliptical with E-ray. For O-ray, the crystal is
isotropic and homogeneous, hence O-ray travels with the same velocity in all directions and
therefore, the wavefront corresponding to it is spherical. For E-ray the crystal is anisotropic
(different properties in different directions) hence its velocities varies with the directions i.e.
wavefront can’t be spherical it is ellipsoid.
2. The two wave fronts corresponding to E-ray and O-ray touch each other at the two points.
The direction of the line joining these two points is called optic axis. As light advances
through the crystal the two wave surfaces travel in different direction in the crystal therefore
two refracted rays corresponding to E–ray and O-ray respectively emerges out of crystal.

11.5.1 Positive and Negative Crystals

Because of the two different types of wavefronts, two possibilities are there

Figure 11.3
Case 1: The spherical wavefront of O-ray is enclosed by the ellipsoidal wavefront of E-ray.
Such crystals are called negative crystals e.g. calcite. Obviously the diameter of the sphere is
equal to the minor axis of the ellipsoid (figure 11.3(a)). For such crystals the velocity of
ordinary ray is constant in all directions while velocity of extraordinary ray varies as the
radius vector of ellipsoid. The velocity of E-ray is minimum and equal to O-ray along optic
axis and a maximum in a direction perpendicular to direction of optic axis.
i.e., ve = vo parallel to optic axis

ve > vo in other direction

In this case, the refractive index for O-ray is greater than the refractive index for the E-ray

i.e., µe < µo

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We can see that in negative crystals, extra ordinary wave surface behaves as if it were
repelled away from the optic axis.
Case 2: The ellipsoidal wavefront of E-ray is enclosed by the spherical wavefront of O-ray.
Such crystals are called positive crystals e.g., quartz. Obviously the diameter of the sphere is
equal to the major axis of the ellipsoid (figure 11.3 (b)). For such crystals the velocity of
ordinary ray is constant in all directions while velocity of extraordinary ray varies as the
radius vector of ellipsoid. The velocity of E-ray is maximum and equal to O-ray along optic
axis and a minimum in a direction perpendicular to direction of optic axis.
i.e., ve = vo parallel to optic axis
ve < vo in other direction
In this case, the refractive index for the E-ray is greater than the refractive index for the O-ray
i.e., µe>µo
In contrast to negative crystals we can see that in positive crystals extra ordinary wave
surface behaves as if it were attracted towards the optic axis.

11.6 NICOL PRISM

Nicol prism is an optical device fabricated from calcite crystal for producing and
analyzing plane polarized light named after its inventor William Nicol who designed it in
1820. Its action is based on phenomenon of double refraction. It’s constructed in such a way
that O-ray is eliminated by total internal reflection and we get only the plane polarized E-ray
coming out of the Nicol.
11.6.1 Construction

Figure 11.4
A rhomb of calcite crystal with length AB three times as long as its breadth CD is
obtained by cleavage from the original crystal. The ends faces are grounded until they make
an angle of 680 instead of 710 in natural crystal. Then the crystal is cut into two parts along a
plane perpendicular to the principal axis as well as to the new end surfaces AB and CD

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(figure 11.4). Thus the two parts are grounded and polished optically flat than again cemented
together with a layer of Canada balsam. Canada balsam is a clear transparent material whose
refractive index (µcb = 1.55) lies between the refractive index for calcite for the O-ray (µo =
1.66) and E-ray (µe = 1.486) for sodium yellow light of mean wavelength λ = 5893 Å.
11.6.2 Action of Nicol Prism
When unpolarised light is incident on the prism parallel to AB. it suffers double
refraction and splits into O-ray and E-ray as shown in figure. When these ray strikes at
Canada balsam layer (which is denser than calcite for E-ray and less dense for O-ray) O-ray
travels from an optically denser to a rarer medium it is totally reflected in case the angle of
incidence is greater than a certain critical value (critical angle). This reflected ray is
completely absorbed as the tube containing the crystal is coated black. E–ray is not reflected
as it travels from an optically rarer to a denser medium hence E-ray is transmitted through the
prism. Thus Nicol prism acts as a polarizer.
Now the reason behind natural angle 710 is reduced to 680 and choosing length three times to
its width enables the O-ray to fall at the Canada balsam layer at an angle greater the critical
angle.
Let θ is the critical angle for the O-ray. Than refractive index of O-ray with respect to Canada
𝜇𝑜 1.658
balsam layer is = .
𝜇𝑐𝑏 1.55

1 1.55
The condition for total internal reflection is sin θ = = = 0.935
𝜇 1.658

Hence 𝜃 = sin−1 (0.935) = 690

Thus, if the angle of incidence for the O-ray is greater than 690 it is totally internal reflected
and absorbed by blackened wall.

11.6.3 Nicol Prism as Polarizer and Analyzer

When two nicol prisms P and A are arranged coaxially adjacent to each other. First
Nicol P acts as a polarizer and other acts as analyzer. Such a combination of polarizer and
analyzer is called polariscope.
If unpolarized ray of light is incident on the first Nicol P, E-ray is transmitted with its
vibration directly lying in the principal section of P. The state of the polarization of the light
can be analyzed by another Nicol called analyzer. If principal section of analyzer A is parallel
to principal section of polarizer P, E-ray is transmitted through analyzer A without any
hindrance. In the case of Parallel Nicols the intensity of emergent E-ray is maximum (figure
11.5(a)). If the analyzer A is gradually rotated the intensity of E-ray decreases in accordance
with Malus law.
In case the angle between principal sections of two Nicol is 900 they are called crossed Nicol.
In the case E-ray when it enters the analyzer acts as O-ray inside the prism hence totally
reflected at balsam layer hence no light comes out of the analyzer A (figure 11.5(b)). If A is
further rotated the intensity of emergent light from A will go on increasing and is becomes
maximum when its principal section is again parallel to that of P.

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Figure 11.5
In this way the Nicol prism A is used to analyze the plane polarized light produced by
Nicol prism P hence named Analyzer and polarizer respectively.
11.6.4 Limitation of Nicol Prism
A Nicol prism cannot be used for highly convergent and divergent beams. It is found
that the angle of incidence is limited for140 above which the O ray is also transmitted. It is
also found that E- ray also totally reflected if the angle is greater than 140. Thus to avoid the
transmission of O-ray and total internal reflection of E-ray the angle between the extreme
rays of the incoming beam is limited to 2x14 = 280.

11.7 ACTION OF POLARIZER ON LIGHT OF DIFFERENT

TYPE OF POLARIZATIONS
Whenever light of different types of polarizations is incident on polarizer its response is
as follows
a) In case of unpolarized light, polarizer transmits half the intensity of light incident on it and
shows no variation in intensity on rotation of polarizer.
b) In case of partially polarized light, intensity of transmitted light depends on the direction
of transmission axis of the polarizer. It also shows variation in intensity on rotation of
polarizer from maximum to minimum non zero value.
c) In case of plane polarized light, intensity of transmitted light varies two times from
maximum to zero in one full rotation of the polarizer.
d) In case of circularly polarized light, intensity of transmitted light remains constant on
rotation of polarizer like unpolarised light. This can be explained as follows: the circular

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vibrations can be resolved into two mutually perpendicular linear variations of equal
amplitude .on incidence on polarizer the vibration parallel to its transmission axis passes
through the polarizer while perpendicular component is blocked. It happens in all positions of
the Polarizer in its rotation therefore transmitted light shows no variation in intensity on
rotation of polarizer.
e) In case of elliptically polarized light, intensity of transmitted light varies two times from
maximum to non zero minimum in one full rotation of the polarizer like partially polarized
light. A maximum occurs when the transmission axis coincides with the semi major axis and
minima when it coincides with the semi minor axis of the ellipse.

Example 11.1: Calculate the velocities of E-ray and O-ray in calcite (i) In a plane
perpendicular to the optic axis (ii) along the optic axis. (Given µe = 1.486, µo = 1.658 and C =
3⨯108)
Solution: (i) velocity of E-ray in calcite in a plane perpendicular to the optic axis
𝐶 3 ⨯ 108
𝑣𝑒 = = = 2.02 × 108 𝑚/𝑠𝑒𝑐
𝜇𝑒 1.486
Similarly, velocity of O-ray in calcite in a plane perpendicular to the optic axis
𝐶 3⨯108
𝑣𝑜 = 𝜇 = = 1.8 × 108 𝑚/𝑠𝑒𝑐
0 1.658

Example 11.2: Two Nicols are first crossed and then one of them rotated through 450.
Calculate what percentage of incident light will be transmitted.
Solution: Let the intensity of incident unpolarized light on first Nicol be I0.On entering the
first nicol it is broken up into O-ray and E-ray each of intensity I0/2. First Nicol only
transmits E-ray while O-ray is lost by total internal reflection. Therefore, I0/2 will be the
intensity of light incident on second Nicol.
Initially when two Nicols are in crossed positions, the angle between their principal planes is
900. If one of them is rotated through 450 from the crossed positions, the angle between the
principal planes of transmission is = 90 ± 45 i.e. 1350 or 450.
According to Malus law intensity of emerging beam from second Nicol is
I1 = I Cos2θ = I0/2Cos2135 or I0/2Cos245
(As I0/2 is the intensity of light incident on second Nicol)
Now Cos 1350 = Cos 450 = 1/√2
Hence I1 = I0/2× (1/√2)2 = I0/4
𝑒𝑚𝑒𝑟𝑔𝑒𝑛𝑡 𝑙𝑖𝑔ℎ𝑡 𝑓𝑟𝑜𝑚 𝑠𝑒𝑐𝑜𝑛𝑑 𝑛𝑖𝑐𝑜𝑙
∴ Percentage of light transmitted from the system = × 100
𝐼𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑙𝑖𝑔ℎ𝑡 𝑜𝑛 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚

I /4
=0 × 100 = 25 %
I0

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11.8 HUYGEN’S CONSTRUCTION FOR DOUBLE

REFRACTION IN UNIAXIAL CRYSTAL
Let us explain the phenomenon of double refraction in uniaxial crystal using Huygens
theory. According to Huygen’s theory O-ray and E-ray gives rise to spherical and ellipsoidal
wavefront respectively. Except along optic axis (along which E- and O-ray travels with same
velocity) E-wave travels with different velocity and the difference is maximum for direction
perpendicular to the optic axis. The extraordinary wavefront depends upon how the surface of
the crystal is cut relative to the optic axis. A thinner rectangular cross section can be cut from
a bigger crystal in such a way that the optic axis lies
1. Inclined to the refracting edge.
2. Parallel to the refracting edge.
3. Perpendicular to the refracting edge.
Let us explain all possible 3 cases for the purpose of tracing the paths of O-ray and E-ray
inside a negative uniaxial crystal.
11.8.1 When Optic Axis is Inclined to the Refracting Edge of Calcite Crystal
Let ABCD represent the calcite (negative crystal). Let PQ represent the incident plane
wavefront incident normal to the crystal surface and crystal is so cut that the optic axis lies in
the plane of incidence. According to Huygens, The Phenomenon of double refraction
involves splitting of unpolarised wavefront to two wavefronts; one O-wave having spherical
wavefront and other E-wave having ellipsoidal wavefront. Two wavefronts touch each other
along the optic axis of the crystal. In negative crystal O-wave surface (sphere) lies within the
E-wave (ellipsoid) (figure 11.6).

Fig. 11.6
Let us consider that parallel beam of light PQ falls normally in the surface of negative
crystal. According to Huygen’s theory as soon as the parallel beam strikes the crystal
boundary, each point on the wavefront becomes a source of secondary disturbance and point

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R and S produce elliptical and spherical wavelets. After time T the new position of wavefront
can be determined as follows. A circle of radius vot is drawn taking R and S as centre. In
negative crystal the elliptical wavefront can be drawn by taking major axis 2vet and minor
axis 2vot. Therefore, circular and elliptical wavefront touch each other along optic axis as
required. If we now draw the common tangents to the secondary wavelets they represent the
plane wavefront corresponding to the two rays thus XX’ is the tangent to spherical wavefront
and YY’ is the tangent to the ellipsoidal wavefront.
Now the line joining the point of origin of wavelets to the point of tangency, give the
direction of propagation of O- and E-ray. Therefore, O-ray travels along RX and SX’
direction while E-ray travels along RY and SY’ direction. This shows in this case O-ray and
E–ray travels along different direction with different velocities.

11.8.2 (A) When Optic Axis is in the Plane of Incidence and Parallel to the
Refracting Edge of Calcite Crystal
Let ABCD represent the calcite (negative crystal). Let PQ represent the incident plane
wavefront incident normal to the crystal surface and crystal is so cut that the optic axis is
parallel to the refracting edge CD and lies in the plane of incidence (figure11.7 (a)).

Fig. 11.7
According to Huygens, because of double refraction the unpolarized wavefront splits into two
wavefront; one O-wave having spherical wavefront and other E-wave having ellipsoidal
wavefront. Two wavefronts touch each other along the optic axis of the crystal. In negative
crystal O-wave surface (sphere) lies within the E-wave (ellipsoid).
Let us consider that parallel beam of light PQ falls normally in the surface of calcite
crystal. According to Huygen’s theory as soon as the parallel beam strikes the crystal
boundary, each point on the wavefront becomes a source of secondary disturbance and point

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R and S produce elliptical and spherical wavelets. After time t the new position of wavefront
can be determined as follows. A circle of radius vot is drawn taking A and B as centre. In
negative crystal the elliptical wavefront can be drawn by taking major axis 2 v et and minor
axis 2vot. Therefore, circular and elliptical wavefront touch each other along optic axis as
required.
If we now draw the common tangents to the secondary wavelets they represent the
plane wavefront corresponding to the two ray thus XX’ is the tangent to spherical wavefront
and YY’ is the tangent to the ellipsoidal wavefront. Now the line joining the point of origin of
wavelets to the point of tangency, give the direction of propagation of O- and E-ray.
Therefore, O-ray travels along RX and SX’ direction while E-ray travels along RY and SY’
direction. This shows in this case although the O- and E-rays are not separated and travel
along same direction, yet there is a double refraction as O-ray and E–ray travels with
different velocities. As the E-ray travels faster than O-ray inside the crystal these ray comes
out with a certain path difference. This property is used in the construction of quarter wave
and half wave plate.

11.8.2(b) When Optic Axis is Perpendicular to the Plane of Incidence and

Parallel to the Refracting Edge of Calcite Crystal
As shown in (figure 11.7 (b)) let ABCD represent the calcite (negative crystal) and PQ
represent the incident plane wavefront incident normal to the crystal surface and crystal is so
cut that the optic axis is parallel to the refracting edge CD but perpendicular to the plane of
incidence. According to Huygens, because of double refraction unpolarised wavefront splits
into two wavefront; one O wave having spherical wavefront and other E-wave having
ellipsoidal wavefront. Two wavefronts touch each other along the optic axis of the crystal. In
negative crystal O-wave surface (sphere) lies within the E wave (ellipsoid).
Let us consider that parallel beam of light PQ falls normally in the surface of calcite
crystal. According to Huygen’s theory as soon as the parallel beam strikes the crystal
boundary, each point on the wavefront becomes a source of secondary disturbance and point
R and S produce elliptical and spherical wavelets. As optic axis is perpendicular to the plane
of incidence it means the spherical and ellipsoidal wavelets touch each other along a line
perpendicular to the plane of incidence therefore in the plane of incidence both the wavelets
appears spherical. After time t the new position of wavefront can be determined as follows.
For spherical wavefront A circle of radius vot is drawn taking R and S as centre. In
negative crystal the elliptical wavefront can be drawn by taking major axis 2 v et and minor
axis 2vot.Therefore circular and elliptical wavefront touch each other along optic axis as
required which in this case is perpendicular to the surface AB. If we now draw the common
tangents to the secondary wavelets they represent the plane wavefront corresponding to the
two ray thus XX’ is the tangent to spherical wavefront and YY’ is the tangent to the
ellipsoidal wavefront. Now the line joining the point of origin of wavelets to the point of
tangency, give the direction of propagation of O- and E-ray. Therefore O-ray travels along
RX and SX’ direction while E-ray travels along RY and SY’ direction. This shows in this

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case although the O- and E- rays are not separated and travel along same direction, but with
different velocities.
11.8.3 When Optic Axis is Perpendicular to the Refracting Edge and Lying
in the Plane of Incidence of Calcite Crystal
As in previous case let ABCD represent the calcite (negative crystal) and PQ represent
the incident plane wavefront incident normal to the crystal surface and crystal is so cut that
the optic axis is parallel to the refracting edge CD but perpendicular to the plane of incidence
(figure 11.8). According to Huygens, because of double refraction unpolarised wavefront
splits into two wavefronts; one O wave having spherical wavefront and other E-wave having
ellipsoidal wavefront. Two wavefront touches each other along the optic axis of the crystal.
In negative crystal O-wave surface (sphere) lies within the E-wave (ellipsoid).

Figure 11.8
Let us consider that parallel beam of light PQ falls normally in the surface of calcite
crystal. According to Huygen’s theory, as soon as the parallel beam strikes the crystal
boundary, each point on the wavefront becomes a source of secondary disturbance and point
R and S produce elliptical and spherical wavelets. As the spherical and ellipsoidal wavelets
touch each other along optic axis they touch at point X and X’. It is also evident from figure
that section of ellipsoid lies outside the section of the circle. After time t the new position of
wavefront can be determined as follows. For spherical wavefront A circle of radius vot is
drawn taking R and S as centre.
In negative crystal the elliptical wavefront can be drawn by taking major axis 2 v et and
minor axis 2vot. Therefore circular and elliptical wavefront touch each other along optic axis
as required which in this case is perpendicular to the refracting edge AB. If we now draw the
common tangents to the secondary wavelets they represent the plane wavefront
corresponding to the two ray thus XX’ is the common tangent to spherical wavefront and the
ellipsoidal wavefront. Now the line joining the point of origin of wavelets to the point of

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tangency, give the direction of propagation of O-and E-ray. Therefore O-ray as well as E-ray
travels along RX and SX’. This shows in this case the O- and E- rays are not separated and
travel along same direction with same velocity.
Note: All the above cases can also be applied to positive uniaxial crystal like quartz. For
them µe>µo and the spherical wavefront of ordinary wave lies outside the ellipsoidal
wavefront of extraordinary wave.

Example 11.3: A plane polarized light is incident on a plate of calcite crystal with its faces
parallel to the optic axis. If the light is incident with vibrations at an angle 600 on the face of
the crystal, calculate the ratio of the intensities of ordinary and extraordinary ray.
Solution: Let A be the amplitude of the incident vibrations making an angle of 600 face of
the crystal with. As optic axis is parallel to the faces ordinary and extraordinary ray vibration
will be perpendicular and parallel to the optic axis. Therefore amplitude of ordinary and
extraordinary ray will be A Sinθ and A Cosθ and intensity will be A2 Sin2θ and A2 Cos2θ
where θ is given 600.
Hence the ratio of the intensities of ordinary and extraordinary ray
2
3
A2 Sin2 60 (√ )
2
= = 1 = 3:1
A2 Cos2 60 ( )2
2

Example 11.4: A 600 quartz prism is cut with its faces parallel to the optic axis. Calculate the
angle of minimum deviation for yellow light for ordinary and extraordinary ray. (Given µo =
1.5422, µe = 1.5533, and λ = 5890 Å )
Solution: The refractive index of prism for ordinary and extraordinary ray is given by
𝐴+𝛿𝑚
sin
2
𝜇= 𝐴
sin
2

where 𝛿𝑚 is angle of minimum deviation and A is the angle of prism = 600 (given)

(i) For ordinary ray µo = 1.5422

𝐴+𝛿𝑚 0 60+𝛿𝑚 0
sin sin
2 2
So 𝜇0 = 𝐴 or 1.5422 = 60
sin sin
2 2

600 +𝛿𝑚 0 1
or sin = 1.5422 sin 300 = 1.5422 × = 0.7711
2 2

600 +𝛿𝑚 0
or sin = sin(500 27′ )
2
600 +𝛿𝑚 0
or = 500 27′
2

or 𝛿𝑚 0 = 1000 54′ − 600 = 400 54′

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(ii) For extraordinary ray µe = 1.5533

𝐴+𝛿𝑚 𝑒 60+𝛿𝑚 𝑒
sin sin
2 2
So 𝜇𝑒 = 𝐴 or 1.5533 = 60
sin sin
2 2

600 +𝛿𝑚 𝑒
or sin = 1.5533 sin 300 = 0.7766
2

600 +𝛿𝑚 𝑒
or sin = sin(500 57′ )
2

or 𝛿𝑚 𝑒 = 1010 54′ − 600 = 410 54′

11.9 DOUBLE IMAGES POLARIZING PRISMS

In Nicol prism O-ray is completely absorbed by total internal reflection and only the E-
ray is transmitted. Also Nicol prism cannot be used in ultra violet light as Canada balsam
layer absorbs these radiations. Double image prisms are optical devices used if two widely
separated images due to both the ray on the emergent side of the prism are desirable. The
Rochon prism and the Wollaston prism are two such double image prisms made by double
refracting crystals (either quartz or calcite).
11.9.1 Rochon Prism
Rochon prism consists of two right angled prism ABC and BCD made of quartz or
calcite and cemented together with glycerin or castor oil to enable them to be used with
ultraviolet light so as to form a rectangular block with their refracting angles equal (figure
11.9 (a)). The prism ABC is cut such that the optic axis is parallel to the base AB and the
incident ray (i.e. parallel to the plane of the paper and perpendicular to the refracting edge).
While the other prism BCD has its optic axis perpendicular to the plane of incidence (i.e.
perpendicular to the plane of the paper and parallel to the refracting edge). Light ray incident
normally on the face AC of the prism passes without deviation up to boundary BC (O-ray and
E-ray are not separated till the ray reach the boundary BC as optic axis is parallel to the plane
of the paper.

Fig 11.9
On interring the prism BCD at P it is split up into O-ray and E-ray and O-ray passes without
deviation while the E-ray is deviated as well as dispersed. If the prism is made of Quartz the

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E-ray is deviated towards the base of the prism BCD because for quartz µ0 < µe. In case of
calcite the E-ray is deviated to the other side as for calcite µ0>µe. Here the emergent E-ray is
chromatic while O-ray is achromatic.
11.9.2 Wollaston Prism
In construction like rochon prism Wollaston prism is also made of two right angled
prism ABC and BCD made of quartz or calcite and cemented together with glycerin or castor
oil to enable them to be used with ultra violet light so as to form a rectangular block with
their refracting angles equal but here the optic axis of first prism ABC is perpendicular to
base AB (figure 11.9 (b)). If a ray of light is made to incident normally on the face AC, then
O-ray and E-ray travel along the same direction but with different speeds. After passing BC
they exchange their nature i.e. ordinary ray behaves as the extraordinary and extraordinary as
ordinary wave since the optic axis here is at right angles to that in ABC. Both rays are
deflected in opposite direction as refractive index for one ray increases and for other ray
decreases. In this way they form two widely separated plane polarized beams with mutually
perpendicular plane of vibration. This prism is particularly useful in determining the
percentage of polarization in a partially polarized beam.

Fig. 11.10
Double image prisms are mostly used in telescopes for finding the angular diameter of
planets, spectrophotometers and pyrometers.

11.10 DICHROISM OR SELECTIVE ABSORPTION

As explained earlier certain crystal like calcite, tourmaline etc. shows double refraction
in which beam of unpolarised light splits up into E-ray and O-ray having vibration in the
plane of incidence and perpendicular to plane of incidence. Some of the doubly refracting
crystals or minerals absorbs either of E-ray or O-ray to a greater extent than the other.
Tourmaline is one of such crystal which shows the property of selective absorption and
absorbs O-ray to a greater extent than E-ray. Even a few mm thick crystal of tourmaline
absorbs the O-ray completely and transmits only E-ray which is completely plane polarized
with its vibration in the plane of incidence which is completely plane polarized. The
phenomenon of selective absorption of one of the either ray (O-ray or E-ray) is called

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dichroism and crystals showing this property are called dichroic crystals. However the use of
dichroic tourmaline crystal as a polarizer is limited as it also absorbs E-ray partially and the
absorption of E-ray varies with wavelength. Therefore, whenever a white unpolarised light is
incident on dichroic tourmaline crystal, emergent light is strongly chromatic.

11.11 POLAROIDS
In 1928, E.H. Land utilized the phenomenon of selective absorption in the
construction of polaroids. Polaroids are basically large sized polarizing films mounting
between two thin glass sheets. As these are inexpensive therefore widely used as a polarizer
for commercial purposes.

Figure 11.11
The Polaroid sheets can be fabricated in different manner.
1. Ultramicroscopic crystals of herpathite (an organic compound of iodine and quinine
sulphate) are embedded in a thin sheet of nitrocellulose in such a manner that optic axis of all
of them are parallel so that they function as single crystal of large dimensions. These
herphatite crystals exhibits dichroism and even a very small thickness (≈ 0.005 inch) can
absorb one of the doubly refracting ray completely.
2. H Polaroid: A sheet of polyvinyl alcohol (PVA) is heated and then stretched 3 to 8 times at
its original length. During the stretch process the molecule of PVA are aligned along the
direction of the stress and the material under stress becomes doubly refracting. It is than
exposed to iodine vapor. The iodine atoms attach themselves to the straight long chain of
PVA molecule and consequently form long parallel conducting chain. Electrons from iodine
can move easily along the aligned chain but not perpendicular to them. When unpolarised
light is incident on the Polaroid sheet it absorbs E-ray because of the dissipative effects of the
electron motion in the chain and O-ray is completely transmitted.
3. K Polaroid: If instead of iodine vapor the stretched film of PVA is heated in the presence
of dehydrating agent it becomes strongly dichroic and very stable too.
For the protection of the Polaroid sheets these are generally mounted between plates of glass.
11.11.1 Uses of Polaroids
1. Polaroids are used to produce and analyze plane polarized light in modern polarizing
instruments as they are relatively lower cost than Nicol.

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2. They are widely used as polarized sun glasses in goggles to cut off glare produced by light
reflected from polished and shining surface just as road and car hoods etc. to reach the eyes.
The light reflected from such types of surfaces is partially plane polarized by reflection
having more horizontal vibration than vertical. Polaroids with a vertical transmission plane
avoids these horizontal vibrations and reduce glare considerably.
3. They are also used to control the intensity of light in aeroplane and train. For this one
Polaroid is fixed outside the window and other is rotable and fitted inside. By rotation of
inner Polaroid the intensity of light entering inside the train or aeroplane can be adjusted.
4. Polaroid glasses are used to produce 3D moving pictures.
5. They are also used improve the color contrast in old oil painting as well as in photography.

11.12 POLARIZATION BY SCATTERING

If a narrow beam of natural unpolarized light passes through a transparent medium
containing a suspension of ultramicroscopic particle, the scattered light is found to be
partially polarized. When incident light passes through the scattering medium, Electrons in
the scattering medium starts to vibrate and emits most light in a direction perpendicular to its
vibration. No light is emitted in a direction parallel to its vibration. Hence light scattered
through about 900 to the incident direction is strongly polarized. The direction of vibration of
light vector will be perpendicular to the plane of the paper. When ordinary unpolarized light
passes through the atmosphere containing extremely small suspended particles of dust, smoke
etc. it is scattered in all directions. The light is partially polarized. Blue colour of sky and red
colour of sunrises and sunsets is due to the scattering of light ray from small atmospheric
particles having dimensions less wavelength of light.

Figure 11.12

11.13 SUMMARY

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Splitting of a beam of unpolarized light into two refracted beam named ordinary ray (O-
ray) and extraordinary ray (E-ray), when falls upon calcite or quartz like crystals, is called
double refraction or birefrigerence. Both E-ray and O-ray are plane polarized with their plane
of polarization are at right angles to each other with O-ray vibrations perpendicular to the
plane of paper while vibrations of E-ray is parallel to the plane of paper. E-ray and O-ray
travel with the same speed and in the same direction along the optic axis. Crystal which has
only one optic axis is known as uniaxial crystal and those two optic axes are known as biaxial
crystal. However in case of biaxial crystal both the refracted rays are extraordinary. In order
to explain the double refraction exhibited by uniaxial crystal Huygens extended his wave
theory of secondary wavelets to explain double refraction in uniaxial crystals.
Double refraction is used to construct an optical device Nicol prism fabricated from
calcite crystal for producing and analyzing plane polarized light. It’s constructed in such a
way that O-ray is eliminated by total internal reflection and we get only the plane polarized
E-ray coming out of the Nicol.
The main results of Huygens construction for double refraction into uniaxial crystal
using Huygens theory can be summarized as under. When Optic axis is inclined to the
refracting edge of calcite crystal, O-ray and E–ray travels along different direction with
different velocities. In other case E-ray travels faster than O-ray inside the crystal. Thus these
ray comes out with a certain path difference when optic axis in the plane of incidence and
parallel to the refracting edge of calcite crystal.
If Optic axis is perpendicular to the plane of incidence and parallel to the refracting
edge of calcite crystal although the O- and E-rays are not separated and travel along same
direction, but with different velocities. However if Optic axis is perpendicular to the
refracting edge and lying in the plane of incidence of calcite crystal O- and E-rays are not
separated and travel along same direction with same velocity.
Double image prisms are optical devices used if two widely separated images due to
both the ray on the emergent side of the prism is desirable. The Rochon prism and the
Wollaston prism are two such double image prisms made by double refracting crystals (either
quartz or calcite). .
Some of the doubly refracting crystals or minerals absorbs either of E-ray or O-ray to a
greater extent than the other. Tourmaline is one of such crystal which shows the property of
selective absorption and absorbs O ray to a greater extent than E-ray. Polaroids are basically
large sized polarizing films mounting between two thin glass sheets, show the phenomenon
of selective absorption and are of multiple uses.
Polarized light can also be obtained by scattering of unpolarized light through a
transparent medium containing a suspension of ultramicroscopic particle, Blue colour of sky
and red colour of sky in sunrises and sunset are natural examples of polarized light from
scattering.

11.14 GLOSSARY

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Double Refraction: Splitting of a beam of unpolarised light into two refracted beam is called
double refraction or birefringence.
Ordinary and Extra-ordinary Ray: In double refraction the refracted ray which obeys the
ordinary laws of refraction is called ordinary ray or O-ray while the refracted ray which do
not obeys the ordinary laws of refraction is called extraordinary ray or E- ray.
Optic Axis: It is a direction along which or parallel to it a ray does not exhibit double
refraction i.e. E-ray and O-ray travel with the same speed and in the same direction along the
optic axis. Any line parallel to optic axis is also an optic axis,
Uniaxial and Biaxial Crystal: Crystal which has only one optic axis is known as uniaxial
crystal e.g., calcite, quartz, tourmaline While Borax, mica, aragonite & selenite, there are two
optic axes. Such crystals are known as biaxial crystal. However in case of biaxial crystal both
the refracted rays are extraordinary.
Principal Section and Principal Plane: A plane containing the optic axis and perpendicular
to the two opposite faces of crystal is called the principal section of the crystal for that pair of
plane. The principal plane of the ordinary ray is as the plane containing optic axis and O-ray
and the plane containing optic axis and E-ray is called principal plane for E-ray.
Positive and Negative Crystals: Because of splitting of ordinary ray into two different ray
both have different wavefront. When the spherical wavefront of O-ray is enclosed by the
ellipsoidal wavefront of E-ray, crystals are called negative crystals e.g., calcite. If the
ellipsoidal wavefront of E-ray is enclosed by the spherical wavefront of O-ray, crystals are
called positive crystals e.g., quartz.
Nicol Prism: Nicol prism is an optical device fabricated from calcite crystal and constructed
in such a way that O-ray is eliminated by total internal reflection and we get only the plane
polarized E-ray coming out of the Nicol.
Double Image Prisms: Double image prisms are optical devices used if two widely
separated images due to both the ray on the emergent side of the prism is desirable. The
Rochon prism and the Wollaston prism are two such double image prisms made by double
refracting crystals (either quartz or calcite).
Dichroism: Some of the doubly refracting crystals or minerals absorbs either of E-ray or O-
ray to a greater extent than the other. The phenomenon of selective absorption of one of the
either ray is called dichroism and crystals showing this property are called dichroic crystals.
Polaroids: Polaroids are basically large sized polarizing films mounting between two thin
glass sheets. Polaroids are widely used as polarizer and analyzer for plane polarized light in
modern polarizing instruments as they are relatively lower cost than nicol. They are also used
in polarized sun glasses in goggles, in aeroplane and train, to produce 3D moving pictures
and photography etc.

11.15 REFERENCE BOOKS

1. Optics by Ajoy Ghatak

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2. A textbook of Optics by Brij Lal and Dr. N. Subrahmnyam

3. Optics by Dr. S.P. Singh and Dr. J.P. Agarwal

11.16 SUGGESTED READINGS

1. Fundamental of Optics by F. A. Jenkins and H. E. White.
2. The Feynman Lectures on Physics by Richard Feynman
3. Optics by Eugene Hecht

11.17 TERMINAL QUESTIONS

11.17.1 Short Answer Type Questions
1. What is double refraction?
2. What are ordinary and extraordinary rays?
3. Distinguish between i) Positive and negative crystal. ii) Uniaxial and Biaxial crystals
4. Unpolarized light falls on two polarizing sheets in crossed position so no light is
transmitted from the combination. Can any light be transmitted if a third polarizing sheet
is placed between them? Explain.
5. Write short notes on
i) Double refraction ii) Dichroism iii) Polarized light by scattering
iv) Principal section and principal plane v) Optic axis

11.17.2 Long Answer Type Questions

1. What is double refraction? Give the Huygens theory of double refraction in uniaxial
crystal?
2. Explain the construction and working of Nicol prism? How it can be used as polarizer and
analyzer.
3. Explain the construction and working of double image prism. Compare its properties with
a Nicol prism
4. Explain the propagation of ordinary and extraordinary wavefronts in a calcite crystal for
normal incidence with optic axis
(i) Parallel to refracting edge (ii) Normal to the refracting edge
(iii) Inclined to the refracting edge
5. What is dichroism? Explain the construction and uses of Polaroids.
11.17.3 Numerical Questions
1. Two nicols have parallel setting hence intensity of transmitted light is maximum.
Through what angle should either Nicol be rotated in order to drop the intensity to one
half of its maximum value? (Ans.± 450)
2. 2. Two plane polarized adjacent light beam A and B having mutually perpendicular plane
of vibration are analyzed by a Nicol prism. In one position of analyzer beam A shows
zero intensity. From this position when Nicol is rotated through angle 30 0, intensity of
both beams becomes same .Calculate the intensity ratio of both the beam.

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(Ans. IA/IB=2:1)
3. 3. Two Nicols are first crossed and then one of them rotated through 600.Calculate what
percentage of incident light will be transmitted. (Ans. 37.5 %)
4. 4. A plane polarized light is incident on a plate of calcite crystal with its faces parallel to
the optic axis. If the light is incident with vibrations at an angle 450 on the face of the
crystal. Calculate the ratio of the intensities of ordinary and extraordinary ray.
(Ans. 1:1)
5. A partially polarized light beam is passed through a Nicol. the intensity of beam changes
to 60% of maximum intensity when nicol is rotated through 900. Find the degree of
polarization. (Ans. 25%)
11.17.4 Objective Questions
1. Plane polarized light produced by Nicol prism is due to
a. Reflection b. Total internal reflection
c. Refraction d. None of the above
Ans. Option ‘b’
2. Calcite crystal is an example of
a. Positive uniaxial crystal b. Negative biaxial crystal
c. Positive biaxial crystal d. Negative uniaxial crystal
Ans. Option ‘d’
3. Phenomenon of selective absorption of either of E-ray or O-ray is called
a. Double refraction b. Diffraction
c. Dichroism d. Total internal reflection
Ans. Option ‘c’
4. E-ray and O-ray travels with the same velocity along the
a. Along the optic axis b. along all the direction
c. Perpendicular to optic axis d. none of the above
Ans. Option ‘a’

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UNIT 12: PLANE, CIRCULARLY AND

ELLIPTICALLY POLARIZED LIGHTS

CONTANTS
12.1 Introduction
12.2 Objectives
12.3 Retardation plates
12.3.1 Quarter Wave Plate
12.3.2 Half Wave Plate
12.4 Application of Retardation Plates: Theory of the Production of Polarized Light
12.4.1 Production of Plane Polarized Light
12.4.2 Production of Elliptical and Circular Polarized Light
12.5 Experimental Arrangements for the Production of Polarized Light
12.6Application of Retardation Plates: Analysis of Polarized Light
12.6.1 Analysis of Plane Polarized Light
12.6.2 Analysis of Circular Polarized Light
12.6.3 Analysis of Elliptical Polarized Light
12.7Rotatory Polarization
12.8 Summary
12.9 Glossary
12.10 References
12.11 Suggested Readings
12.12 Terminal Questions
12.13 Answers

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12.1 INTRODUCTION
As of now, you will be well familiarized with the phenomenon of birefringence (or
double refraction) in birefringent crystals (or doubly refracting crystals). Birefringence is a
phenomenon of the production of two refracted rays instead of one after a beam of ordinary
unpolarized light is allowed to pass through the birefringent crystals or doubly refracting
crystals. These two refracted rays are generally known as ordinary and extraordinary rays are
plane polarized and having perpendicular plane of vibrations. The ordinary rays (O-rays)
obey the ordinary laws of refraction while the extraordinary rays (E-rays) do not obey the
laws of refraction. These two rays travel with different speed in the birefringent crystals. The
difference between the speeds of these two light rays produces the difference in the refracting
angle.
For negative birefringent crystals, the refractive index for ordinary light is greater than
extraordinary light (μo>μe), in contrast to positive crystal where the refractive index for
extraordinary light is greater to ordinary light (μo<μe). For a given thickness of the
birefringent crystal, a path difference or phase difference has been produced between the
ordinary rays (O-rays) and extraordinary rays (E-rays) due to the different speeds inside the
crystal. In this unit, we will study the various applications of birefringence property of doubly
refracting crystals to produce and analyze plane, circularly and elliptically polarized light.

12.2 OBJECTIVES
After studying this unit, you should be able to
 Understand the concept of retardation plates
 Solve problems on retardation plates
 Apply the properties of retardation plates in producing plane, circular and elliptically
polarized waves.
 Analyze the plane, circular and elliptically polarized waves using retardation plates.
 Understand the concept of rotatory polarization.

12.3 RETARDATION PLATES

A retardation plate, also known as plate retarders or wave plates, is an optically
transparent birefringent crystal which resolves a beam of unpolarized light into two
orthogonal components (ordinary light rays and extra ordinary light rays); change the relative
phase difference between the components; then recombines the components into a single
beam with new polarization characteristics. These plates are very useful to produce different
kind of polarized light and convert one type of polarized light to other.
A retardation plate is generally a plane-parallel plate of a birefringent crystal like
quartz, mica, magnesium fluoride and sapphire, with the optic axis in the plane of the surface.
It is oriented so that incident polarized light may be resolved into components projected along

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the optic axis and perpendicular to it. These two components will experience a relative phase
shift (retardation) proportional to the thickness of the plate. When the fractional part of this
retardation is a nonzero value, the waveplate modifies the polarization state of incident
polarized light polarizations from one state to another. A waveplate does not polarize light,
but modifies the state of polarized light. Further, a relative phase shift produced by the
retardation plate is subject to the availability of both components (E- rays and O- rays) of
incident light which are parallel and perpendicular to its optic axis.
The amount of retardation can be expressed as birefringence times thickness.
Birefringence (μo – μe) varies with temperature and wavelength. For a given temperature and
wavelength, one can form the retardation plate or the wave plate for producing a wanted
phase shift between the two components (ordinary light rays and extra ordinary light rays) for
specific purposes. The retardation plates are mainly of two types as follows.
12.3.1 Quarter Wave Plate
A retardation plate of such a thickness that it produce a path difference of λ/4 (quarter
of the wavelength of incident light) or a phase difference π/2 between the two components
(ordinary light rays and extra ordinary light rays) of incident light beam passing through it, is
known as quarter wave plate. Hence, for a quarter wave plate
Path difference = (μo~μe) x thickness
or λ/4 = (μo~μe) t ,
Where, t is the thickness of the doubly refracting crystal or birefringent crystal
𝜆
𝑡= ....... (12.1)
4(𝜇𝑜 ∼𝜇𝑒 )

12.3.2 Half Wave Plates

If the thickness of the retardation plate is such that it produce a path difference of λ/2
(half of the wavelength of incident light) or a phase difference π between the two components
(E-ray and O-ray) of incident light beam passing through it, then this retardation plate is
known as half wave plate. Hence, for a half wave plate
Path difference = (μo~μe) x thickness
λ/2 = (μo~μe) t ,
Wheret is the thickness of the doubly refracting crystal or birefringent crystal
𝜆
𝑡= ....... (12.2)
2(𝜇𝑜 ∼𝜇𝑒 )

Example 12.1: Plane polarized light is incident on a piece of quartz cut parallel to the axis.
Calculate the least thickness for which the ordinary ray and the extraordinary ray combine to
form the pane polarized light. Given µo = 1.5442, µe = 1.5533, and λ = 5×10-5cm.
Solution: The ordinary ray and the extraordinary ray combine to form the plane polarized
light on emergence if the plate introduce a phase difference of π or a path difference of λ/2

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between the ordinary ray and the extraordinary ray. The plate, which introduces a phase
difference of π or path difference of λ/2 between the ordinary ray and extraordinary ray, is the
half wave plate.

The data given are µo=1.5442, µe=1.5533, λ=5×10-5cm

1 𝜆
Here, 𝑡= ( )
µ𝑜 −µ𝑒 2

is the least thickness of a half-wave plate in positive crystals

1 5×10−5
Therefore, 𝑡= ×( ) cm = 2.75×10-3 cm.
1.5442−1.5533 2

Example 12.2: Calculate the thickness of the mica sheet required to make a quarter-wave
plate and a half-wave plate for λ=5460 Å. The indices of refraction for the ordinary and
extraordinary waves in mica are 1.586 and 1.592 respectively.
Solution: The data given are µo=1.586, µe=1.592, λ=5460× 10-8cm.
Thickness for the quarter- wave plate in the positive crystals is given by
1 𝜆
𝑡4 = µ (4)
𝑒 −µ0

1 5460×10−8
or 𝑡4 = 1.592−1.586 ( ) = 2.275×10-3 cm.
4

Thickness for the half-wave plate in positive crystals is

1 𝜆
𝑡2 = ( )
µ𝑒 −µ0 2

1 5460×10−8
𝑡2 = 1.592−1.586 ( ) = 4.55×10-3 cm.
2

12.4 APPLICATION OF RETARDATION PLATES: THEORY OF

THE PRODUCTION OF POLARIZED LIGHT
You have well understood so far about a birefringent crystal and its property of
birefringence (production of E-rays and O-rays with their plane of vibrations perpendicular to
each other), when an unpolarized light passes through it. Now, consider a beam of plane
polarized light having their vibrations of electric field along one axis only perpendicular to
the direction of propagation of beam, falls normally on a birefringent crystal (like calcite
crystal) cut with optic axis parallel to its faces. Let A = XY be the maximum amplitude of
incident light which makes angle θ with optic axis of crystal. The plane polarized light split
up in two components ordinary light (O-ray) and extra ordinary light (E-ray) with amplitude
XY cos θ along XA and XY sin θ along XB.

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As discussed earlier, these two rays travel with different speeds but in same direction
inside the crystal.

Figure 12.1
Due to this, the E-ray and O-ray develops a path difference or phase difference (δ) after
emerging from the crystal. If A is the amplitude of the incident wave and plane polarized
wave (XY=A in figure 12.1), the E-rays and O–rays vibrates along perpendicular directions
with amplitude Acosθ and Asinθ with a phase difference of δ depending upon the thickness of
the crystal. The form of resultant vibration in the resultant vibration will be given by the
resultant of these two components having simple harmonic vibrations in perpendicular
directions with angular frequency ω and phase difference δ.
Let us consider the equations of these two components as
𝑋 = 𝐴 𝑐𝑜𝑠 𝜃 𝑠𝑖𝑛 (𝜔𝑡 + 𝛿) ....... (12.3)
𝑌 = 𝐴 𝑠𝑖𝑛 𝜃 𝑠𝑖𝑛 𝜔𝑡 ....... (12.4)
Let A cos θ = ɑ and A sin θ = b, the above equation reduces to new form
𝑋 = ɑ 𝑠𝑖𝑛 (𝜔𝑡 + 𝛿) ....... (12.5)
𝑌 = 𝑏 𝑠𝑖𝑛 𝜔𝑡 ....... (12.6)

Y2
From equation (12.6), we have, 𝑠𝑖𝑛𝜔𝑡 = 𝑌/𝑏 and 𝑐𝑜𝑠𝜔𝑡 = √(1- )
b2

X
And from equation (12.5), = sinωt cosδ + cosωt sinδ
ɑ

X Y Y2
= b cos δ + √(1- ) sin δ
ɑ b2

X Y Y2
or - cos δ =√(1 - b2
) sin δ
ɑ b

Squaring on both sides, we get,

X2 Y2 𝑋 Y Y2
2 + 2 cos2 δ – 2( ) . ( ) cosδ = (1 - ) sin2 δ
ɑ b 𝑎 b b2

On simplifying, we obtain,
X2 Y2 𝑋𝑌
+ - 2( ) cos δ = sin2 δ ....... (12.7)
ɑ2 b 2 𝑎𝑏

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Thus the resultant wave equation is a general equation of ellipse. The polarization state
of emerging light beam will depend on δ. We will discuss the special cases of above equation
depending upon the phase difference (δ) between two components after emergence from the
crystal:
12.4.1 Production of Plane Polarized Light
Case 1: When δ = 0, 2π, 4π, 6π . . . . . . , we have, sin δ = 0 and cos δ=1
From equation (12.7), we have,
X2 2

ɑ2
+Yb2 - 2(
𝑋𝑌
𝑎𝑏
)= 0

𝑋 𝑌 2
( − ) =0
𝑎 𝑏

b
or Y= X ....... (12.8)
𝑎
𝑏
This is an equation for straight line with slope . Hence, the resultant light beam will be plane
𝑎
b
polarized with plane of vibration in a direction tan-1( ). If the amplitudes of both components
𝑎
are same (ɑ = b), the resultant polarized light will vibrate at an angle of 450 with x axis.
Case 2: When δ = π, 3π, 5π, 7π . . . . . . then sin δ = 0 and cos δ= - 1
From equation (12.7), we have,
X2 Y2 XY
+ + 2( )= 0
ɑ2 b 2 𝑎b

𝑋 𝑌 2
( + ) =0
𝑎 𝑏
𝑋 𝑌
( + ) =0 ....... (12.9)
𝑎 𝑏

Hence, the resultant light beam will be plane polarized with plane of vibration in a direction
b
tan-1(− ).If the amplitudes of both components are same (ɑ = b), the resultant polarized light
𝑎
will vibrate at an angle of 1350 with x axis.
12.4.2 Production of Elliptical and Circular Polarized Light
Case1: When δ = 3π/2, 7π/2, 11π/2, 15π/2 . . . . . ., we have, sin δ = -1 and cos δ=0.
From equation (12.7), we get,
X2 Y2
+ =1 ....... (12.10)
ɑ2 b 2

This is an equation of symmetrical ellipse with major and minor axes as ɑ and b. Hence,
the resultant light beam will be elliptical polarized. From figure 12.2, we draw some position
of the both of components (E-ray and O-ray) having phase differences of δ = 3π/2, 7π/2,
11π/2, 15π/2....... . When the displacement of one component (X= ɑ sin (ωt + δ)) is

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maximum, the other has zero displacement (Y= b sin ωt = 0). As the displacement of the first
component comes below its maximum displacement, an increase in the displacement of
second component starts with phase differences of δ = 3π/2, 7π/2, 11π/2, 15π/2. This process
continues up to when the displacement of X- component becomes zero and the Y- component
shows maximum displacement. The resultant of these intermediate displacements of both the
component shows a clockwise or right handed rotation. Hence, it can be concluded that the
resultant wave will be right handed elliptically polarized light.

Y= b sin (ωt)
X= ɑ sin (ωt + δ)
Figure 12.2
If the amplitudes of both the components (E–ray and O–ray) are same, i.e., ɑ = b. The above
equation of ellipse reduces to following form:
X2 + Y2 = ɑ2

This is an equation of circle. The emergent light will be right handed (clockwise) circularly
polarized light if the of E-ray and O-ray has same amplitudes and maintains a phase
difference of δ = 3π/2, 7π/2, 11π/2, 15π/2 . . .
Case2: When δ = π/2, 5π/2, 9π/2, 13π/2 . . . . . ., we have, sin δ = -1 and cos δ=0
From equation (12.7), we obtain,
X2 Y2
+ =1
ɑ2 b 2

This is again an equation of symmetrical ellipse with major and minor axes are ɑ and b, but
the resultant is a combination of E- rays and O- rays with a phase differences which are
different to Case 1. In this case, the resultant light beam will be left handed elliptical
polarized. From figure 12.2, we can understand this situation. When the displacement of one
component (X= ɑ sin (ωt + δ)) is maximum, the other has zero displacement (Y= b sin (ωt) =
0). As the displacement of the first component comes below its maximum displacement, an
increase in the displacement of second component will appears with a phase difference of
π/2, 5π/2, 9π/2, 13π/2...... .
Y= b sin (ωt)

X= ɑ sin (ωt + δ)

Figure 12.3
This process continues to the condition where the displacement of X-component becomes
zero and the Y-axes has maximum displacement. The resultant of these intermediate

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displacements of both the component shows an anticlockwise or left handed rotation. Hence,
it can be concluded that the resultant wave will be right handed elliptically polarized light. If
the amplitudes of both the components (E– ray and O– ray) are the same, i.e., ɑ = b then
above equation of ellipse reduces to following form
X2 + Y 2 = ɑ 2

This is an equation of circle. The emergent light will be left handed (anti clockwise)
circularly polarized light if the of E- ray and O- ray has same amplitudes and maintains a
phase difference of δ = π/2, 5π/2, 9π/2, 13π/2..... .

12.5. EXPERIMENTAL ARRANGEMENTS FOR THE

PRODUCTION OF POLARIZED LIGHT
1. Plane Polarized Light
When a beam of unpolarized monochromatic light is allowed to pass through a polarizer
(Nicol Prism), it splits up into two components (O-rays and E-rays). Both rays are plane
polarized with perpendicular plane of vibrations. The O-rays, follows the ordinary laws of
refraction, total internally reflected and get absorbed inside the Nicol prism. The only ray
coming out from the Nicol prism is E-rays, which is plane polarized. The Experimental
arrangement to produce plane polarized light contains a Nicol prism which is placed after the
unpolarized source light (figure 12.4).

Unpolarized light Polarized light

Nicol Prism
PrPrism

Figure 12.4

2. Circularly Polarized Light

Circularly polarized light is the resultant of two mutually perpendicular vibrations of
equal amplitudes with a phase difference of π/2. To obtain circularly polarized light, the
experimental arrangement shown in Figure 12.5.
Circularly or elliptically
Polarized light
Unpolarized light Polarized light

Polarizing Nicole Analyzing Nicole

PrPrism
Rotating QWP
plate
Figure 12.5
The unpolarized light is allowed to fall on a Nicol prism. The emergent light from Nicol
is plane polarized light. Another Nicol prism (Analyzer) is placed at some distances from
polarizing Nicol with crossed position. In this position no light is transmitted from second

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Nicole (Analyzer), hence the field of view appears dark. A quarter wave plate (QWP) is then
introduced between two Nicole in the path of plane polarized light such that polarized beam
falls normally on the rotating quarter wave plate. The amplitude of polarized beam on
entering the quarter wave plate is splits up into two mutually perpendicular components and
receives a phase difference of π/2 after emergence. Now, the quarter wave plate rotated and
the position of the quarter wave plate is such that the vibration in the incident polarized light
make an angle of 450 with the optic axis of the plate. In this position the plane polarized light
on entering the quarter wave plate is split up into two components (E-ray and O-ray) of equal
amplitudes.
In this way the resultant of two perpendicular components with equal amplitudes having
a phase difference of π/2 produces a circularly polarized light after emerging out from the
quarter wave plate.
3. Elliptically Polarized Light
This is a more general case than circular polarization, in which there is a phase
difference of π/2 between the two components. Elliptical polarization is the result when the
components are equal with non-quarter-wave phase difference.
To obtain the elliptically polarized light, the experimental arrangement and undergoing
processes is the same as shown in figure 12.5. The only change is the rotation of quarter wave
plate should maintain such that the vibrations of light incident on it makes any angle other
than 450. This makes two perpendicular components with unequal amplitudes having a phase
difference of π/2 and the resultant wave produces an elliptically polarized light.

12.6 APPLICATION OF RETARDATION PLATES: ANALYSIS

OF POLARIZED LIGHT
12.6.1 Analysis of Plane Polarized Light
When a given light is allowed to pass through a rotating Nicol prism and the intensity of
emergent light varies from zero to maximum. The given light beam is plane polarized.

Intensity varies from

Given light Rotating Nicol zero to maximum Plane Polarized
PrPrism Intensity light
Figure 12.6

12.6.2 Analysis of Circularly Polarized Light

When a given light is allowed to pass through a rotating Nicol prism and the intensity of
emergent light remains unchanged, this shows that incident light beam is either circularly
polarized or un-polarized.

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Circularly Polarized
Intensity remains
Given light Rotating Nicol unchanged or

Unpolarised light
Figure 12.7
Now, given light is first passed through a quarter wave plate (QWP). If the given light is
circularly polarized light, then it splits into two components (E-ray and O-ray) and because of
QWP there is a phase difference of π/2 developed between them in addition to existing phase
difference of π/2 which exists because of it being a circularly polarized light.
Therefore, the total phase difference between the E-rays and O-rays becomes 0 or π, which
results in rectilinear vibrations and the emergent light is now plane polarized. If we pass this
light again through a rotating Nicol prism, the intensity of emergent light will vary with zero
to maximum intensity.

Intensity varies Circularly

with zero to Polarized
Given light Rotating Nicol maximum Intensity

Intensity
QP Unpolarised
remains
unchanged light
Figure 12.8
If the intensity of emergent light will remain unchanged even after passing a quarter wave
plate and rotating Nicole, this shows that the incident light beam is unpolarised light.
12.6.3 Analysis of Elliptically Polarized Light
When beam of incident light is passed through the rotating Nicol prism and the intensity of
emergent light varies from maximum to non-zero intensity, the incident light may be
elliptically polarized or partially plane polarized light.

Figure 12.9
Now, given light is first passed through a quarter wave plate (QWP). If the given light is
elliptically polarized light, then it splits into two components (E-ray and O-ray) and because
of QWP there is a phase difference of π/2 developed between them in addition to existing
phase difference of π/2 which exists because of it being an elliptically polarized light.
Therefore, the total phase difference between the E- rays and O-rays becomes 0 or π, which
results in rectilinear vibrations and the emergent light is now plane polarized.

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If we pass this light again through a rotating Nicol prism, the intensity of emergent light
will vary with zero to maximum intensity. On the other hand, if incident light is a partially
plane polarized light, a mixture of plane polarized and unpolarized light, then after passing
through a quarter wave plate it cannot become plane polarized and thus the intensity through
the Nicol prism will change from maximum to non-zero.

Intensity varies Elliptically

with zero to Polarized
Given light Rotating Nicol maximum Intensity
PrPrism
Intensity Partially
QP remains Plane
unchanged Polarised
Figure 12.10

12.7 ROTATORY POLARIZATION

Rotatory polarization or optical activity is an ability of any substances to rotate the
plane of incident linearly polarized light. The substances rotate the plane of vibration in
clockwise direction are known as dextrorotatory substances and those substances produce a
counterclockwise rotation are known as levorotatory substances. This property was
discovered in quartz in 1811 by Arago. Two different crystalline structures of quartz produce
d-rotatory and l-rotatory behavior. In the case of many naturally occurring organic
compounds such as turpentine, sugar and, tartaric acid optical activity also shows rotatory
polarization in the liquid state. This shows that the activity is associated with the individual
molecules themselves. The detailed studies of rotatory polarization or optical activity are
given in next unit (Unit 14).

Self Assessment Questions

1. How will you convert right circularly polarized light into left circularly polarized light?
2. How will you convert plane polarized light circularly polarized light into left circularly
polarized light?
3. Fill in the blank
(i) When the emergent light is ……… ………polarized, the intensity varies from maximum
to zero minimum after passing through rotating Nicol.
(ii) When a given light is allowed to pass through a rotating Nicol prism and the intensity of
emergent light remains unchanged, this shows that incident light beam is either
………………… or ……………………...

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12.8 SUMMARY
In this unit, you have studied about the application of birefringence or double refraction
in birefringent crystals or doubly refracting crystals. To present the clear understanding of
motion, some basic concepts like ordinary light and extraordinary light have been discussed.
You have studied that the quarter wave plates and half wave plate’s produces phase
differences of π/2 and π respectively having numerous applications. These plates are formed
by cutting the doubly refracting crystal for desired thickness. When a plane polarized light
incident on a half wave plate, the emergent light is plane polarized. When a plane polarized
light incident on a quarter wave plate such that E–rays and O–rays make an angle 450 with
the optic axis, the emergent light is circularly polarized. You have also studied the theory
behind the production of circularly and elliptical polarized lights. The phase difference
between E-ray and O- ray is a deciding factor for clockwise circularly polarized light or
counter clockwise circularly polarized light. By using quarter wave plate and Nicol prism,
you can check about the polarization sate of any given light. You have also introduced about
the rotatory polarization or optical activity. The substance showing this property is known as
optically active substance. These are of two types: laevorotatory and dextrorotatory.

12.9 GLOSSARY
Birefringence: Double refraction
Ordinary light ray: A light ray obeys the ordinary laws of refraction (Snell’ law)
Extraordinary light ray: A light ray does not obey the ordinary laws of refraction (Snell’s
law)
Limited: Restricted
Undergo: Suffer
Maintain: Sustain
Interactions: Exchanges
Resist: Refuses to go along with
Friction: Resistance
Retardation plates: The doubly refracting crystals cut along the thickness to produce a
special phase difference or path difference
Assessment: Evaluation

12.10 REFERENCES
1. Optics, IGNOU, New Delhi
2. Engineering Physics, S.K. Gupta, Krishna Publication, Meerut.
3. Objective Physics, Satya Prakash, AS Prakashan, Meerut.

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4. Optics by E. Hecht, 4th edition, Pearson Education Inc., New Delhi.

5. Physics, Part I, S. L. Gupta, Shubham Publication, Delhi.

12.11 SUGGESTED READINGS

1. Optics, Ajoy Ghatak, Tata McGraw Hill
2. Introduction to Electrodynamics by D.J. Griffith, Pearson Publication.
3. Waves by F.S. Crawford Jr., Berkeley Physics Course Vol. 3.
4. Optics by M.V. Klein and T.E. Furtak, Wiley.

12.12 TERMINAL QUESTIONS

12.12.1ObjectiveType Questions
1. When a plane polarized light is incident on a quarter wave plate with its vibration making
an angle of 450 with the optic axis, the emergent light is
(a) elliptically polarized
(b) plane polarized
(c) a mixture of elliptically and circularly polarized
(d) circularly polarized
2. When a plane polarized light is incident on a quarter wave plate with its vibration making
an angle other than 450 with the optic axis, the emergent light is
(a) elliptically polarized
(b) plane polarized
(c) a mixture of elliptically and circularly polarised
(d) circularly polarized
3. When a plane polarized light is passed through a half wave plate, the emergent light is
(a) elliptically polarized
(b) plane polarized
(c) a mixture of elliptically and circularly polarized
(d) circularly polarized
4. When a plane polarized light is passed through a quarter wave plate, the emergent light is
(a) elliptically polarized
(b) plane polarized
(c) either elliptically or circularly polarized depending on the angle of incidence
(d) circularly polarized

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5. Circularly polarized light is produced if the amplitudes of the ordinary and extraordinary
rays are equal and there is a phase difference of
(a) π
(b) π/2
(c) π/4
(d) 0
6. Elliptically polarized light is produced if the amplitudes of the ordinary and extraordinary
rays are unequal and there is a phase difference of
(a) π
(b) π/2
(c) π/4
(d) 0

7. When elliptically polarized light, after passing through quarter wave plate, is observed
through a rotating Nicol, the emergent light would have shown
(a) the variation of intensity with minimum not zero
(b) no variation in intensity
(c) the variation of intensity with minimum zero
(d) the intensity of incident and emergent light is same
8. When elliptically polarized light is observed through a rotating Nicol, the emergent light
would have shown
(a) the variation of intensity with minimum not zero
(b) no variation in intensity
(c) the variation of intensity with minimum zero
(d) the intensity of incident and emergent light is same
9. When a circularly polarized light, after passing through quarter wave plate, is observed
through a rotating Nicol, the emergent light would have shown
(a) the variation of intensity with minimum not zero
(b) no variation in intensity
(c) the variation of intensity with minimum zero
(d) the intensity of incident and emergent light is same
10. In an elliptically polarized light
(a) amplitude of vibration changes in direction only
(b) amplitude of vibration changes in magnitude only
(c) amplitude of vibration changes in magnitude and direction both
(d) none of above

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12.12.2 Short Answer Type Question

1. What is the polarization state of emergent light if a plane polarized light is incident on a
half wave plate ?
2. A plane polarized light is incident on a quarter wave plate such that O- rays and E-rays
make an angle 450 with the optic axis. What will be polarization state of the emergent
light?
3. A plane polarized light is incident on a quarter wave plate such that O- rays and E-rays
make an angle other than 450 with the optic axis. What will be polarization state of the
emergent light is?
4. What do you mean by rotatory polarization?
12.12.3 Long Answer Type Questions
1. Give the construction and theory of quarter wave plate and half wave plate.
2. Describe how, with the help of Nicole prism and quarter wave plate, plane polarized,
circularly polarized and elliptically polarized lights are produced and detected.
3. What is meant by plane-polarized circularly polarized and elliptically polarized light?
Show that the plane polarized and circularly polarized lights are special case of
elliptically polarized light.
4. How will you find whether a given beam of light is ordinary, plane polarized, circularly
polarized or elliptically polarized?

12.12.4 Numerical Answer Type Questions

1. Calculate the thickness of a calcite plate which would convert plane polarized light into
circularly polarized light. The principal refractive indices are μ0 = 1.658 and μe = 1.486 at
the wavelength of light used as 5890 Å.
2. Calculate the thickness of mica sheet required for making a quarter wave plate for λ =
5460 Å. The indices of refraction for the ordinary and extraordinary rays in mica are
1.586 and 1.592.
3. Calculate the thickness of a double refracting plate capable of producing a path difference
of λ/4 between ordinary and extraordinary waves. (λ = 5890 A, µo = 1.53 and µE = 1 .54).
4. Calculate thickness of quarter wave plate for light of wavelength 5000 Å. Given µo = 1.54
and ratio of velocity extraordinary to ordinary wave is 1.006.
5. We have a QWP of calcite corresponding to 4046 Å. A left circularly polarized light λ=
7065Å is incident on the plate. Obtain the polarization of the emergent beam. Use the
information λ = 4046 Å, 7065 Å, µο = 1.68134, 1.65207, μE = 1.49694, 1.48359.
6. Quartz plate has μο = 1.54 and μe = 1.55 for sodium light. What minimum thickness of the
quartz plate between the crossed polarizer and analyzer will produce annulment of the
light, while the plate is cut parallel to optic axis?
7. (a) A left circularly polarized beam (λ = 5893 Å) is incident normally on a calcite crystal
(with its optic axis cut parallel to the surface) of thickness 0.0005141mm. Find the state
of polarization of the emergent beam. Given μο = 1.65836 and μe = 1.48641.
8. Repeat the above problem for quartz plate where all parameters are same except that μ e =
1.54425 and μe = 1.55356.

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9. A quarter waveplate is meant for wavelength 5893 Å. How much phase retardation it will
show for wavelength 4300 Å?

12.13 ANSWERS
12.13.1 Self Assessment Questions
1. A right circularly polarized light is a combination of two plane polarized light with equal
amplitudes and having a phase difference of 3π/2, 7π/2, 11π/2; while a left circularly
polarized light is a combination of two plane polarized light with equal amplitudes and
having a phase difference of π/2, 5π/2, 9π/2. Hence, if a right circularly polarized light is
allowed to pass through a half wave plate, which provide an additional phase difference of π
between the two plane polarized light, the emergent light will be left circularly polarized
light.
2. After passing through a quarter wave plate, the plane polarized light is converted to
circularly polarized light.
3. (i) plane
(ii) Circularly polarized, unpolarized
12.13.2 Terminal Questions
Objective Type Question
1. (d) , 2. (a) , 3. (b) , 4. (c), 5. (b) , 6. (b) , 7. (c) , 8. (a) , 9. (c) , 10. (c)
Short Answer Type Questions
1. Polarization state of emergent light will be plane polarized light.
2. Polarization state of emergent light will be circularly polarized light.
3. Polarization state of emergent light will be elliptically polarized light.
4. Rotatory polarization or optical activity is an ability of any substances to rotate the plane of
incident linearly polarized light. The substances rotate the plane of vibration in clockwise
direction are known as dextrorotatory substances and those substances produce a
counterclockwise rotation are known as levorotatory substances
Numerical Type Questions
1. Thickness = λ / [4 (μ0–μe)] = 0.856 μm
2. t = λ / 4 (μe - µo )= 5460 x 10-8/ [ 4 (1.592 -1.586) ] =2.275 x 10 -2 cm
3. t = λ / 4 (μe - µO ) = 5.890 x 10-5 / [4 (1.54 -1.53) ] = 1 .47 x 10-3 cm
4. t = λ / 4(μe - µO ) =5000 x 10 -10/ [4 ( 1.54 - 1.53 )] = 1.25 x 10 -5 cm
5. t = λ / 4(μe - µO ) = 4046 x 10 -10/ [4 ( 1.68134 - 1.49694 )] = 4046 x 10 -10/ [0.7376] =
0.54 μm

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For λ = 7065 Å, birefringence = (μe - µO) t = (1.65207 – 1.48359) 0.54 x 10-6 = 90980 Å
& the path difference for λ = 7065 Å is λ / 0.0775
6. t = λ / 2(μe - µO ) = 5893 x 10 -10/ [2 ( 1.55 - 1.54 )] = 5893 x 10 -10/ [0.02] = 29.5 μm
7. Birefringence = (μe - µO) t = (1.65836 – 1.48641) 5141 x 10-10 = 883.99 Å
& the path difference for λ = 5893 Å is λ /6.67
8. Birefringence = (μe - µO) t = (1.55356 – 1.54425) 5141 x 10-10 = 47.86 Å
& the path difference for λ = 5893 Å is λ /123.12
9. t = λ / 4(μe - µO) or t (μe - µO ) = λ / 4 = 5893 Å / 4 = 1473 Å
For λ = 4300 Å, λ/ [t (μe - µO)] =4300 / 1473 = 2.91.
Hence a path difference between E-ray and O-ray is λ/2.91 or a phase change of
(2π/λ) (λ/2.91) = 2π / 2.91 = 123.710

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UNIT 13: OPTICAL ACTIVITY

CONTANTS
13.1 Introduction
13.2 Objectives
13.3 Rotatory Polarization and Optical Activity
13.3.1 Laevo-rotatory Substances
13.3.2 Dextro-rotatory Substances
13.4 Fresnel’s Explanation of Optical Activity
13.5 Specific Rotation
13.6 Polarimeters
13.6.1 Laurent’s Half Shade Polarimeter
13.6.2 Biquartz Polarimeter
13.7 Summary
13.8 Glossary
13.9 References
13.10 Suggested Readings
13.11 Terminal Questions
13.12 Answers

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13.1 INTRODUCTION
When two Nicol prisms (polarizer and analyser) are placed in the line of vision in the
crossed position, no light gets through and the field of view is dark. Now, a crystal of quartz
or a tube of sugar solution is placed between the crossed polarizer and analyzer, the light
reappears. This shows that the quartz or sugar solution has rotated the plane of polarization of
emergent plane polarized light after the first Nicole prism. In this unit, you will study about
this property and its types. The angle through which the plane of polarization is rotated and
the direction of rotation will also be discussed.

13.2 OBJECTIVES
After studying this unit, you should be able to
 Understand the concept of rotatory polarization or optical activity
 Apply the properties of optical activity to measure specific rotation.
 Solve problems on specific rotation
 Understand different kinds of polarimeters.

13.3 ROTATORY POLARIZATION AND OPTICAL ACTIVITY

When a plane polarized light is allowed to pass through certain substance, it is found
that the plane of vibration of the emergent light is not the same as that of the incident light
and it is rotated through a certain angle. This property of rotating the plane of vibration and
plane of polarization is called rotatory polarization or optical activity and the substances
which rotates plane of polarized light are called optically active substances, e.g., sugar
solution, turpentine, sodium chlorate, cinnabar etc. Compounds that are optically active
contain molecules that are chiral. Chirality is a property of a molecule that results from its
structure. Optical activity is a macroscopic property of a collection of these molecules that
arises from the way they interact with light.

Optically
Nicole Prism active Rotating Nicole
PrPrism substance PrPrism

Figure 13.1
This ability of rotating the plane of polarization in certain substances was discovered by
a French Scientist Arago in 1811. In figure 13.1, a Nicol prism (polarizer) is placed with
monochromatic source of light. The emergent light after passing through Nicol prism is plane
polarized, which is again passing through an optically active substance like turpentine. It is
clearly observable from figure 13.1 that the plane of polarization and plane of vibration of

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emergent plane polarized light are different from the incident plane polarized light. This can
be verified by using a second Nicol prism as an analyzer.
The rotation of polarized light by optically active substances has massive applications
in different field of science. It provided insight into the physics of light, the structure of
molecules, and the nature of life etc. The optically active substances are of two types
(i) Dextra rotatory or right handed.
(ii) Laevo rotatory or left handed.

13.3.1 Dextro Rotatory

The substance which rotates plane of polarization of light towards right or in clockwise
Direction are called dextro rotatory or right handed.
13.3.2 Laevo Rotatory
The substance which rotates plane of polarization of light towards left or in anti-
clockwise directions are called laevo rotatory or left handed.

13.4 FRESNEL’S THEORY OF OPTICAL ROTATION

Fresnel’s theory of optical rotation is based on the fact that a linearly plane polarized
light consists of resultant of two circularly polarized vibrations rotating in opposite directions
with same angular velocities. Fresnel’s made the following assumptions
(1) When a beam of plane polarized light enters in a crystal along the optic axis, it is broken
up into two circularly polarized vibrations, one right handed and the other left handed.
(2) When a plane polarized light enters a crystal of an optically inactive substance (like
calcite) along optical axis, it breaks up into two circularly polarized vibrations rotating in
opposite direction with same angular frequency or velocity such that resultant of these two
vibrations at all point of time is along the optic axis. The Vibrations of clockwise direction
rotation in (in figure 13.2 (a)), are represented by OR and vibration rotating in
counterclockwise direction are represented by OL. The resultant OR and OL at all the point
of time will be along AB.
(3) In case of an optically active substance (like Quartz), a linearly polarized light on entering
the crystal is resolved into two circularly polarized vibrations rotating in opposite direction
with different angular velocity or frequency. In case of left handed optically active quartz
crystal, anticlockwise vibrations travel faster, while in case of right handed optically active,
quartz crystal, clockwise vibrations travel faster.
Consider a right handed quartz crystal in which clockwise component travels faster than
left handed component. Suppose at any instant of time, right handed component traverse
angle δ greater than left handed component as shown in figure 13.2 (b). The new position of
resultant of L and OR will be along CD i.e., plane of vibration of light has been rotated

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through angle δ/2 towards right after passing through quartz crystal. The angle δ/2 depends
upon thickness of crystal.

Figure 13.2

(a) For Optically Inactive Crystals: when linearly plane polarized light enters a calcite
crystal it gets resolved into two circularly polarized vibrations. One is moving anticlockwise
with same angular frequency or velocity. As each circularly polarized vibration further
consist of two rectangular components having phase differences π/2, so for clockwise circular
vibration
𝑥1 = ɑ 𝑠𝑖𝑛𝜃 = ɑ 𝑠𝑖𝑛 𝜔𝑡
𝑦1 = ɑ 𝑐𝑜𝑠 𝜃 = ɑ 𝑐𝑜𝑠𝜔𝑡
For anticlockwise circular vibration
𝑥2 = − ɑ 𝑠𝑖𝑛 𝜃 = − ɑ 𝑠𝑖𝑛 𝜔𝑡
𝑦2 = ɑ 𝑐𝑜𝑠 𝜃 = ɑ 𝑐𝑜𝑠 𝜔𝑡
From above, the resultant displacement of vibrations along x-axis and y-axis respectively are
given as,
𝑥 = 𝑥1 + 𝑥2 = ɑ 𝑠𝑖𝑛 𝜃 – ɑ 𝑠𝑖𝑛 𝜃 = 0
𝑦 = 𝑦1 + 𝑦2 = ɑ 𝑐𝑜𝑠 𝜔𝑡 + ɑ 𝑐𝑜𝑠 𝜔𝑡 = 2ɑ 𝑐𝑜𝑠 𝜔𝑡
Hence resultant vibration has amplitude 2a and is plane y-axis, i.e., along original direction
AB. Hence two oppositely circularly polarized vibrations give rise to a plane polarized
vibrations.
(b) For Optically Active Crystal (Quartz): When linearly plane polarized light enters
quartz crystal (right handed), it gets resolved into circularly polarized vibrations moving in
opposite direction with different angular frequency or velocity. In case of right handed crystal

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clockwise vibrations travel faster than anticlockwise vibrations. Let at any instant of time
anticlockwise vibrations has traversed angle e and clockwise vibrations has traversed
angle(𝜃 + 𝛿). Therefore, component of two circular vibrations at that instant of time will be,
for clockwise vibration
𝑥1 = ɑ 𝑠𝑖𝑛 (𝜔𝑡 + 𝛿)
𝑦1 = ɑ 𝑐𝑜𝑠 (𝜔𝑡 + 𝛿)
For anticlockwise circular vibration
𝑥2 = – 𝑎 𝑠𝑖𝑛 𝜔𝑡
𝑦2 = 𝑎 𝑐𝑜𝑠 𝜔𝑡
From resultant displacement of vibrations along x-axis and y-axis respectively are given as
𝑋 = 𝑥1 + 𝑥2 = ɑ 𝑠𝑖𝑛 (𝜔𝑡 + 𝛿) – ɑ 𝑠𝑖𝑛 𝜔𝑡 = 2ɑ 𝑠𝑖𝑛 𝛿 /2 𝑐𝑜𝑠 (𝑤𝑡 + 𝛿/2)
𝑌 = 𝑦1 + 𝑦2 = ɑ 𝑐𝑜𝑠 (𝜔𝑡 + 𝛿) + ɑ 𝑐𝑜𝑠𝜔𝑡 = 2ɑ 𝑐𝑜𝑠 𝛿/2 𝑐𝑜𝑠 (𝜔𝑡 + 𝛿/2)
The resultant vibration along x-axis and y-axis are in same phase, so resultant of these
vibrations is plane polarized and makes an angle of δ/2 with original direction AB. Thus,
plane of vibrations get rotated through angle ő/2 towards right after passing through a right
handed quartz crystal. From above, we get
𝑡𝑎𝑛 𝛿/2 = 𝑦/𝑥 ....... (13.1)
Angle of Rotation
If µ𝑅 and µ𝐿 be the refractive indices of quartz crystal for right handed and left handed
vibrations respectively then optical path difference on passing through a quartz crystal slab of
thickness 𝑡 is given as,
Path difference =(µ𝐿 − µ𝑅 ) 𝑡
If λ be the wavelength of light used, then phase difference,
𝛿 = 2𝜋/𝜆 (µ𝐿 − µ𝑅 ) 𝑡.
Angle of rotation
𝜃 = 𝛿/2 = 𝜋/𝜆 (µ𝐿 − µ𝑅 ) 𝑡 ....... (13.2)
If vL and vR be the velocities of the left handed and right handed circular vibrations
respectively, then, µL = c/vL and µR = c/vR where c is the velocity of light.
𝜋𝑡 𝑐 𝑐
𝜃= ( ∼ )
𝜆 𝑣𝐿 𝑣𝑅

𝜋𝑐𝑡 1 1
Or 𝜃= ( ∼ )
𝜆 𝑣𝐿 𝑣𝑅

If T be the time period, then c = λ/T or 1/T = c/ λ

𝜋𝑡 1 1
𝜃= ( ∼ )
𝑇 𝑣𝐿 𝑣𝑅

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In case of left handed substances (case of dextrorotatory substance), 𝑣𝑅 > 𝑣𝐿

𝜋𝑡 1 1
𝜃𝐿 = ( − ) ....... (13.3)
𝑇 𝑣𝐿 𝑣𝑅

In case of left handed substances (case of laevorotatory substance), 𝑣𝐿 > 𝑣𝑅

𝜋𝑡 1 1
𝜃𝑅 = ( − ) ....... (13.4)
𝑇 𝑣𝑅 𝑣𝐿

In case of non-optically active substances, θ = 00. Hence the direction of vibrations remains
unchanged. Such substance does not exhibit the phenomenon of optical rotation.
Experimental Verification
Fresnel’s verified his hypothesis by arranging a number of right handed and left handed
prism of quartz to form a parallelepiped. Axes of the prism are parallel to the bases. When a
plane polarized light is incident normally on one face, the beam splits into two circularly
polarized beams which are widely separated. These are analyzed by a rotating Nicol. It was
observed that the intensity of emergent beam is constant. This establishes the fact that both
beams are circularly polarized as assumed by Fresnel.

Example 13.1 The refractive indices for right- and left-handed vibrations are 1.5580 and
1.55821 respectively for quartz for sodium light of wavelength 5890 Å. Find the optical
rotation for the same light by quartz of thickness 1.00 mm when its faces are cut
perpendicular to the optic axis.
Solution: The angle of rotation of the plane of vibration is given as
𝜋𝑡
𝜃 = (µ𝑅 − µ𝐿 )
𝜆
The data given are
t = 1.00 mm = 0.100 cm, λ = 5890 Å = 5890×10-8 cm, µR = 1.55810, µL = 1.55821
Putting these data into the aforementioned equation, we get the angle of rotation as

𝜋 × 0.1
𝜃= (1.55821 − 1.55810)
5890 × 10−8

= 0.5870 radian = 33.630

13.5 SPECIFIC ROTATION

When a linearly plane polarized light is passed through an optically active medium/
substance, the plane of linearly polarized light gets rotated through certain angle either
towards left or right. The angle through which plane polarized light get rotated depends upon
(i) thickness of the medium
(ii) density of active substance or concentration of solution

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(iii) wavelength of light

(iv) temperature.
Hence, mathematically θ L,
θ C,
θ λ,
and θ t
 θ L C λ t
or θ=SLCλt
or S=θ/(LCλt)
Where S is proportionality constant and is known as specific rotation. θ = rotation in degree,
L = length of tube in decimeter, C = concentration of solution in gm/ cc, t = temperature and
λ. = wavelength of linearly polarized light
For given wavelength and temperature,
S = θ / ( LC) (when L is in decimeter) ....... (13.5)
or S = 10θ / ( L C ) (when L is in centimeter) ....... (13.6)
Hence, specific rotation is defined as the rotation produced by 1 decimeter long solution of
concentration 1 g/cc at given temperature for given wavelength. The rotation produced by
optically active/medium/substance can be measured by polarimeter.

Example 13.2: The plane of polarization of a plane polarized light is turned through an angle
12.60 passing through a 10% sugar solution of length 20 cm. Calculate the specific rotation.
Solution- The data given are Ɵ = 12.60, L = 20 cm, C = 10% = 0.1 gm/cm3
10𝜃 10×12.6
The specific rotation S is given by 𝑆 = = = 630 cm2/gm
𝐿𝐶 20×0.1

13.6 POLARIMETER
Polarimeters are the optical instruments designed to measure the angle of rotation of
plane polarized light after passing through an optically active substance. When used for
finding the optical rotation of sugar it is called a Saccharimeter. If the specific rotation of
sugar is known, the concentration of sugar solution can be determined. Generally, there are
two types of polarimeters used
1. Laurent’s half-shade polarimeter
2. Bi-quartz polarimeter.

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13.6.1 Laurent’s Half-Shade Polarimeter

Construction: The optical parts of a Laurent’s half shade polarimeter are shown in figure
13.3. A source of monochromatic light (usually a sodium lamp) is placed at the focus of a
convex lens, which makes the emerging light rays parallel. This parallel light beam is allowed
to fall on a polarizer (usually Nicol prism P) such that the emerging light become plane
polarized light. This plane polarized light is then passing through the half shade device H
(called Laurent’s), a glass tube T containing the solution of optically substance and an
analyzer (Nicol prism A). The light is viewed through a telescope E. The analyzing Nicol A
can be rotated about the axis of the tube and its rotation can be measured with the help of a
vernier scale on the graduated circular scale C divided in degrees and mounted on telescope.

Figure 13.3
Working: The Laureate half shade device is a combination of two semicircular plates of
glass (ADB) and quartz (ACB) shown in figure 13.4. The quartz plate is cut such that its
optic axis becomes parallel to the line of separation AOB. The thickness of the quartz plate is
chosen such that it works as half wave plate and offers a phase difference of π between the O-
rays and E-rays. The thickness of the glass plate is such that it absorbs the equal amount of
light that by the quartz half wave plate.

Figure 13.4
The source light is first allowed to pass through the polarizer P and emergent plane
polarized light with vibrations along OP is incident normally on the half-shade plate (figure
13.4). On passing through the glass half (right half) the vibrations will remain along OP, but
on passing through the quartz half (left half), the vibrations along OP split up into two
perpendicular components (E -rays and O-rays) along OQ (in left half or through quartz half)
and perpendicular to OQ (in right half or glass half). The analysis of the emergent light after
Laurent half shade can be done with the help of another Nicol as follows

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(1) If the analyzing Nicol is fixed with its principal plane parallel to OP, the plane polarized
light through glass half will pass and hence it will appear brighter than the quartz half from
which light is partially obstructed.
(2) If the principal plane of the Nicol is parallel to OQ the quartz will appear brighter than the
glass half due to the same reason.
(3) When the principal plane of the analyzing Nicol is parallel to AOB, the two halves will
appear equally bright. It is because the vibrations emerging out of the two halves are equally
inclined to its principal plane and hence two components will have equal intensity.
(4) When the principal plane of the analyzer is at right angle to AOB again the components of
OP and OQ are equal. The two halves are again equally illuminated, but as the intensity of
the components passing through is small as compared to that in the previous case, the two
halves are said to be equally dark.
The eye can easily detect a slight change when the two halves are equally dark. The readings
are, therefore, taken for this position.
Application: The Laurent half shade polarimeter can be used to measure the concentration
of sugar solution. For this purpose, the following steps to be followed:
(1) Fill the polarimeter tube with water and find the reading on a circular scale corresponding
to equally dark position of the half shade device.
(2) Now fill the tube completely with the given sugar solution and again find the reading on
the circular scale for equally dark positions of the half shade device.
(3) The difference between the scale readings gives the optical rotation θ produced by the
given length l in decimeters of the sugar solution.
(4) If S is the specific rotation of sugar for the same wavelength and at the same temperature,
then concentration
C = θ/(LS) g/cc ....... (13.7)
Polarimeter is of great importance in the industries for estimating the quantity of sugar in the
presence of an optically inactive impurity. A polarimeter calibrated to read directly the
percentage of cane-sugar in the solution is called as saccharimeter.
Merits and Demerits of Laurent’s Half Shade Polarimeter
 This instrument is suitable for monochromatic light source. It is usually constructed for
sodium source.
 When position is adjusted for equally dark halves, it gives fairly accurate observation as
slight rotation of the analyzer changes the intensity of two halves.
13.6.2BiquartzPolarimeter
It is also an instrument used for finding the optical rotation of certain optical active
solutions. The arrangement of bi-quartz polarimeter is similar to that of a Laurent’s half-
shade polarimeter except a white light is used instead of monochromatic light.

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Construction: The experimental arrangement for a bi-quartz polarimeter is shown in

figure13.5. According to the figure 13.5, the light from a source S (white light) which is just
an ordinary bulb after passing through a slit is incident on a convex lens L which converts the
emergent light into a parallel beam. This parallel beam of light falls on a NicolP (polarizer)
and the emergent light from P become plane polarized light.

Figure 13.5
This plane polarized light is then passed through the Biquartz plate B, a glass tube T
containing the solution of optically active substance and an analyzer (Nicol prism A). The
light is viewed through a telescope E. The analyzing Nicol A can be rotated about the axis of
the tube and its rotation can be measured with the help of a vernier scale on the graduated
circular scale C divided in degrees and mounted on telescope E.
Working: A bi-quartz plate is made up of two semicircular plates out of which one is of left
handed quartz and other of right handed quartz. The quartz plates are cut perpendicular to
their optic axes and cemented along a diameter (YZ) as shown in figure 13.6. In order to
remove double refraction, the optic axes of both semicircular quartz plates are made
perpendicular to their planes. The thickness of each plate is nearly 4mm which is so much
that each plate can rotate the plane of polarization of yellow light through 90°. One clockwise
rotation is through right handed quartz and other anticlockwise rotation is through left handed
quartz.
The working of biquartz polarimeter may be divided into following points
(1) A white light is incident on Nicol P and the emergent light becomes plane polarized. This
plane polarized light is then allowed to fall on the bi-quartz plate B normally and it travels
in the direction of optic axis. In the plate the rotatory dispersion of every color takes place
in opposite direction. (∵ 𝜃 𝛼 1/𝜆²).
(2) If the vibrations of incident plane polarized light after passing through polarizer are along
YZ direction (figure 13.7), then the emergent light from the bi-quartz plate for yellow

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Figure 13.6

color the vibrations will be perpendicular to YZ because the vibrations are rotated through
90° by bi-quartz, whereas the red light is rotated through the smallest amount and that of
violet light is rotated through the largest angle.

(a) (b)
Figure 13.7

(3) If the principal section of is placed parallel to YOZ, the yellow colour vibrations
terminates and due to the mixture of red and blue, both the halves appear to the similar and
has grayish violet tint, called the tint of passage [figure13.7(b)].
(4) Now if the principal section of A is rotated through a small angle from his position as
shown R1B2 in figure13.8(a), then right half appears more bluish and left more red. If the
principal section of Nicol N2 is rotated in opposite direction R2B1 [figure13.8 (b)] we see an
observation just opposite to the above.
The tint is very sensitive and depends on position. Hence, it can be used for accurate
determination of optical rotation.

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(a) (b)
Figure 13.8
Application: Determination of Specific Rotation of Sugar Solution
(1) The position of analyzer (A) is adjusted to (a sensitive tint) without using optically active
solution. The position of the analyzer is recorded using a mounted circular scale on the
telescope E.
(2) Now place the optically active solution (sugar solution) in the tube T and the position of
A is again adjusted so as to get again sensitive tint (grayish shade) and its position noted
again. The difference between the two positions of A gives the optical rotation produced by
the optically active solution.
(3) The length L of solution is found in decimeter and the concentration C of the solution is
found in g/cc by using the formula S = θ/(LC), specific rotation of optically active solution,
such as sugar, can be determined.
Merits of Bi-quartz Polarimeter
 As the transition from red to blue is very rapid, the zero position can be obtained very
accurately.
 For this instrument, white light can also be used.
 It is highly sensitive device for measuring optical rotation.
Demerits of Bi-quartz Polarimeter
 This instrument does not give accurate result for colorless optically active substance.
 It is not possible for a color blind person to use this instrument.
Self Assessment Question
1. Specific rotation is measured using _____
2. Two unknown sample liquids are measured in a polarimeter. They are found to have the
same observed rotation. What is the relationship of these two liquids?
3. For which type of molecules can you measure specific rotation?
4. What is saccharimeter?
5. The substances that rotate the plane of vibration in the clockwise direction is known as

13.7 SUMMARY
In this unit, you have studied about the substances that rotate the plane of polarization
of light which is passed through them. These substances are called optically active substances

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and the property is called as optical activity. So, this was discussed along with the concept of
specific rotation. To present the clear understanding of optical activity and specific rotation,
Fresnel’s theory was discussed which explains the reason of rotatory polarization and its
dependency of the speed of left handed and right handed components of plane polarized
light. The polarimeters are used to produce polarized light and also to determine the rotation
of its plane of polarization when passed through an optically active substance e. g. sugar
solution. Specifically Laurent half shade polarimeter and biquartz polarimeter with their
working and principal were discussed. The concept of polarimeter may be extended to
Saccharimeter, an instrument to determine the concentration of sugar solution based on the
measurement of angle of rotation of the plane of polarization of light. Many solved examples
are given in the unit to make the concepts clear. To check your progress, self assessment
questions (SAQs) are given place to place.

13.8 GLOSSARY
Optical rotation: rotation in plane polarized light when passing through optically active
substances
Ordinary light ray: A light ray obeys the ordinary laws of refraction (Snell’ law)
Extraordinary light ray: A light ray does not obey the ordinary laws of refraction (Snell’s
law)
Limited: restricted
Undergo: suffer
Maintain: sustain
Polarimeter: device to measure optical rotation
Assessment: evaluation

13.9 REFERENCES
1. Optics, IGNOU, New Delhi
2. Engineering Physics, S.K. Gupta, Krishna Publication, Meerut
3. Objective Physics, Satya Prakash, AS Prakashan, Meerut
4. Optics by E. Hecht, 4th edition, Pearson Education Inc., New Delhi
5. Physics, Part I, S. L. Gupta, Shubham Publication, Delhi

13.10 SUGGESTED READINGS

1. Optics, Ajoy Ghatak, Tata McGraw Hill
2. Introduction to Electrodynamics by D.J. Griffith, Pearson Publ

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3. Waves by F.S. Crawford Jr., Berkeley Physics Course Vol. 3

4. Optics by M.V. Klein and T.E. Furtak Wiley

13.11 TERMINAL QUESTIONS

13.11.1 Objective type
11. The substance that show the phenomenon of optical rotation are said to be
(e) optically active (b) optically inactive
(c) crystals (d) polaroids
12. In optically inactive substances, the relation between the velocities of right handed and
left handed circularly polarized lights is
(a) vR > vL (b) vR < vL
(c) vR = vL (d) none of these
13. In a dextro-rotatary substances, the relation between the refractive indices of right handed
and left handed circularly polarized lights is
(a) μR > μL (b) μR < μL
(c) μR = μL (d) none of these
14. In a leavo-rorotatary substances, the relation between the refractive indices of right
handed and left handed circularly polarized lights is
(a) μR > μL (b) μR < μL
(c) μR = μL (d) none of these
15. Choose the incorrect statement. The amount of rotation of the plane of polarization is
(e) Directly proportional to the concentration of the solution
(f) inversely proportional to the concentration of the solution
(g) least for violet and greatest for red
(h) directly proportional to the length of optically active substance
16. The device which measures the angle through which the plane of polarization of a plane
polarized beam is rotated by a given medium is called
(e) Babinet compensator (b) Polarimeter
(c) Refractometer (d) Polaroid
17. The device based on the measurement of angle of rotation of the plane of polarization of
light and measures the concentration of sugar solution is called
(a) Babinet compensator (b) Saccharimeter
(c) Refractometer (d) Polaroid
18. Dextro rotatory optically active substance rotates the plane of vibrations

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(a) In clockwise direction (b) In anticlockwise direction

(c) By 1800 (d) None of these
19. Laevo rotatory optically active substance rotates the plane of vibrations
(a) In clockwise direction (b) In anticlockwise direction
(c) By 1800 (d) None
20. Polarimeter is the instrument designed for
(a) measures the angle of rotation produce by a substance
(b) to study nature of the polarized light
(c) to study the phase difference between circularly polarized light and elliptically polarized
light.
(d) none of these
21. A tube 2 decimeter long is filled with a cane sugar solution 20 gm in 100 cm3. The
specific rotation of cane sugar is 600. The angle of rotation of plane polarized light
passing through it is
(a) 150 (b) 200
(c) 240 (d) 300
22. A sugar solution in a tube of length 20 cm produces optical rotation of 130. The solution
is then diluted to one third of the previous concentration. The optical rotation produced by
a 30 cm long tube containing the diluted solution is
(a) 5.50 (b) 6.50
(c) 7.50 (c) 8.50
13.11.2 Short Answer Type Questions
1. What do you mean by optical rotations?
2. Define plane of vibration and plane of polarization
3. For which type of molecules can you measure specific rotation?
4. Which device is used to measure the optical rotation?
5. Which device is used to measure the concentration of sugar solution?

13.11.3 Long Answer Type Questions

1. Give two differences between Laurent’s half shade polarimeter and biquartz polarimeter.
2. Define specific rotation. Describe the construction and working of Laurent’s half shade
polarimeter. Discuss the relative merit of biquartz polarimeter and half shade polarimeter.
3. What is optical activity? Describe the construction, theory and working of biquartz
polarimeter to find the optical rotation of a solution and also discuss the action of biquartz
plate in it.

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4. What do you understand by specific rotation? Describe the construction and working of
biquartz polarimeter to find the specific rotation of sugar solution and discuss the utility of
biquartz plate in it.

13.11.4 Numerical Answer Type Questions

1. The plane of polarization of plane polarised light is rotated through 6.50 in passing through
a length of 2.0 decimeter of sugar solution of 5 % concentration. Calculate the specific
rotation of sugar solution. A quarter wave plate is meant for wavelength 5893 Å. How
much phase retardation it will show for wavelength 4300 Å?
2. A tube of sugar solution 20 cm long is placed between crossed Nicols and illuminated with
light of wavelength 6 x 10-5 cm. If the optical rotation produced is 130 and the specific
rotation is 650/dm/gm/cm3, determine the strength of the solution.
3. Determine the specific rotation of a given sample of sugar solution if the plane of
polarization is turned through 13.2°. The length of the tube containing 10% sugar solution
is 20 cm.
4. On introducing a polarimeter tube 25 cm long and containing sugar solution of known
strength, it is found that the plane of polarization is rotated through 10°. Find the strength
of sugar solution in g/cm3. Given that specific rotation of sugar solution is 60° per
decimeter per unit concentration.
5. A tube of 20 cm long with solution of 15 gm of cane sugar in 100 cc of water is placed in
the path of polarization if the specific rotation of cane sugar is 66°.
6. 80 gm of impure sugar when dissolved in a litre of water gives an optical rotation of 9.9 0.
When placed in a tube of length 20 cm. If the specific rotation of sugar is 660, find the
percentage purity of the sugar solution.
7. Calculate the specific rotation if the plane of polarization is turned through 26.40,
traversing 20 cm length of 20 % sugar solution.
8. A Sugar solution in a tube of length 20 cm produces optical rotation of 130. The solution is
then diluted to one-third of its previous concentration. Find the optical rotation produced
by 30 cm.

13.12 ANSWERS
13.12.1 Self Assessment Questions
1. Polarimeter
2. They are the same compound.
3. Chiral molecules or optically active substances
4. A polarimeter calibrated to read directly the % age of cane sugar in the solution is called
saccharimeter.
5. Right handed or dextro-rotatary
13.12.2 Objective Type Questions
1. (a), 2. (c), 3. (b), 4. (a), 5. (c), 6. (b), 7. (b), 8. (a), 9. (b), 10. (a), 11. (c), 12. (b)

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13.12.3 Short Answer Type Questions

1. The rotations produced by a decimeter long column of the liquid containing 1 gm of active
substance in 1 cc of the solution.
2. The plane of polarization is that plane in which no vibrations occur and the plane in which
vibrations occur known as plane of vibration. The vibrations occur at the right angle to the
plane of polarization.
3. Optically active substances e.g. sugar solution.
4. Polarimeter
5. Saccharimeter

13.12.4 Numerical Type Questions

1. S = θ/ (LC) = 65/(2 x 0.05) = 650 (dm)-1 (g/cc)-1
2. S = θ/ (LC), C = θ/(LS) = 13/(2 x 65) = 0.1 g / cc = 10 % .
3. S = 10θ /L .C = 10 x 13.2/20 x 0.1 = 660(dm)-1 (g/cc)-1
4. S = 10θ/ L.C = 60°; C =10θ /L.C =10 x 10 / 25 x 60 =1/15 = 0.0679 g/m3.
5. S = 10θ/L.C; θ = SLC/L.C = 66 x 20 x 15 /10 x 100 = 19.80
6. S = θ/ (LC), C = θ/(LS) = 9.9/(2 x 66) =0.0751 g / cc = 75 g/L and purity % = 93.75 %
7. 20 % = 0.2 g/cc, S = θ/(LC) =26.4/(2 x 0.2) = 66 (dm)-1 (gm/cc)-1
8. S = 10 θ/(LC) = 10 θ’/(L’C’), θ’= 6.50

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UNIT 14: OPTICAL INSTRUMENTS

Structure
14.1 Introduction
14.2 Objectives
14.3 Human eye
14.3.1 Sectional View and Different Parts of Human Eye
14.3.2 Action of Human Eye
14.4 Field of View
14.5 Need of Multiple Lens Eyepieces
14.5.1 Positive and Negative Eyepiece
14.6 Ramsden’s Eyepiece
14.6.1 Construction
14.6.2 Working
14.6.3 Cardinal Points of Ramsden’s Eyepiece
14.7 Huygen’s Eyepiece
14.7.1 Construction
14.7.2 Working
14.7.3 Cardinal Points of Ramsden’s Eyepiece
14.8 Gaussian Eyepiece
14.8.1 Construction
14.8.2 Uses
14.9 Comparison of Huygen’s and Ramsden’s Eyepieces
14.10 Spectrometer
14.10.1 General Layout of a Spectrometer
14.10.2 Types of Spectrometer
14.11 Electron Microscope
14.11.1 Types of Electron Microscopes
14.11.2 Use of Electron Microscope
14.12 Summary

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14.13 Glossary
14.14 Reference Books
14.15 Suggested Readings
14.16 Terminal Questions
14.16.1 Short Answer Type Questions
14.16.2 Long Answer Type Questions
14.16.3 Numerical Questions
14.16.4 Objective Questions

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14.1 INTRODUCTION
An eye is one of the most important organ of our body which works as an optical
instrument and enables us to view all things around us. Optical instruments process light
wave so that image of object is enhanced as well as clearer. Generally optical instrument are
used to look things bigger and to see the fine detail of any object. Optical instruments can be
broadly divided into two categories. One forms real images of an object which is projected
onto a screen. Image can be viewed simultaneously by many observers, e.g., projectors.
Other kind of optical instruments forms virtual images of an object and only one
observer can see the image. The virtual image formed by the instrument is transformed by the
eye into a real image, e.g., spectrometer, microscopes, telescopes.

14.2 OBJECTIVES
After study of unit the student will be able
 To understand the concept of optical instruments
 To explain working of human eye
 To explain different types of eye piece
 To compare different types of eye pieces
 To describe about spectrometer and electron microscopes

14.3 HUMAN EYE

Our eyes are not only vital for seeing the world around us but also an essential part of
all optical instruments. The eye is a slightly asymmetrical sphere in shape and about one inch
(2.5 cm) in diameter. The detailed structure of front part of the eye (the part we see in the
mirror) is discussed below.
14.3.1 Sectional View and Different Parts of Human Eye
The Sclera: It is the white part of the eye which is opaque, fibrous, tough, protective outer
layer of the eye that is directly continuous with the cornea in front and with the sheath
covering the optic nerve behind . It is lined inside with vascular tissue which consists of blood
vessel feeding the eye.
The Iris: It is the pigmented muscular ring and can be of different color contributes to eyes
with different color, i.e., green, blue, brown or black.
The Cornea: It is a clear dome over the iris which acts as an entrance lens for the eye. It is
the clear, transparent front part of the eye that covers the iris, pupil and anterior chamber and

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provides most of an eye’s optical power (if too flat = hyperopia/far sightedness; if too steep =
myopia/near sightedness). It needs to be smooth, round, clear, and tough. It is like a
protective window. The function of the cornea is to let light rays enter the eye and converge
the light rays.
The Pupil: It is the black circular opening with variable diameter in the iris, which lets light
in and opens and closes to adapt to changing light intensity.

Figure 14.1

The Conjunctiva: It is an invisible, clear layer of tissue covering the front of the eye,
except the cornea.
Crystalline Lens: The lens is the natural biconvex lens of the eye behind the iris.
Transparent, biconvex intraocular tissue that helps brings rays of light to focus on the retina
(It bends light, but not as much as the cornea). The lens is held in place by fine ligaments
(zonules) attached between ciliary processes. The lens contains a fibrous jelly, hard at center
and progressively softer at the outer portions.
Anterior Chamber: anterior chamber is in front of the lens and behind the cornea filled
with Aqueous Humor. The eye receives oxygen through the aqueous. Its function is to
nourish the cornea, iris, and lens by carrying nutrients; it removes waste products excreted
from the lens, and maintains intraocular pressure and thus maintains the shape of the eye.
Vitreous Chamber: It is the transparent, colorless gelatinous mass that fills rear two-thirds
of the eyeball, between the lens and the retina. It has to be clear so light can pass through it
and it has to be there or eye would collapse.
Retina: Vitreous Chamber is lined with a sensory layer called retina. The retina is the light
sensitive nerve tissue in the eye that converts images from the eye’s optical system into
electrical impulses that are sent along the optic nerve to the brain, to interpret as vision. The
retina has the shape of hemisphere consists of layers that include two types of light receptor
cells called rods and cones. The human eye has a total of 125 million rod and 6.5 million
cones.

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Cones: The cones are the light-sensitive retinal receptor cell that provides the sharp visual
acuity (detail vision) and color discrimination; most numerous in macular area. Function
under bright lighting.
Rods: The light-sensitive, specialized retinal receptor cell that works at low light levels
(night vision). The rods function with movement and provide light/dark contrast. It makes up
peripheral vision.
Optic Nerve: The optic nerve is the largest sensory nerve of the eye. It carries impulses for
sight from the retina to the brain. Composed of retinal nerve fibers that exit the eyeball
through the optic disc, traverse the orbit, pass through the optic foramen into the cranial
cavity, where they meet fibers from the other optic nerve at the optic chiasm.
Blind Spot: This is the sightless area within the visual field of a normal eye. It is caused by
absence of light sensitive photoreceptors where the optic nerve enters the eye.
Fovea: Contrary to blind spot the fovea is the central pit in the macula that produces the
sharpest vision. It contains a high concentration of cones within the macula and no retinal
blood vessels. The remaining area of the retina is occupied mainly by rods.
Macula: It is the yellow spot in the small (3°) central area of the retina surrounding the
fovea. It is the area of acute central vision (used for reading and discriminating fine detail and
color). Within this area is the largest concentration of cones.
Choroid: The vascular (major blood vessel), central layer of the eye lying between the
retina and sclera. Its function is to provide nourishment to the outer layers of the retina
through blood vessels. It is part of the uveal tract.
Extra Ocular Muscles: There are six extra ocular muscles in each eye.
Rectus Muscles: There are four Rectus muscles that are responsible for straight movements:
Superior (upward), Inferior (lower), Lateral (toward the outside, or away from the nose), and
Medial (toward the inside, or toward the nose).
Oblique Muscles: There are two Oblique muscles that are responsible for angled movements.
The superior oblique muscles control angled movements upward toward the right or left.
Inferior oblique muscles control angled movements downward toward the right or left.
14.3.2 Action of Human Eye
The refractive index of both the aqueous humor and vitreous humor are about 1.336.
The crystalline lens has an average index of 1437. Most of the refraction of light entering the
eye occurs at the outer surface of the cornea. Refraction at the cornea and the surfaces of the
lens produces a real and inverted image of the object on the retina. The optic nerve sends a
signal to the brain.
The image must be formed exactly at the location of retina for the object to be seen
clearly. The eye adjust different object distances by changing the focal length f of its lens by
varying its radii of curvature with the help of ciliary muscle without changing lens to retina
distance. The process of adjustment is called accommodation. However, it is the ability of

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human eye to accommodate in terms of the far point and the near point of the eye. For the
normal eye the far point of the eye is infinity and the near point of the eye is 25 cm [normal
distance of distinct vision (NDDV)]. Range of accommodation diminishes with age as the
crystalline lens grows through a person’s life and the ciliary muscle is less able to distort a
larger lens.

14.4 FIELD OF VIEW

Field of view of an optical instrument may be defined as the extent or range of its visual
area. For an eyepiece it is the widest dimension of an object which is visible through the
eyepiece. Generally it is expressed as the width in feet at 1000 yards or in degree of field.

(a) (b)
Figure 14.2
The field of view is shown without a lens (figure 14.2 (a)) and with a lens (figure 14.2 (b))
When the field of view is expressed in feet, it is called the linear field of view. When it is
shown in degree, it is known as angular field of view. We can convert one to other by using a
formula
1 angular degree of view = 52.5 feet.
As shown in figure light is passing through the aperture AB and reaching the eye E. The eye
can view objects in the angle range MEN. The widest possible angle MEN which we can see,
is called field of view. If a concave lens is added in plane AB from a wider field M’EN’ can
reach the eye. We say the field of view is enlarged. On the other hand convex lens narrow the
field of view.

14.5 NEED OF MULTIPLE LENS EYEPIECES

The basic purpose of optical instrument is to produce a magnified image free from
aberrations and enlarged field of view. A single lens fails to fulfill both the requirements as
the image formed by single lens as it suffers from chromatic and spherical aberration. Also
the field of view is small and with the increase in magnification of optical instruments it
became even smaller as the image formed by marginal rays refracted through the peripheral
portion of eye lens can’t simultaneously enter the small aperture of the pupil of eye placed

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near the eye lens. Hence only the part of image nearer to the axis will be seen and final image
will cover a small field of view.
Field of view is also decreased with the increase in distance between the objective and
eye lens. As the magnifying power depends on distance so field of view is also decreased as
the instrument is set for higher magnifying power by adjusting the distance between objective
lens and eye lens. Now to overcome the shortcomings of a single lens the eye lens is replaced
by multiple lens eyepiece in optical instruments. In general, in an eyepiece there are two lens
made by suitable material and type separated by a suitable distance. The extra lens facing the
objective of instruments is known as field lens. Field lens and eye lens are made and kept in
such a way that their combination minimized chromatic and spherical aberrations. Also the
field lens gathers more of the rays from the objective towards the axis of the eyepiece and
covers all the rays from the image to enter the eye lens. In this way it increases the field of
view.
14.5.1 Positive and Negative Eyepiece
Eyepiece in which the first focal plane of an eyepiece lies in the object space outside the
eyepiece is called positive eyepiece. In such eyepiece a real object can be placed on first focal
plane to be in focus with final image, i.e., Ramsden’s eyepiece. Eyepiece in which the first
focal plane of an eyepiece lies within eyepiece i.e. between eye lens and field lens where no
real object can be place is called negative eyepiece, i.e., Huygen’s eyepiece.

14.6 RAMSDEN’S EYEPIECE

14.6.1 Construction
Ramsden’s Eyepiece consists of two planoconvex lenses made of same material with
their curved (convex) side facing each other inwards (figure 14.3). In this type of eyepiece
focal length of field lens and eye lens is same and distance between field lens and eye lens is
where f is focal length of eye lens. The field lens is a little larger rand is placed close to
intermediate image to allow maximum possible light to pass through it. The eye lens has a
smaller diameter but caries out the actual magnification.

Figure 14.3

Condition of Achromatism
For achromatism the distance d between two lenses should be

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In case of Ramsden’s eyepiece

d=f

But the distance between field lens and eye lens in Ramsden’s eyepiece is kept which is
slightly less than f, therefore, not completely free from chromatic aberration. If the distance
between field lens and eye lens is kept f the field lens will be at the focal plane of eye lens. In
this position any dust particle or scratch would be magnified and final image will not be
clear.
Condition for Minimum Spherical Aberration: The distance between the two lenses
for minimum spherical aberration should be

In case of Ramsden’s eyepiece

It means that it doesn’t satisfy condition for minimum spherical aberration. However to
reduce spherical aberration in Ramsden’s eyepiece two plano convex lenses with their convex
lens facing each other are used.

14.6.2 Working
When eye piece is adjusted for normal vision, the final image formed by it is at infinity.
For this the image formed, the field lens should lie in the first focal plane of the eye lens. The
objective forms the real inverted image I of a distant object. This acts as an object for the
field lens. This gives rise to a virtual image I1. I1 in turns serves as an object for the eye lens
therefore it must be at distance equal to focal length f from eye lens to make final image at
infinity. Hence image I1 lies at a distance f/3 to the left of the field lens.

Figure 14.4
For field lenses the image I1 formed by the objective of the instrument (in which eye-piece is
used) acts as an object for eye lens.
If u is the distance of I1 from the field lens, than from lens formula

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Since here = and , we have,

or = - , Hence

That is, the image I formed by objective lies to the left of the field lens. The rays coming
from I, after emerging from the field lens appear to come from I1 at a distance f/3 left to field
lens. These rays emerge from the eye lens as a parallel beam.
Equivalent Focal Length
Equivalent focal length of Ramsden’s eyepiece is given by

or

When eye piece is adjusted for normal vision, the final image formed by it should be at
infinity. For this the image formed by the equivalent lens must be at infinity. For this I1
should lies in the first focal plane of the equivalent lens.
14.6.3 Cardinal Points of Ramsden’s Eyepiece
Position of Focal Points
The distance of first focal point F1 from the field lens L1 is given by

Where negative sign indicates that first focal point F1 lies at a distance of f/4 to the left of
field lens L1. The distance of second focal point F2 from the eye lens L2 is given by

Where positive sign indicates that second focal point F2 lies at a distance of f/4 to the right of
eye lens L2.
Position of Principal Points
The distance of first principal point H1 from the field lens L1 is given by

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Where positive sign indicates that the first principal point H1 lies at a distance of f/2 to the
right of field lens L1. The distance of second principal point H2 from the eye lens L2 is given
by

Where negative sign indicates that the second principal point H2 lies at a distance of f/2 to the
left of eye lens L2.
Position of Nodal Points
As the medium on either side of the eye piece is same (air), the nodal points N1, N2 coincide
with the principal points H1, H2 respectively.
The position of cardinal points for Ramsden’s eyepiece can be plotted as:

Figure 14.5

Position of Cross Wires

The cross wires, if used, must be placed at the position where the image due to
objective is formed so that they would be in focus with the final image and magnified by both
the lens of eyepiece. The cross wire placed in first focal plane satisfies both the conditions.
Therefore cross wire must be placed at a distance of f/4 in front of field lens. As it is outside
both the lenses in object space and is real hence it is used to examine a real object or real
image hence it is called a positive eyepiece. Therefore this eyepiece is used in optical
instruments where accurate quantitative measurements of distances and angles are made.

Example 14.1: In Ramsden’s eyepiece focal length of eye lens is 6 cm. Locate the position of
cardinal points in a diagram.
Solution: In a Ramsden’s eyepiece field lens and eye lens having same focal length f are
separated by a distance (2/3) f.
Given focal length of eye lens is f = 6 cm. So focal length of field lens is also 6 cm and
separation between lens is, d = (2/3) f = (2/3) 6 = 4 cm.

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The equivalent focal length of Huygen’s eyepiece is

Here

or = cm.

Position of Focal Points

The distance of first focal point F1 from the field lens L1 is given by

Negative sign indicates that first focal point F1 lies at a distance 1.5 cm to the left of field lens
L1. The distance of second focal point F2 from the eye lens L2 is given by

Positive sign indicates that second focal point F2 lies at a distance of 1.5 cm. to the right of
eye lens L2.
Position of Principal Points
The distance of first principal point H1 from the field lens L1 is given by

Positive sign indicates that first principal point H1 lies at a distance of 3 cm to the right of
field lens L1. The distance of second principal point H2 from the eye lens L2 is given by

Negative sign indicates that second principal point H2 lies at a distance of 3 cm. to the left of
eye lens L2.
Position of Nodal Points
As the medium on either side of the eye piece is same (air), the nodal points N1, N2 coincide
with the principal points H1, H2 respectively.
The position of cardinal points for Ramsden’s eyepiece can be plotted as:

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Figure 14.6

 , , ,

Example 14.2: In Ramsden’s eyepiece equivalent focal length is 9 cm. Calculate the position
of cardinal points. If an object is situated at 9 cm. in front of the field lens, find the position
of the image formed by the eyepiece.
Solution: In a Ramsden’s eyepiece field lens and eye lens having same focal length f are
separated by a distance (2/3) f. Given equivalent focal length of eyepiece is F = 9 cm.
The equivalent focal length of Huygen’s eyepiece is

Hence,

So focal length of eye lens is f= 12 cm, focal length of field lens is f = 12 cm and separation
between lens is d = (2/3) f i.e. d = (2/3) x 12 = 8 cm.

Position of Focal Points

The distance of first focal point F1 from the field lens L1 is given by

Negative sign indicates that first focal point F1 lies at a distance 3 cm to the left of field lens
L1. The distance of second focal point F2 from the eye lens L2 is given by

Positive sign indicates that second focal point F2 lies at a distance of 3 cm to the right of eye
lens L2.
Position of Principal Points
The distance of first principal point H1 from the field lens L1 is given by

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Positive sign indicates that first principal point H1 lies at a distance of 6 cm to the right of
field lens L1.
The distance of second principal point H2 from the eye lens L2 is given by

Negative sign indicates that second principal point H2 lies at a distance of 6 cm to the left of
eye lens L2.
Position of Nodal Points:
As the medium on either side of the eye piece is same (air), the nodal points N1, N2 coincide
with the principal points H1, H2 respectively.
The position of cardinal points for Huygen’s eyepiece can be plotted as:

Figure 14.7

Position of Image: Given that object is situated at a distance 9 cm. in front of the field lens.
If u and v are the distance of the object O and image I from the first and second principal
point H1 and H2 .
Then u= - H1O= - (H1 L1+ L1O) = - (6+9) = -15 cm., F = 9 cm.
If u is the distance of I1 from the field lens, than from lens formula

Hence, L2I = H2I - H2 L2 = 22.5 – 6 = 16.5 cm.

Thus the image lies at a distance of 16.5 cm. to the right of eye lens L2.
, , , ,

Also, position of image formed by eyepiece, lies at a distance of 16.5 cm to the right of eye
lens L2.

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14.7 HUYGEN’S EYEPIECE

14.7.1 Construction
Huygens’s eyepiece consist of a combination of two coaxial plano convex lenses having
focal length in the ratio 3:1 separated by the distance between them is equal to the difference
in their focal length. The focal length and the positions of the two lenses are such that the
system is free from chromatic as well as spherical aberrations. The field and eye lenses are
placed with their convex surface towards the incident ray.

Figure 14.8

Condition of Achromatism: For achromatism the distance d between two lenses should
be

In case of Huygen’s eyepiece

This is the distance between field lens and eye lens. Hence Huygen’s eyepiece is free from
chromatic aberration
Condition for Minimum Spherical Aberration: The distance between the two lenses
for minimum spherical aberration should be

In case of Huygen’s eyepiece

This is the distance between field lens and eye lens in Huygen’s eyepiece hence Huygen’s
eyepiece is free from spherical aberration.

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14.7.2 Working
Like Ramsden’s eyepiece when eye piece is adjusted for normal vision, the final image
formed by it is at infinity. For this the image formed by the field lens should lies in the first
focal plane of the eye lens i.e. at a distance f to the left of eye lens or at a distance f to the rigt
of field lens as the distance between eye and field lens is 2f. The inverted image I1 of a distant
object, formed by objective, acts as an object for the field lens. This gives rise to a virtual
image I2. I2 in turns serves as an object for the eye lens therefore it must be at distance equal
to focal length f from eye lens to make final image at infinity.
For field lens if u is the distance of I1 from the field lens, than from lens formula

Figure 14.9

Since here = and we have

The positive sign indicates that image I1 formed by field lens as well as image I2 formed by
objective lies on the same side, i.e., the field lens focused the rays at I2 which otherwise
would be focused at I1 by objective. The rays coming from I2 emerge from the eye lens as a
parallel beam.
Equivalent Focal Length
Equivalent focal length of Huygen’s eyepiece is given by:

Here

or

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When eye piece is adjusted for normal vision, the final image formed by it should be at
infinity. For this, the image formed by the equivalent lens must be at infinity. .For this
equivalent lens must be placed at a distance 3f/2 to the right of I1 or at a distance f to the right
of eye lens.
14.7.3 Cardinal Points of Huygen’s Eyepiece
Position of Focal Points:
The distance of first focal point F1 from the field lens L1 is given by

Positive sign indicates that first focal point F1 lies at a distance of 3f/2 to the right of field
lens L1. The distance of second focal point F2 from the eye lens L2 is given by

Positive sign indicates that second focal point F2 lies at a distance of f/2 to the right of eye
lens L2.
Position of Principal Points
The distance of first principal point H1 from the field lens L1 is given by

Positive sign indicates that first principal point H1 lies at a distance of 3f to the right of field
lens L1. The distance of second principal point H2 from the eye lens L2 is given by

Negative sign indicates that second principal point H2 lies at a distance of f to the left of eye
lens L2.
Position of Nodal Points
As the medium on either side of the eye piece is same (air), the nodal points N1, N2 coincide
with the principal points H1, H2 respectively.
The position of cardinal points for Huygen’s eyepiece can be plotted as:

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Figure 14.10

Position of Cross Wires

The cross wires if used must be placed at the position where the image due to objective
is formed so that they would be in focus with the final image and magnified by both the lens
of eyepiece. The cross wire placed in first focal plane satisfies both the conditions. However
in Huygen’s eyepiece first focal plane lies between the lenses of eyepiece and is virtual.
Therefore, crosswire must be placed at a distance of 3f/2 in right side of field lens and f/2 left
to eye lens, i.e., in between the lenses of eyepiece. So it is magnified by eye lens only while
the image is magnified by both the lenses. It is called a negative eyepiece as the first focal
plane lies within the eyepiece where no real objects can be placed. Therefore this eyepiece is
used to examine a virtual image like in microscope. Therefore generally eyepiece is not used
in this type of eyepiece.

Example 14.3: In Huygen’s eyepiece focal length of field lens is 6 cm. Locate the position of
cardinal points in a diagram.
Solution: In a Huygen’s eyepiece field lens and eye lens having focal length 3f and f are
separated by a distance 2f.
Given focal length of field lens is = 2 cm, i.e., 3f = 6cm, hence f = 6/3 = 2 cm.
So focal length of eye lens is = 2 cm and separation between lens is = 2f, i.e., d = 2x2 = 4 cm.
The equivalent focal length of Huygen’s eyepiece is

Here

or

or F = 3 cm
Position of Focal Points
The distance of first focal point F1 from the field lens L1 is given by

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Positive sign indicates that first focal point F1 lies at a distance 3 cm. to the right of field lens
L1. The distance of second focal point F2 from the eye lens L2 is given by

Positive sign indicates that second focal point F2 lies at a distance of 1cm to the right of eye
lens L2.
Position of Principal Points
The distance of first principal point H1 from the field lens L1 is given by

Positive sign indicates that first principal point H1 lies at a distance of 6 cm to the right of
field lens L1. The distance of second principal point H2 from the eye lens L2 is given by

Positive sign indicates that second principal point H2 lies at a distance of 2 cm to the left of
eye lens L2.
Position of Nodal Points
As the medium on either side of the eye piece is same (air), the nodal points N1, N2 coincide
with the principal points H1, H2 respectively.
The position of cardinal points for Huygen’s eyepiece can be plotted as

Figure 14.11

 , , , ,

Example 14.4: In Huygen’s eyepiece equivalent focal length of Huygen’s eyepiece is 9 cm.
Calculate the position of cardinal points. If an object is situated at 9 cm. in front of the field
lens, find the position of the image formed by the eyepiece.
Solution: In a Huygen’s eyepiece field lens and eye lens having focal length 3f and f are
separated by a distance 2f.

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Given equivalent focal length of field lens is F= 9 cm

The equivalent focal length of Huygen’s eyepiece is

Here,

so 3f = 18 cm.

So focal length of eye lens is f = 6 cm, focal length of field lens is 3f = 18 cm and separation
between lens is d = 2f = 12 cm.

Position of Focal Points:

The distance of first focal point F1 from the field lens L1 is given by

Positive sign indicates that first focal point F1 lies at a distance 9 cm to the right of field lens
L1. The distance of second focal point F2 from the eye lens L2 is given by

Positive sign indicates that second focal point F2 lies at a distance of 3 cm to the right of eye
lens L2.
Position of Principal Points
The distance of first principal point H1 from the field lens L1 is given by

Positive sign indicates that first principal point H1 lies at a distance of 18 cm to the right of
field lens L1. The distance of second principal point H2 from the eye lens L2 is given by

Negative sign indicates that second principal point H2 lies at a distance of 6 cm to the left of
eye lens L2.
Position of Nodal Points
As the medium on either side of the eye piece is same (air), the nodal points N1, N2 coincide
with the principal points H1, H2 respectively. The position of cardinal points for Huygen’s
eyepiece can be plotted as

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Figure 14.12

Position of Image: Given that object is situated at a distance 9 cm in front of the field lens.
If u and v are the distance of the object O and image I from the first and second principal
point H1 and H2 .
Then u = - H1O = - ( H1 L1+ L1O) = - (18+9) = - 27 cm, F = 9 cm.
If u is the distance of I1 from the field lens, than from lens formula

Hence, L2I = H2I - H2 L2 = 13.5 – 6 = 7.5 cm.

Thus the image lies at a distance of 7.5 cm to the right of eye lens L2.
, , , ,

Position of image formed by eyepiece, therefore, lies at a distance of 7.5 cm to the right of
eye lens L2.

14.8 GAUSSIAN EYEPIECE

14.8.1 Construction
It is a modification of Ramsden’s eyepiece. Like the Ramsden’s eyepiece, it consists of
a field lens and eye lens having equal focal length f separated by a distance equal to two third
of focal length f. A thin plane glass plate G inclined at an angle 450 to the axis of the lens
system is placed between the field lens and eye lens to illuminate the field of view. The light
from the source S enter the tubes from opening given for the purpose and reflects from the
glass plate G along the axis towards the objective of the telescope and illuminates the field of
view. The cross wire C is kept at a distance f/4 in front of the field lens like Ramsden’s
eyepiece.

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14.8.2 Uses
The eye piece is often used in spectrometer telescopes and is very helpful in focusing
the telescope and collimator of the spectrometer for parallel rays and adjusting the axis of
telescope and microscope perpendicular to the axis of the instrument.

Figure 14.13

(i) To Focus the Telescope and Collimator of the Spectrometer for Parallel Rays
To adjust the axis of the telescope perpendicular to the axis of the spectrometer the
telescope is first focused for parallel rays, the turn table (with the reflecting glass surface) is
rotated through 1800. Now the leveling screws of the table are adjusted until the image of the
cross wires are at the same place as before rotating the table. Now the inclination of the
telescope is altered until the image of the cross wires coincides with cross wires. At this
position the axis of the telescope is perpendicular to the axis of the spectrometer.
To focus the telescope for parallel rays the eye-piece is first focused on the cross wires.
Then a plane reflecting surface is placed on the turn table in front of the objective. Now the
distance between the objective and cross wires is so adjusted that their image and cross wires
are in sharp focus without parallax. In this position the cross wires are in the focal plane of
the objectives means telescope is focused for parallel rays.
Now to focus the collimator for parallel rays the slit of the collimator is illuminated and
a telescope focused for parallel rays is brought along the line of the collimator. Now the
distance between the slit and crosswire is so adjusted that the image of the slit is formed on
the cross wires without parallax. In this position collimator is focused for parallel rays.
(ii) To Adjust the Axis of the Telescope and Microscope Perpendicular to the
Axis of the Spectrometer
To adjust the axis of the collimator perpendicular to the axis of the spectrometer the
inclination of the collimator is altered until the image of the slit is formed on the cross wires
in such a way that half of the image is up the intersection of cross wires and half is down the
intersection of the cross wires. At this position the axis of the collimator is perpendicular to
the axis of the spectrometer.

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14. 9 COMPARISON OF HUYGEN’S AND RAMSDEN’S

EYEPIECES

S.N. Huygen’s Eyepiece Ramsden’s Eyepiece

1. Construction: Construction:
a) Lens Type: Two planoconvex lenses a) Lens Type: Two planoconvex lenses
made of same material with their convex made of same material with their convex
side facing the incident light. side facing each other inwards.
b) Ratio of focal length of field lens and b) Ratio of focal length of field lens and eye
eye lens is 3:1. lens is 1:1.
c) Distance between field lens and eye c) Distance between field lens and eye lens
lens is 2f where f is focal length of eye is 2/3f where f is focal length of eye lens.
lens.

2. Huygen’s eyepiece is a negative eye piece Ramsden’s eyepiece is a positive eye piece
as the image formed by objective lens of as the image formed by objective lens of the
the instrument lies between field lens and instrument lies in front of the field lens.
eye lens.

3. In this eyepiece generally crosswire In this eyepiece generally crosswire can be

cannot be used. Cross wire if used is used. Cross wire and the image of the object
magnified by eye lens; while the image of is magnified by both field lens and eye lens
the object is magnified by both field lens hence magnification is same for two.
and eye lens hence magnification is
different for two.

4. As the cross wires are put outside the Cross wires if used has to be placed
eyepiece it involves no mechanical between the field lens and eye lens which
difficulty. causes mechanical difficulty to fit them.

5. The condition for minimum spherical The condition for minimum spherical
aberration (d=f1-f2) is completely aberration (d=f1-f2) is not satisfied. But it is
satisfied. reduced by using the planoconvex lenses
with their convex surface facing each other.

6. The condition for minimum chromatic The condition for minimum chromatic
aberration [d = (f1+f2) / 2] is completely aberration (d= (f1+f2)/2) is not satisfied.
satisfied. Therefore, can be used for white Therefore generally used for
colour. monochromatic (single) colour. Even if
used for heterogeneous light measurement

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is made for a particular colour at a time.

7. It exhibits other type of aberration like Other types of aberration are better
coma and distortion. eliminated. Coma is absent and distortion is
less than Huygen’s eyepiece.

8. It is used for qualitative purposes in It is used in microscopes and telescopes for

microscopes and telescopes. accurate quantitative measurements.

9. It cannot be used as a simple magnifier. It can be used as a simple magnifier.

Self Assessment Questions

6. What is an eyepiece?
7. What is the condition of minimum spherical aberration in Huygens’ eyepiece?
8. A Huygens eyepiece has an eyelens of 4cm focal length. What will be the focal length of
field lens and the distance between the two lenses.
9. If eyepiece is Ramsden’s then?
10. Discuss in brief the specific arrangement of lenses in Ramsden’s eyepiece.
11. Write the conditions of achromatism and minimum spherical aberration in Ramsden’s
eyepiece.

14.10 SPECTROMETER
A spectrometer is a measuring device that collects light waves. It uses these light waves
to determine the material that emitted the energy, or to create a frequency spectrum.
Astronomers make the most frequent use of spectrometers to determine the makeup of stars
or other celestial bodies. The concept of a spectrometer now encompasses instruments that
particles, atoms, and molecules by their mass, momentum, or energy. These types of
spectrometers are used in chemical analysis and particle physics.
When objects are hot enough, they emit visible light at a given point or points on the
electromagnetic spectrum. Spectrometers split the incoming light wave into its component
colors. Using this, they can determine what material created the light.
14.10.1 General Layout of a Spectrometer
The most basic design of a modern spectrometer is an assembly of a slitted screen, a
diffraction grating and a photo detector. The screen allows a beam of light into the interior of
the spectrometer, where the light passes through the diffraction grating. The grating splits the
light into a beam of its component colors, similar to a prism. Many spectrometers also have a
collimating mirror that makes the light waves parallel and coherent, thus making it more
focused. This applies especially to spectrometers used in telescopes. The light then reflects
onto a detector that picks up individual wavelengths.

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14.10.2 Types of spectrometer

Optical Spectrometer
Optical spectrometers (often simply called spectrometers), in particular, show the
intensity of light as a function of wavelength or of frequency. The deflection is produced
either by refraction in a prism or by diffraction in a diffraction grating. It consists of a slit, a
collimator to make light rays parallel to the axis, a table in which prism or grating mounted
and a detector (e.g. telescope).
These spectrometers utilize the phenomenon of optical dispersion. The light from a
source can consist of a continuous spectrum, an emission spectrum (bright lines), or an
absorption spectrum (dark lines). Because each element leaves its spectral signature in the
pattern of lines observed, a spectral analysis can reveal the composition of the object being
analyzed.
Mass Spectrometer
A mass spectrometer is an analytical instrument that is used to identify the amount and type
of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of
gas-phase ions.
Time-of-Flight Spectrometer
The energy spectrum of particles of known mass can also be measured by determining
the time of flight between two detectors (and hence, the velocity) in a time-of-flight
spectrometer. Alternatively, if the velocity is known, masses can be determined in a time-of-
flight mass spectrometer.

14.11 ELECTRON MICROSCOPE

The electron microscope is a powerful microscope that uses a beam of electrons instead
of light beam and an electron detector instead of our eyes to create an image of the specimen.
It is capable of much higher magnifications and has a greater resolving power than a light
microscope, allowing it to see much smaller objects in finer detail. They are large, expensive

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Figure 14.14
pieces of equipment, generally standing alone in a small, specially designed room and
requiring trained personnel. An electron has the properties of a wave with a wavelength that
is much smaller than visible light (a few trillionths of a meter!). With this wavelength we can
distinguish features down to a fraction of a nanometer.
14.11.1 Types of Electron Microscopes
Scanning Electron Microscope (SEM)
In a scanning electron microscope or SEM, a beam of electrons scans the surface of a
sample. The electrons interact with the material in a way that triggers the emission of
secondary electrons. These secondary electrons are captured by a detector, which forms an
image of the surface of the sample. The direction of the emission of the secondary electrons
depends on the orientation of the features of the surface. There, the image formed will reflect
the characteristic feature of the region of the surface that was exposed to the electron beam.
Transmission Electron Microscope (TEM)
In a transmission electron microscope or TEM, Transmission electron microscopy
(TEM) involves a high voltage electron beam emitted by a cathode and formed by magnetic
lenses hits a very thin sample (≤100 nm thick). The electrons are transmitted through the
sample. The spatial variation in this information (the "image") is then magnified by a series
of magnetic lenses until it is recorded by hitting a fluorescent screen, photographic plate, or
light sensitive sensor such as a CCD (charge-coupled device) camera. The image detected by
the CCD may be displayed in real time on a monitor or computer.
This process is similar to working of movie projector. In a projector, a film has the
negative image that will be projected. The projector shines white light on the negative and the
light transmitted forms the image contained in the negative. Transmission electron
microscopes produce two-dimensional, black and white images.

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Figure 14.15
Scanning Transmission Electron Microscope (STEM)
A scanning transmission electron microscope or STEM combines the capabilities of
both an SEM and a TEM. The electron beam is transmitted across the sample to create an
image (TEM) while it also scans a small region on the sample (SEM). The ability to scan the
electron beams allows the user to analyze the sample with various techniques such as
Electron Energy Loss Spectroscopy (EELS) and Energy Dispersive X-ray (EDX)
Spectroscopy which are useful tools to understand the nature of the materials in the sample.
14.11.2 Use of Electron Microscope
Electron microscope is used to examine biological materials (such as microorganisms
and cells), a variety of large molecules, medical biopsy samples, metals and crystalline
structures, and the characteristics of various surfaces. Outside research, they can be used in
the fabrication of silicon chips, or within forensics laboratories for looking at samples such as
gunshot residues. They are also used for fault diagnosis and quality control, e.g., they can be
used to look for stress lines in engine parts or simply to check the ratio of air to solids in ice
cream.

14.12 SUMMARY
Optical instruments are devices which process light waves so that the image of object is
enhanced as well as become clearer, e.g., microscope, telescope etc. Our eyes are not only
vital for seeing the world around us but also an essential part of all optical instruments. Most
of the refraction of light entering the eye occurs at the outer surface of the cornea. Refraction
at the cornea and the surfaces of the lens produces a real and inverted image of the object on
the retina. The optic nerve sends a signal to the brain.
Field of view and aberration are two main concerns for optical instruments. Field of
view of an optical instrument may be defined as the extent or range of its visual area. The
basic purpose of optical instrument is to produce a magnified image free from aberrations and

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enlarged field of view. An eyepiece is used in place of single eye lens for enlarged field of
view and reduces aberrations in final image formed by eyepieces.
Ramsden’s Eyepiece consists of two plano convex lenses made of same material with
their curved (convex) side facing each other inwards. In this type of eyepiece focal length of
field lens and eye lens is same and distance between field lens and eye lens is where f is
focal length of eye lens.
Huygens’s eyepiece consist of a combination of two coaxial planoconvex lenses having
focal length in the ratio 3:1 separated by the distance equal to the difference in their focal
length. The focal length and the positions of the two lenses are such that the system is free
from chromatic as well as spherical aberrations. It is a modification of Ramsden’s eyepiece. It
consists of a field lens and eye lens having equal focal length f separated by a distance equal
to two third of focal length f.
The eye piece is often used in spectrometer telescopes and is very helpful in focusing
the telescope and collimator of the spectrometer for parallel rays and adjusting the axis of
telescope and microscope perpendicular to the axis of the instrument.
A spectrometer is a measuring device that collects light waves. It uses these light waves
to determine the material that emitted the energy, or to create a frequency spectrum.
Astronomers make the most frequent use of spectrometers to determine the makeup of stars
or other celestial bodies.
The electron microscope is a powerful microscope that uses a beam of electrons instead
of light beam and an electron detector instead of our eyes to create an image of the specimen.
It is capable of much higher magnifications and has a greater resolving power than a light
microscope, allowing it to see much smaller objects in finer detail.

14.13 GLOSSARY
Optical Instruments: Optical instruments are devices which process light waves so that
image of object is enhanced as well as clearer.
Field of View: Field of view of an optical instrument may be defined as the extent or range
of its visual area.
Eyepiece: An eyepiece is an optical instrument consisting of two lenses i.e. field lens and eye
lens, made by suitable material. It is preferred in place of single lens. It enlarges field of view
and reduces chromatic aberration and spherical aberrations.
Ramsden’s Eyepiece: Ramsden’s Eyepiece consists of two plano convex lenses made of
same material and same focal length with their curved (convex) sides facing each other.
Distance between field lens and eye lens is 2/3 of focal length of either lens. It is an example
of positive eye piece and we can use cross wire in it.
Huygen’s Eyepiece: Huygens’s eyepiece consists of a combination of two coaxial plano
convex lenses having focal length in the ratio 3:1. The distance between them is equal to the

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difference in their focal length. The focal length and the positions of the two lenses are such
that the system is free from chromatic as well as spherical aberrations. The field and eye
lenses are placed with their convex surface towards the incident ray. It is an example of
negative eye piece and we generally don’t use cross wire in it.
Gaussian eyepiece: It is a modification of Ramsden’s eyepiece. It consists of a field lens and
eye lens having equal focal lengths f separated by a distance equal to two third of f. A thin
plane glass plate G inclined at an angle 450 to the axis of the lens system is placed between
the field lens and eye lens to illuminate the field of view. The cross wire C is kept at a
distance f/4 in front of the field lens.

14.14 REFERENCE BOOKS

1. Optics by Ajoy Ghatak
2. A textbook of Optics by Brij Lal and Dr. N.Subrahmnyam
3. Optics by Dr. S.P. Singh and Dr. J.P. Agarwal

14.15 SUGGESTED READINGS

1. Fundamental of Optics by F. A. Jenkins and H. E. White.
2. The Feynman Lectures on Physics by Richard Feynman
3. Optics by Eugene Hecht

14.16 TERMINAL QUESTIONS:

14.16.1 Short Answer Type Questions
1. What is field of view? Why we need multiple lens eye piece.
2. Give the name and construction which satisfies the condition for achromatism?
3. Give the name and construction of eyepiece in which cross wire is used.
4. 4. What types of eyepieces should be used in a) spectrometer b) low power microscope?
5. Give the construction and working of electron microscope.
14.16.2 Long Answer Type Questions
1. Explain the construction and working of Huygen’s eyepiece. How chromatic and
spherical aberrations minimized in the eyepiece? Calculate the position of cardinal points
and indicate them in a diagram. Why crosswire is not used in Huygen’s eyepiece?
2. Explain the construction and working of Ramsden’s eyepiece. How chromatic and
spherical aberrations minimized in the eyepiece? Calculate the position of cardinal points
and indicate them in a diagram. How it is modified in Gaussian eyepiece?

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3. Explain the sectional view and working of different parts of human eye.
4. Write short notes on
a) Spectrometer b) Comparison of eye piece
c) Positive and negative eyepiece d) Gaussian eyepiece
14.16.3 Numerical Questions
3. In Huygen’s eyepiece the focal length of lenses are 4 cm and 12 cm. Find and plot the position of
cardinal points in a diagram. If an object is situated at 6 cm in front of the field lens, what will be
the position of the image formed by the eyepiece? (Ans. L1H1 = L1N1 = +12cm, L2H2 = L2N2 =
+0.4cm, L1F1 = 6 cm, L2F2 = +2cm, image will be at 0.5 cm behind eye lens)
4. A Ramsden’s eyepiece designed using two plano convex lens of focal length 4 cm. each.
Find the equivalent focal length and position of cardinal points. (Ans. equivalent focal
length = 3 cm, L1H1 = L1N1 = +2cm, L2H2 = L2N2 = -2cm, L1F1 = -1 cm, L2F2 = +1cm)
5. The equivalent focal length of Huygen’s eyepiece is 5 cm. Calculate the focal length and
distance between field lens and eye lens. Also locate the position of cardinal points. (Ans.
focal length of field lens and eye lens = +10 cm and +10/3 cm and distance between field
lens and eye lens = +20/3cm, L1H1 = L1N1 = +10/3 cm, L2H2 = L2N2 = -10/3 cm, L1F1 =
+5 cm, L2F2 = +5/3 cm)
6. The focal length of each lens of a Ramsden’s eyepiece is 10 cm. Calculate the equivalent
focal length of the eyepiece.
7. The focal length of the more convergent lens of an Huygens’ eyepiece is 0.5cm. Calculate
the focal length of the eyepiece and locate on a diagram the positions of its focal points.
14.16.4 Objective Questions
1. Huygen’s eye piece consists of
a. Two plano convex lens of focal length F and 3F separated by a distance 2F.
b. Two plano convex lens of equal focal length F separated by a distance 2/3F.
c. Two plano convex lens of equal focal length F separated by a distance F.
d. Two plano convex lens of focal length F and 3F separated by a distance 2/3F.
Ans. Option ‘a’
2. Ramsden’s eye piece is
a. Negative eyepiece b. Positive eye piece
c. Null eye piece d. Can’t say
Ans. Option ‘b’
3. Spectrometer consists of
a. A collimator b. Telescope
c. Prism Table d. All of the above
Ans. Option ‘d’
4. The conditions of achromatism and minimum spherical aberration are fully satisfied in:
a. Ramsden’s eye piece b. Huygen’s eyepiece
c. both Ramsden’s and Huygen’s eyepieces

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d. Neither Ramden’s nor Huygen’s eyepiece.

Ans. Option ‘b’
5. In Ramsden’s eyepiece fitted in a telescope, the position of the equivalent lens is:
a. before the objective
b. between the field lens and the eye lens
c. between the objective and the field lens
d. behind the eye lens
Ans. Option ‘b’

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UNIT 15: ABERRATIONS

CONTANTS
15.1 Introduction
15.2 Objectives
15.3 Aberrations in Images
15.4 Chromatic Aberration of Lens
15.4.1 Longitudinal or Axial Chromatic Aberration
15.4.2 Lateral or Transverse Chromatic Aberration
15.5 Achromatic Combination of Lenses
15.6 Monochromatic Aberration
15.7 Spherical Aberration and Its Elimination
15.7.1 Spherical Aberration
15.7.2 Reduction of Spherical Aberration
15.8 Other Monochromatic Aberration and Their Elimination
15.8.1 Coma
15.8.2 Curvature of the Field
15.8.3 Astigmatism
15.8.4 Distortion
15.9 Spherical Mirrors
15.10 Schmidt Corrector Plate
15.11 Oil Immersion Lens
15.12 Summary
15.13 Glossary
15.14 Answer to check your progress/Possible Answers to SAQ
15.15 Reference Books
15.16 Suggested Readings
15.17 Terminal Questions
15.17.1 Short Answer Type Questions
15.17.2 Long Answer Type Questions
15.17.3 Numerical Questions
15.17.4 Objective Questions

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15.1 INTRODUCTION
The purpose of using a lens or a system of lenses is to obtain an image that is exact
magnified replica of object. But in general, it is not possible due to various reasons. These
reasons will be discussed in detail, in this unit. Any deviation produced in image from object
is called aberrations in the image. The focal length of the lens depends on refractive index and
refractive index is different for different wavelengths. Thus, if the object is illuminated by
white light then due to prismatic action of the lens after refraction through the lens, image
looks coloured and blurred. Even if we use monochromatic light to illuminate object, image is
not completely free from defects because generally during formulae derivation for object and
image distances and magnifications produced by a lens, we assumed that a point object gives
a point image, the aperture of lens is small also the angles made by light rays with the
principal axis are small. But in actual practice the objects are bigger and lens is used to form a
perfect, bright and magnified image of points situated off the axis also. For this we have to
consider wide angle rays passing through the extremes of lens (known as peripheral or
marginal ray) as well as the rays passing near the axis (known as paraxial rays). Generally the
image formed by these two kinds of rays does not meet at a single point after refraction
through the lens causing imperfection in image.

15.2 OBJECTIVES
After study of unit the student will be able
 To understand the concept of aberration in images.
 To explain chromatic aberration, it’s cause and methods to remove them.
 To explain different types of monochromatic aberration and their elimination
 To describe construction and working of spherical mirrors
 To explain construction and use of Schmidt corrector plate
 To explain construction and use of oil immersion lens

15.3 ABERRATIONS IN IMAGES

Any departure of real images from the objects in respects to shape, orientation, colour
etc. is called aberrations. It is not caused by faulty construction of lens or impurity in lens
material but an inherent shortcoming arises due to the failure of lens to behave precisely
according to the formulae derived.
Aberrations are derived broadly into two categories. The aberrations produced due to
variation of refractive index, wavelength or dispersion of light (i.e., object illuminated by
white light) is called chromatic aberrations. While the defects due to wide angle incidence and
peripheral incidence (due to marginal ray) even if illuminated with monochromatic light is
called monochromatic aberrations. Monochromatic aberrations are again classified as
1) Spherical Aberration

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2) Coma
3) Astigmatism
4) Curvature
5) Distortion

15.4 CHROMATIC ABERRATION OF LENS

The image of an object illuminated by white light made by a lens is generally coloured
and blurred this is called chromatic aberration. The chromatic aberration arises due to
prismatic action of lens material. If a parallel beam of white light is incident on a convex lens,
because of prismatic action it splits into its constituent colours and focus separately. Hence the
position as well as size of image for different colours will be different for different constituent
colours.

Figure15.1
It is generally of two types.
1. Longitudinal or axial chromatic aberration
2. Lateral or transverse chromatic aberration

15.4.1. Longitudinal or Axial Chromatic Aberration

Since the deviation for the red colour is minimum and for the violet colour is maximum,
the focal length of lens for red colour will be different from that due to violet colour. Hence
the images for different colours will be formed at different position along the axis. The
formation of images of different colours at different positions along the axis is called
longitudinal chromatic aberration. If the object is placed at infinity longitudinal chromatic
aberration will be equal to the difference in focal lengths for red and violet rays. The
difference 𝑓𝑟 − 𝑓𝑣 measures axial or longitudinal chromatic aberration.
Expression of Longitudinal Chromatic Aberration for Object Situated at Infinity
1 1 1
The focal length of a lens of radii of curvature R1 and R2 is given by = (𝜇 − 1) ( − )
𝑓 𝑅1 𝑅2

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Let the focal lengths and refractive indices of the lens for red, yellow, and violet colours are
𝑓𝑟, 𝑓𝑦 𝑎𝑛𝑑 𝑓𝑣 and 𝜇𝑟, 𝜇𝑦 𝑎𝑛𝑑 𝜇𝑣 respectively.

Figure 15.2
So above equation takes the form for different colours as follows
1 1 1
= (𝜇𝑟 − 1) (𝑅 − 𝑅 ) ....... (15.1)
𝑓𝑟 1 2

1 1 1
= (𝜇𝑦 − 1) (𝑅 − 𝑅 ) ....... (15.2)
𝑓𝑦 1 2

1 1 1
= (𝜇𝑣 − 1) (𝑅 − 𝑅 ) ....... (15.3)
𝑓𝑣 1 2

Subtracting equation (15.1) from (15.3), we get

1 1 1 1
− = (𝜇𝑣 − 𝜇𝑟 ) ( − )
𝑓𝑣 𝑓𝑟 𝑅1 𝑅2

𝑓𝑟 −𝑓𝑣 (𝜇𝑣 −𝜇𝑟 ) 1 1

= (𝜇𝑦 −1)
(𝜇𝑦 − 1) (𝑅 − 𝑅 )
𝑓𝑣 𝑓𝑟 1 2

𝑓𝑟 −𝑓𝑣 1
= 𝜔. ....... (15.4)
𝑓𝑦 2 𝑓𝑦

(𝜇𝑣 −𝜇𝑟 )
Where 𝜔 = is the dispersive power of lens material.
(𝜇𝑦 −1)

or 𝑓𝑟 − 𝑓𝑣 = 𝜔𝑓𝑦 ....... (15.5)

Expression of Longitudinal Chromatic Aberration for Object Situated at Finite

Distance
1 1 1
Lens formula for thin lens is − = where symbols have their usual meanings. Let
𝑣 𝑢 𝑓
us take the object is at fix point i.e. u is constant will vary with f. Differentiating, we get,
𝑑𝑣 𝑑𝑓
= ....... (15.6)
𝑣2 𝑓2

Let the focal length and image distance for red and violet ray be 𝑓𝑟, 𝑓𝑣, 𝑓𝑦 and 𝑣𝑟, 𝑣𝑣 , 𝑣𝑦
respectively than 𝑑𝑣 = 𝑣𝑟 − 𝑣𝑣 and 𝑑𝑓 = 𝑓𝑟 − 𝑓𝑣 . Therefore,
𝑣𝑟 − 𝑣𝑣 𝑓𝑟 − 𝑓𝑣
=
𝑣2 𝑓2

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Now 𝑓𝑟 − 𝑓𝑣 = 𝜔𝑓𝑦

Figure 15.3

𝑣𝑟 − 𝑣𝑣 𝜔𝑓𝑦 𝜔
= =
𝑣𝑦 2 𝑓𝑦 2 𝑓𝑦
𝜔
or 𝑣𝑟 − 𝑣𝑣 = 𝑓 ....... (15.7)
𝑦

This shows that the longitudinal chromatic aberration depends upon the focal length and
image distance for mean ray and the dispersive power of the lens material.
15.4.2 Lateral or Transverse Chromatic Aberration
𝑓
The magnification [𝑚 = ( )] produced by a lens depends on the focal length and
𝑢+𝑓
focal length of the lens is different for different colours. Hence magnification of lens will also
vary with colour. As a result for a white light illuminated object the size of different colours
will be different. The difference of sizes of images for violet and red colours is a measure of
lateral chromatic aberration.

Figure 15.4
Analytically chromatic aberration is expressed as the rate of change of size of image with
wavelength. Thus if x and y represents the axial and transverse distance then
𝑑𝑥
Longitudinal chromatic aberration =
𝑑𝜆
𝑑𝑦
Transverse chromatic aberration =
𝑑𝜆

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15.5 ACHROMATIC COMBINATION OF LENSES

As we discussed earlier image of a white object formed by a single lens is generally
coloured and blurred this defect is called chromatic aberration. The removal or minimize
chromatic aberration is called achromatism. Generally it is achieved for two prominent
colours by following way
A) A combination of two lenses one concave and other convex made from suitable different
material placed in contact to each other. This combination is called achromatic doublet.
Convex lens of crown and concave lens of flint glass are popularly used.
B) By a combination of two convex lenses made by same material separated by a distance
equal to average of focal length of two lenses.
A) Achromatic Doublet: The focal length for a thin lens having radii of curvature R1, R2 is
given by relation
1 1 1
= (𝜇 − 1) ( − )
𝑓 𝑅1 𝑅2

Now focal length f varies with 𝜇, differentiating above equation, we get,

− ∂f 1 1 ∂𝜇 1 1 ∂𝜇 1
= ∂𝜇 ( − )= (𝜇−1) (𝜇 − 1) ( − ) = (𝜇−1)
f2 𝑅1 𝑅2 𝑅1 𝑅2 𝑓

∂𝜇
But = ω = dispersive power of lens
(𝜇−1)

− ∂f
 =ω ....... (15.8)
f

Figure 15.5
Let f1 and f2 be the focal length and ω1 and ω2 of constituent lens of achromatic doublet.
Equivalent focal length for lens combination in contact is given by
1 1 1
= +
F f1 f2

− ∂F ∂f1 ∂f2 ∂f1 𝟏 ∂f2 𝟏 ω1 ω2

Differentiating it =- - =- . - . = +
F2 f1 2 f2 2 f1 f1 f2 f2 f1 f2

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For achromatism the focal length of the combination for all colours must be the same i.e.
∂F = 0
ω1 ω2 ω1 ω2 f1 ω1
+ =0 or =− or =− ....... (15.9)
f1 f2 f1 f2 f2 ω2

Let us discuss the condition

(i) As ω1 and ω2 both are positive therefore f1 and f2 must have opposite sign to satisfy
condition for achromatism. Thus if one lens is convex other should be concave.
ω1 ω2 1 1
(ii) If ω1 = ω2 i.e. both lenses are made of same material then + = 0 becomes + =0
f1 f2 f1 f2
1
or =0 or F=∞
F

It means that the achromatic doublet will behave like a plane glass plate and not as a lens
hence for an achromatic doublet material of both the lenses should be different.
(iii) If the combination is to behave like a convergent lens, then power of convex lens should
1
be greater than that of concave lens. But, 𝑃𝑜𝑤𝑒𝑟 𝑜𝑓 𝑎 𝑙𝑒𝑛𝑠 ∝
𝐹𝑜𝑐𝑎𝑙 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑙𝑒𝑛𝑠

Therefore focal length of convex lens should be smaller than the focal length of the convex
f1 ω1
lens. From equation =− if 𝑓1 < 𝑓2 than ω1 should also be less than ω2 . It means that
f2 ω2
convex lens should be made of crown lens and concave lens should be made of flint glass.
(iv) This type of combination is perfectly aromatic for two specified colours. If a number of
thin lenses are placed in contact to form an achromatic lens, the condition of achromatism is
given by
𝜔
∑ =0 ....... (15.10)
𝑓

B) Achromatism by Combination of Two Convex Lenses Made By Same Material

Separated By a Suitable Distance: Let f1 and f2 be the focal length of two convex lens
made from same material separated by a distance d.
1 1 1 d
If F be the focal length of combination then = + −
F f1 f2 f1 f2

Differentiating it, we get,

− ∂F ∂f1 ∂f2 𝟏 ∂f1 𝟏 ∂f2
=- 2 - 2 − d [− . − . ]
F2 f1 f2 𝐟𝟐 f1 2 𝐟𝟏 f2 2

∂f1 𝟏 ∂f2 𝟏 d ∂f1 ∂f2

=- . - . − [ − ]
f1 f1 f2 f2 f1 f2 f1 f2

∂f ∂f1 ∂f2
But − = ω, the dispersive power of the lens medium. Here we have, − = = ω as
f f1 f2
both the lenses are made by same material.
− ∂F ω ω d
Hence, = + − [ω + ω]
F2 f1 f2 f1 f2

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ω ω 2ωd
= + −
f1 f2 f1 f2

For chromatic aberration to be minimum the focal length of the combination for all colour
must be the same, i.e., ∂F = 0
ω ω 2ωd f1 +f2
 + − = 0 or 𝑑 = ....... (15.11)
f1 f2 f1 f2 2

Thus the distance between two lenses must be equal to the average of their individual focal
lengths.

Figure 15.6
i) Since d is always positive 𝑓1 + 𝑓2 must be positive. It means either both the lenses or the
one with greater focal length are convex.
ii) The achromatic doublet has the same focal length for all colours as the expression for d is
independent of 𝜔.
However the condition d = (f1 + f2)/2 does not mean the coincidence of focal points but
simply means the focal length for different colours is same. However, the deviation produced
by lens will be same as focal length for all the colours is same therefore the combination is
apparently free from lateral chromatic aberration.

Example 15.1: A converging achromatic doublet of focal length 50 cm is to be constructed

out of thin crown and flint glass lens. The radius of curvature of the surface is in contact is 15
cm.Find the radius of curvature of the second surface of each lens. (Given that 𝜔𝑐𝑟𝑜𝑤𝑛 =
0.015, 𝜔𝑓𝑙𝑖𝑛𝑡 = 0.030 and 𝜇𝑐𝑟𝑜𝑤𝑛 = 1.5, 𝜇𝑓𝑙𝑖𝑛𝑡 = 1.7)
𝜔𝑐𝑟𝑜𝑤𝑛 𝜔𝑓𝑙𝑖𝑛𝑡 𝜔𝑐𝑟𝑜𝑤𝑛 𝑓𝑐𝑟𝑜𝑤𝑛
Solution: For an achromatic doublet + =0 or =−
𝑓𝑐𝑟𝑜𝑤𝑛 𝑓𝑓𝑙𝑖𝑛𝑡 𝜔𝑓𝑙𝑖𝑛𝑡 𝑓𝑓𝑙𝑖𝑛𝑡

Given that, 𝜔𝑐𝑟𝑜𝑤𝑛 = 0.015, 𝜔𝑓𝑙𝑖𝑛𝑡 = 0.030 and 𝜇𝑐𝑟𝑜𝑤𝑛 = 1.5, 𝜇𝑓𝑙𝑖𝑛𝑡 = 1.7
𝑓𝑐𝑟𝑜𝑤𝑛 0.015 1 1
 =− =− or 𝑓𝑐𝑟𝑜𝑤𝑛 = − 𝑓𝑓𝑙𝑖𝑛𝑡
𝑓𝑓𝑙𝑖𝑛𝑡 0.030 2 2

1 1 1
Also equivalent focal length of two lenses in contact + =
𝑓𝑐𝑟𝑜𝑤𝑛 𝑓𝑓𝑙𝑖𝑛𝑡 𝐹

Here focal length of combination is= 30 cm

1 1 1 2 1 1
∴ + = or − + = as 𝑓𝑓𝑙𝑖𝑛𝑡 = −30
𝑓𝑐𝑟𝑜𝑤𝑛 𝑓𝑓𝑙𝑖𝑛𝑡 50 𝑓𝑓𝑙𝑖𝑛𝑡 𝑓𝑓𝑙𝑖𝑛𝑡 30

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1
 𝑓𝑐𝑟𝑜𝑤𝑛 = − 𝑓𝑓𝑙𝑖𝑛𝑡 = 15 𝑐𝑚
2
1 1 1
For the crown lens, = (𝜇𝑐𝑟𝑜𝑤𝑛 − 1) ( − )
𝑓𝑐𝑟𝑜𝑤𝑛 𝑅1 𝑅2

1 1 1
or = (1.5 − 1) ( − ) as 𝜇𝑐𝑟𝑜𝑤𝑛 = 1.5 , 𝑓𝑐𝑟𝑜𝑤𝑛 = 15 𝑐𝑚 and 𝑅2 = −15𝑐𝑚
15 𝑅1 −15
1 1 1 2−1 1
or = − = = , i.e., 𝑅1 = 15 𝑐𝑚
𝑅1 7.5 15 15 15

1 1 1
Similarly for flint glass = (𝜇𝑓𝑙𝑖𝑛𝑡 − 1) ( − )
𝑓𝑓𝑙𝑖𝑛𝑡 𝑅1 𝑅2

1 1 1
or = (1.7 − 1) (−15 − 𝑅 ) as 𝜇𝑓𝑙𝑖𝑛𝑡 = 1.7, 𝑓𝑓𝑙𝑖𝑛𝑡 = −30 𝑐𝑚 and 𝑅2 = −15𝑐𝑚
−30 2

1 1 1 5−7 2
or = + = =− , i.e., 𝑅2 = −52.5 𝑐𝑚
𝑅2 21 −15 105 105

Thus the radius of curvature of the second surface of crown lens is = 15 cm and the radius of
curvature of the second surface of flint lens is −52.5 𝑐𝑚.
Example 15.2: An achromatic converging combination of focal length 30 cm is made by two
lenses having dispersive power in the ratio 3:5. What are the focal lengths of the lenses?
𝜔1 𝜔2 𝜔1 𝑓1
Solution: For an achromatic doublet + =0 or =−
𝑓1 𝑓2 𝜔2 𝑓2

𝜔1 3 𝑓1 3 3
Given, = . Hence, =− or 𝑓1 = − 5 𝑓2
𝜔2 5 𝑓2 5
1 1 1
Also equivalent focal length of two lenses in contact + =
𝑓1 𝑓2 𝐹

Here focal length of combination is = 30 cm

1 1 1 3 1 1 3
∴ + = or − + = as 𝑓1 = − 5 𝑓2
𝑓1 𝑓2 30 5𝑓2 𝑓2 30
2 1 3
= or 𝑓2 = 20 𝑐𝑚, hence 𝑓1 = − × 20 = −12 𝑐𝑚
4𝑓2 40 5

So the focal lengths of the lenses are 𝑓1 = −12 𝑐𝑚 and 𝑓2 = 20 𝑐𝑚

Self-Assessment Questions (SAQ)

1. Chromatic aberration is due to the prismatic action of lens. True/False
2. Image of an object illuminated by white light is generally coloured and blurred. This is
called mono chromatic aberration. True/False
3. For Achromatism the distance between two lenses must be equal to the sum of their
individual focal lengths. True/False
4. For achromatism the focal length of the combination for all colours must be the same.
True/False

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15.6 MONOCHROMATIC ABERRATION

Chromatic aberration is automatically removed if monochromatic light is used to illuminate
an object even than the image formed by lenses is not free from other defects. Such kinds of
defects are called monochromatic aberrations. Prominent spherical aberrations are
1) Spherical aberration
2) Coma
3) Astigmatism
4) Curvature of field
5) Distortion

15.7 SPHERICAL ABERRATION AND ITS ELIMINATION

15.7.1 Spherical Aberration
It is an optical defect observed in spherical surfaces (lens, mirror etc.) having large
aperture. In spherical aberration parallel light ray that pass through the central region or near
the axis of the lens (known as paraxial ray) focus at different point to the rays that passes to
the edge or periphery of the lens (known as marginal ray).

Figure 15.7
As shown in figure 15.7 a point object O on the axis is imaged as Im and Ip where Im and Ip are
the image formed by marginal and paraxial ray. Im Ip is the measure of axial or longitudinal
spherical aberration.
15.7.2 Reduction of Spherical Aberration
Generally spherical aberration is minimum for lenses having small aperture or the total
deviation produced by the system is equally divided on all refracting surfaces. Based on these
facts different methods are used to minimize spherical aberrations.
1) By using plano convex lens
2) By crossed lens

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3) By using stops: In this method the aperture of the lens is limited by using a stop to cut off
marginal or paraxial ray. We can use a circular shutter or blackened the marginal or paraxial
parts.
By Using a Convex and Concave Doublet: The image due to marginal ray is closer to
the lens than the image due to paraxial rays while for the concave lens the image due to
paraxial rays is closer to the lens than the image due to marginal ray. Thus by taking a suitable
combination the image due to paraxial rays coincide to the image due to marginal ray to
minimize spherical aberration. To form a real image the doublet must behave as convex.
By Using Aplanetic Lens: The image formed by aplanetic lenses exhibit low aberration. If
𝑅
the distance of point object on the axis from centre of curvature is than the distance of the
𝜇
point image on the axis will be 𝜇R.Such two points are called aplanetic points.
By Using Two Convex Lenses Separated By Suitable Distance: Let two convex
lenses of focal length f1 and f2 are separated by a distance d. A ray of light AB, parallel to the
principal axis, is incident on the first lens L1 at a height h1.after deviating though a small angle
δ from the first lens the ray is incident on the second lens at height h2 on the second lens L2
after deviating by an angle δ2
From the second lens again the ray meets the principal axis at F2, The second focus of the
system. If the second lens is not there the ray AB should meet the principal axis at F1 after
deviation from L1 hence L1F1 is equal to the focal length of the first lens i.e.f1.
ℎ
The deviation produced by single lens is given by 𝛿 =
𝑓
ℎ1
Hence deviation produced by lens L1 is given by 𝛿1 =
𝑓1

ℎ2
And deviation produced by lens L2 is given by 𝛿2 =
𝑓2

Now for minimum spherical aberration the deviation produced by two lenses must be equal,
i.e.,
ℎ1 ℎ2 ℎ1 𝑓1
𝛿1 = 𝛿2 𝑜𝑟 = or = ....... (15.12)
𝑓1 𝑓2 ℎ2 𝑓2

Figure 15.8
From similar triangle BL1F1 and C L2F2, we have

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ℎ1 𝐿1 𝐹1 𝐿1 𝐹1 𝑓1
= = = ....... (15.13)
ℎ2 𝐿2 𝐹1 𝐿1 𝐹1 −𝐿1 𝐿2 𝑓1 −𝑑

𝑓1 𝑓
From equations 15.12 and 15.13, we have, = 𝑓 −𝑑1 or 𝑓2 = 𝑓1 − 𝑑
𝑓2 1

d = 𝑓1 − 𝑓2 ....... (15.14)
That is for minimum spherical aberration the distance between the two lenses should be equal
to the difference of their focal length.

Example 15.3: A convergent doublet of lenses separated by a distance 5 cm. suitable for
minimum spherical aberration. The equivalent focal length of the system is 15 cm. Find out
the focal length of lenses.
Solution: Let the focal length of convergent doublet of lenses are 𝑓1 𝑎𝑛𝑑 𝑓2 separated by
distance d.
For minimum spherical aberration 𝑑 = 𝑓1 − 𝑓2

Equivalent focal length of two lenses of focal length 𝑓1 𝑎𝑛𝑑𝑓2 separated by a distance d is
1 1 1 𝑑 𝑓1 𝑓2
= + − or F=
𝐹 𝑓1 𝑓2 𝑓1 𝑓2 𝑓1 +𝑓2 −𝑑

For minimum spherical aberration, 𝑑 = 𝑓1 − 𝑓2 so 𝑓2 = 𝑓1 − 𝑑

𝑓1 𝑓2 𝑓1 𝑓2 𝑓1 𝑓2 𝑓1
 𝐹=𝑓 = = = or 𝑓1 = 2𝐹
1 +𝑓2 −𝑑 𝑓2 +𝑓2 2𝑓2 2

Given focal length of combination is F = 15 cm, hence, 𝑓1 = 2 × 15 = 30 cm and 𝑓2 = 𝑓1 −

𝑑 = 30 − 15 = 15 𝑐𝑚.
Example 15.4: A combination of two lenses made by same material and of focal length
𝑓1 𝑎𝑛𝑑𝑓2 separated by a distance d satisfies the condition for no chromatic aberration and
minimum spherical aberration .If the equivalent focal length of combination is 30 cm find the
focal length of both the lenses as well as distance between them.
𝑓1 +𝑓2
Solution: For no chromatic aberration, 𝑑 =
2

And for minimum spherical aberration, 𝑑 = 𝑓1 − 𝑓2

3𝑑 𝑑
Solving these two equations, we get, 𝑓1 = and 𝑓2 =
2 2

Equivalent focal length of two lenses of focal length 𝑓1 𝑎𝑛𝑑𝑓2 separated by a distance d is
1 1 1 𝑑
= + −
𝐹 𝑓1 𝑓2 𝑓1 𝑓2

Here the focal length of combination is 30 cm

3𝑑 𝑑
𝑓1 𝑓2 × 3𝑑
2 2
or 30 = = 3𝑑 𝑑 = or d = 40 cm.
𝑓1 +𝑓2 −𝑑 + −𝑑 4
2 2

Hence, 𝑓1 = 60 cm and 𝑓2 = 20 𝑐𝑚

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15.8 OTHER MONOCHROMATIC ABERRATION AND THEIR

ELIMINATION
15.8.1 Coma
The image of a point object lying just away from the axis is not a point but it looks like a
comet. Hence the aberration is called coma. Coma is the result of asymmetrical zones of the
lens for the points away from the axis and varying magnification for rays refracted through
different zones of the lens. Because of asymmetrical zones for a point object lying away from
the axis is imaged in the form of discs by different zones. As shown in Figure 15.9 Object
point A is imaged as Q by paraxial ray 2, 2, as R by paraxial ray 3, 3 and as S by paraxial ray
4, and so on. As we pass from paraxial to marginal zones the disc shaped image becomes
wider and wider because the focal length for paraxial zones is greater than marginal.

Figure 15.9
Coma is said to be positive if the magnification of the image due to outer zone is larger
than the inner zones and if the magnification of the image due to outer zone is smaller than the
inner zones it is called negative.
Removal of Coma: A lens satisfying the Abbe’s sine condition 𝜇1 𝑦1 sin 𝜃1 =
𝜇2 𝑦2 sin 𝜃2 will be free from coma. Where 𝜇1 , 𝑦1 , 𝜃1 and 𝜇2 , 𝑦2 , 𝜃2 refer to the refractive index,
height of the object above the axis, and the slope of the incident ray and refracted ray of light
respectively.
𝑦2
Now as The magnification of the image is given by . Hence from above equation
𝑦1

𝑦2 𝜇1 sin 𝜃1
=
𝑦1 𝜇2 sin 𝜃2

Now for elimination of coma transverse magnification should be constant for all rays of light
𝜇1 𝑦2 sin 𝜃1
as is constant so for will be constant only if = constant.
𝜇2 𝑦1 sin 𝜃2
ℎ
For distant object 𝑢 → ∞ and sin 𝜃 → ℎ. So = constant
sin 𝜃2

A lens that satisfies the above condition is called aplanetic lens. So use of aplanetic lens not
only removes spherical aberration but also coma.

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15.8.2 Curvature of the Field

Generally the image of a point object made by a single lens free from other
monochromatic aberration is a point. But as the paraxial focal length is greater than the
marginal focal length the image of an extended or flat plane object in front of lens is curved
one instead of flat surface.
The central portion nearer to the axis is in focus but same is not true for the outer
portion. As shown in figure 15.10 the real images formed by convex lens curves towards the
lens while a virtual image curves away from the lens.

Figure 15.10

Removal of Curvature of the Field

a) It can be eliminated by using stops in the suitable position in front of single thin lens.
b) For a system of thin lens the curvature of image is theoretically given by
1 1
=∑
𝑅 𝜇𝑛 𝑓𝑛

Where R is the radius of curvature of the final image, 𝜇𝑛 and 𝑓𝑛 are the refractive index and
1
focal length of nth lens. ∑ is called the Petzwal sum and depends only upon the refractive
𝜇𝑛 𝑓𝑛
indices and radii of curvature of the surfaces of the lenses. It does not depend on the thickness
and separation of the constituent lens. For the image to be flat, R must be infinity i.e.
1 1 1
=∑ = =0
𝑅 𝜇𝑛 𝑓𝑛 ∞

Figure 15.11

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Therefore if lens system consists of two lenses placed in air, the condition for no curvature of
1 1
field will be + =0
𝜇1 𝑓1 𝜇2 𝑓2

This is known as Petzwal condition for no curvature. The condition holds good for the
lenses in contact as well as separated by a distance. As 𝜇1 and 𝜇2 are positive this condition
will be satisfied only if 𝑓1 and 𝑓2 are of opposite signs, i.e., for eliminating curvature if one
lens is convex, the other must be concave. A suitable combination of convex and concave lens
of different materials can be used to eliminate the curvature of field.
15.8.3 Astigmatism
Like coma, if a point object is far off the axis, its image consists of two mutually
perpendicular lines separated by a finite distance and lying in perpendicular planes. This
defect of the image is called as astigmatism.

Figure 15.12
As shown in figure 15.12, let O is an object point away from the principal axis. A plane
T1T2 passing through O and the principal axis is known as tangential or meridonial plane
another plane S1S2 perpendicular to meridonial plane and passing through the object point O is
known as sagittal plane. The rays passing through these two planes do not meet at the same
point. Thus a line image T is obtained passing through meridonal plane and other image S is
obtained by rays passing through sagittal plane. If screen is moved from image S due to
sagittal plane towards T, a circle of least confusion will be observed at a place where the
diverging beam from first and converging beam to second focal lines intersect. The distance
between these two lines is called astigmatic difference. In fig curved focal lines corresponding
to tangential and sagittal planes are shown as T and S.The astigmatism is said positive if T is
before S, If S is before T it is called negative. On axis evidently astigmatism is zero.
Removal of Astigmatism: In case of convex lens the astigmatism is positive while it is
negative in case of concave lens.Therfore a suitable convex and concave lens separated by a
proper distance may be used to remove astigmatigm.Such combination of two lenses is called
anastigmat.
15.8.4 Distortion
Any variation in shape of the object is called distortion. Magnification produced by a
lens for different axial distances is different because of this generally image of a square is not

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perfect square as every point on the square is imaged by different zone of the lens and
therefore differently magnified. This causes the distortion. It is generally of two types: 1)
Barrel shaped, 2) Cushion shaped.
In barrel shaped distortion, the magnification decreases with the increase in axial
distance. The image of the square looks as barrel shaped (figure 15.13 B). In cushion shaped
distortion, the magnification increases with the increase in axial distance and the image of the
square appears as cushion shaped (Figure 15.13 C)

Figure 15.13
However a single thin lens exhibits no distortion the presence of stops limits the cone of
rays or light striking the lens thus causing distortion. If a stop is placed before the lens the
distortion is barrel shaped and if a stop is placed after the lens the distortion is cushion type.
so use of single thin lens without stops is prefeered.however as stops are useful to reduce
other type of monochromatic aberration therefore to eliminate distortion stop is placed in
between two symmetrical lenses. So that cushion type distortion produced by first lens will be
compensated by the barrel shaped distortion produced by second lens. As the optical
instruments intended mainly form visual observation it is difficult to remove all kind of
aberrations simultaneously but every effort should be made to eliminate distortion in
photographic camera lens. In general a compromise is made as per requirement and use of the
optical instruments.

SAQ 5: Magnification produced by a lens for different axial distances is different therfore
causes variation in shape of the object. This is called Coma. True/False
SAQ 6: For minimum spherical aberration the distance between the two lenses should be
equal to the difference of their focal length. True/False
1 1
SAQ7: + = 0is known as Petzwal condition for no curvature. True/False
𝜇1 𝑓1 𝜇2 𝑓2

SAQ 8: Chromatic aberration is an optical defect observed in spherical surfaces (lens, mirror
etc.) having large aperture. True/False

15. 9 SPHERICAL MIRRORS

A spherical mirror is a mirror which has the shape of a piece cut out of a spherical surface.
There are two types of spherical mirrors: concave and convex (Figure 15.14). The most

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commonly occurring examples of concave mirrors are shaving mirrors and makeup mirrors.
As is well-known, these types of mirrors magnify objects placed close to them. The most
commonly occurring examples of convex mirrors are the passenger-side wing mirrors of cars.
These type of mirrors have wider fields of view than equivalent flat mirrors, but objects which
appear in them generally look smaller (and, therefore, farther away) than they actually are.

Figure 15.14

15.10 SCHMIDT CORRECTOR PLATE

In case of large telescopes, large spherical mirrors are used to form images and the
images are recorded on a curved film. In such cases although the image is free from
astigmatism and coma but because of large aperture of lens spherical aberration occurs. To
remove spherical aberration in such large spherical mirror or lens Schmidt designed a special
system in 1929 known as Schmidt corrector plate.
Construction and Working
A thin glass plate having a shallow toroidal surface known as Schmidt corrector plate
(Figure 15.15) before the spherical mirror could help minimizing the spherical aberration in a
mirror. Schmidt corrector plate is used with large aperture reflector telescopes. The surfaces
of the corrector plate facing large spherical aperture is plane and the second surface is convex
towards the axis and concave towards marginal ends.

Figure 15.15
We know that the marginal rays are focused nearer than the paraxial rays for large
aperture reflecting surfaces hence causing spherical aberration by behaving as a concave lens
for the marginal rays and as a convex lens for the paraxial rays. The paraxial rays are refracted
towards the axis and the marginal rays are refracted outwards. All the refracted rays (paraxial
as well as marginal) from spherical surface come to focus at F and spherical aberration is

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minimized. The Schmidt plate is generally placed at the curvature of the reflecting surface so
that working of plate is not affected.

15.11 OIL IMMERSION LENS

In light microscopy, oil immersion is a technique used to increase the resolving power of
a microscope. This is achieved by immersing both the objective lens and the specimen in a
transparent oil of high refractive index, thereby increasing the numerical aperture of the
objective lens of microscope.
Why Use Microscope Immersion Oil?
When light passes from a material of one refractive index to another (for example: from
glass to air), it bends. In the space between the microscope objective lens and the slide (where
air is), light is refracted, the light scatters and it is lost. The refractive index of air is
approximately 1.0, while the refractive index of glass is approximately 1.5. When light passes
through both glass and air it is refracted. Light of different wavelengths bend at different
angles, so as objects are magnified more, images become less distinct. Basically when using
lower magnification microscope objective lenses (4x, 10x, 40x) the light refraction is not
usually noticeable. However, once you use the 100x objective lens, the light refraction when
using a dry lens is noticeable. If you can reduce the amount of light refraction, more light
passing through the microscope slide will be directed through the very narrow diameter of a
higher power objective lens. In microscopy, more light = clear and crisp images. By placing a
substance such as immersion oil with a refractive index equal to that of the glass slide in the
space filled with air, more light is directed through the objective and a clearer image is
observed.
Oil immersion lenses make use of aplanatic surfaces for obtaining a very wide angled
pencil of light from each point of the object. In case of single spherical surface these points
are at distance R/μ and Rμ from the optic centre. If object is placed at one aplanetic centre its
image will be formed at other aplanatic point also the image will be free from spherical
aberration and coma.

15.12 SUMMARY
The purpose of using a lens or a system of lens is to obtain an image exact magnified
replica of object but in general it is not possible due to various reasons. Any deviation
produced in image from object is called aberrations in the image. Aberrations are derived
broadly into two categories, chromatic aberrations and mochromatic aberrations. The image of
an object illuminated by white light made by a lens is generally coloured and blurred this is
called chromatic aberration. It arises basically because of the prismatic action of lens. To
remove or minimize chromatic aberration is called achromatism.
Generally it is achieved either by using achromatic doublet or by a combination of two
convex lenses made by same material separated by a distance equal to average of focal length

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of two lenses. Even if monochromatic light is used to illuminate an object image is not free
from defects. Such aberration is called monochromatic aberrations. Prominent spherical
aberrations are spherical aberration, coma, astigmatism, curvature of field and distortion. In
spherical aberration paraxial rays focus at different point to the marginal rays while in coma
the image of a point object lying just away from the axis has a comet like structure. Also if a
point object is far off the axis, its image made by sagital plane and tangential plane which are
perpendicular to each other are not same it is called astigmatism.
The image of an extended or flat plane object in front of lens is curved one instead of
flat surface due to the aberration known as curvature of field. While any variation in shape of
the image with respect to object. It is generally of two types; barrel shaped and cushion
shaped.

15.13 GLOSSARY
Aberration: Any departure of actual image with respect to size, shape, position, colour with
respect to ideal image is called aberration.
Chromatic Aberration: The image of an object illuminated by white light made by a lens is
generally coloured and blurred this is called chromatic aberration.
Achromatism: To remove or minimize chromatic aberration is called achromatism.Generally
it is achieved either by using achromatic doublet or by a combination of two convex lenses
made by same material separated by a distance equal to average of focal length of two lenses.
Monochromatic Aberration: Kinds of defects occurred in image even if monochromatic
light is used to illuminate an object are called monochromatic aberrations. Prominent
spherical aberrations are spherical aberration, coma, astigmatism, curvature of field and
distortion
Spherical Aberration: It is an optical defect observed in spherical surfaces (lens, mirror etc.)
having large aperture. In spherical aberration paraxial rays focus at different point to the
marginal rays.
Coma The image of a point object lying just away from the axis looks like a comet such
aberration is called coma.
Astigmatism: A point object is far off the axis, its image made by sagittal plane and maridonal
plane is not same. Such defect is called astigmatism.
Curvature of Field: Generally the image of a point object made by a single lens free from
other monochromatic aberration is a point. But as the paraxial focal length is greater than the
marginal focal length the image of an extended or flat plane object in front of lens is curved
one instead of flat surface.
Distortion: Any variation in shape of the image with respect to object. It is generally of two
types: 1) Barrel shaped, 2) Cushion shaped.

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15.14 ANSWERS OF SAQ’S

1. True, 2.False, 3. False, 4. True, 5. False, 6. True, 7. True, 8. False

15.15 REFERENCE BOOKS

1. Optics by Ajoy Ghatak
2. A textbook of Optics by Brij Lal and Dr. N. Subhrahmnyam
3. Optics by Dr. S.P. Singh and Dr. J.P. Agarwal

15.16 SUGGESTED READINGS

1. Fundamental of Optics by F. A. Jenkins and H. E. White.
2. The Feynman Lectures on Physics by Richard Feynman
3. Optics by Eugene Hecht

15.17 TERMINAL QUESTIONS

15.17.1 Short Answer Type Questions
1. What is meant by monochromatic aberration? What are different types of monochromatic
aberration?
2. With the suitable diagram explain longitudinal and lateral spherical aberration in lens.
3. What is the difference between coma and astigmatism?
4. Define astigmatism, distortion and radius of curvature.
5. What do you mean by meniscus lens?

15.17.2 Long Answer Type Questions

1. Explain Chromatic aberration, its type and expression for its magnitude. how two lenses may be
combined for making achromatic doublet?
2. What do you mean by aberration? Discuss its type and the way for their elimination.
3. Explain spherical aberration and various ways for its removal.
4. Discuss aplanetism. What is an aplanetic lens? Find the aplanetic foci for a spherical refracting
surface? How they are utilized for the construction of oil immersion objective lens
5. Write short notes on
a) Spherical mirrors b) Oil immersion lens
c) Schmidt corrector plate d) Aplanetic points

15.17.3 Numerical Questions

1. A double convex lens has radii of curvature of 20 cm and 5 cm. Find the longitudinal chromatic
aberration of an object at infinity? Given that 𝜇𝑣 = 1.52 and 𝜇𝑟 = 1.51

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2. An achromatic doublet made by crown glass lens and flint glass lens of 40 cm focal
length have a common radius of 25 cm for the surfaces in contact. Calculate the radius of
curvature of the second face of each lens. (Given that 𝜔𝑐𝑟𝑜𝑤𝑛 = 0.017, 𝜔𝑓𝑙𝑖𝑛𝑡 = 0.034
and 𝜇𝑐𝑟𝑜𝑤𝑛 = 1.5, 𝜇𝑓𝑙𝑖𝑛𝑡 = 1.7) (Ans. The radius of curvature of the second surface of
crown glass lens is = 16.67 cm and the radius of curvature of the second surface of flint
glass lens is −233.33 𝑐𝑚)
3. The ratio of dispersive power of a converging achromatic doublet is in the ratio 1:2. If the
equivalent focal length of the doublet is 40 cm. Find the focal length of each lens. (Ans. f1
= 20 cm and f2 = 40 cm)
4. An oil immersion objective has lens of refractive index 1.67. Find out the semi angle of
the incident and emergent rays. (Ans. 590, 310)
5. Two thin lenses separated by a distance 5 cm form a combination free from spherical
aberration. If the system has an equivalent focal length of 14 cm, calculate the focal length
of its component lenses. (Ans. f1 = 28 cm and f2 = 23cm)

15.17.4 Objective Questions

1. Which of the following is Monochromatic aberration?
a. Spherical aberration b. Astigmatism
c. Coma d. All of the above
Ans. Option ‘d’
2. Two plano convex lens of focal length f1 and f2 separated by a distance d. The condition of
achromatism is
a. d= f1 - f2 b. d= f1 + f2
c. d= (f1 + f2)/2 d. d= f1 / f2
Ans. Option ‘c’
3. Spherical mirrors are free from
a. Chromatic aberration b. Spherical aberration
c. both chromatic and spherical aberration d. None of these
Ans. Option ‘a’
4. Both chromatic and spherical aberration are minimized for two plano convex lens of focal
length f1 and f2 separated by a distance d .then the ratio of their focal length will be
a. 2:1 b. 3:1
c. 1:2 d. 1:4
Ans. Option ‘b’

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LABORATORY COURSE

PHY (N)-220L

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EXPERIMENT 1: STUDY OF MALUS LAW

CONTENTS
1.1 Objectives

1.2 Introduction

1.3 Apparatus

1.4 Theory and Formula Used

1.5 Procedure

1.6 Observation

1.7 Calculation

1.8 Result

1.9 Precautions and Sources of Error

1.10 Summary

1.11 Glossary

1.12 References

1.13 Viva-voce Questions and Answers

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1.1 OBJECTIVES
To verify the Law of Malus for plane polarized light with the help of photovoltaic cell.

1.2 INTRODUCTION
According to Malus, when completely plane polarized light is incident on the analyzer, the intensity I
of the light transmitted by the analyzer is directly proportional to the square of the cosine of angle
between the transmission axes of the analyzer and the polarizer.

1.3 APPARATUS
Optical bench, halogen lamp, double convex lens, polarizer, analyzer, photo voltaic cell and micro-
ammeter (0-50mA).

1.4 THEORY AND FORMULA USED

According to Malus Law, when a beam of completely plane polarized light is incident on the
analyzer, then the intensity I of the emergent light is given by

I= Io cos2 ϕ

Where Io = Intensity of plane polarized light incident on the analyzer.

Φ= angles between planes of transmission of polarizer and the analyzer

Figure 1: Experimental Setup for studying Malus Law

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To verify this law, light from the analyzer is made to enter in a photovoltaic cell. The current output
of photovoltaic cell is connected to microammeter.

1.5 PROCEDURE
1. The experimental setup is arranged as shown in Figure 1. In this arrangement, the source S,
convex lens, Polarizer P, Analyzer A and the window of Photovoltaic cell should be at the
same height.
2. Now switch on the incandescent bulb. Light from the source S rendered parallel with the help
of convex lens L is allowed to fall on polarizer P.
3. For any orientation of the polarizer P, the polarized light passes through analyzer A. The
analyzer A is rotated till there is maximum deflection in the micro-ammeter. The position of
analyzer is noted on the circular scale. The corresponding micro-ammeter deflection is also
recorded. The position of analyzer corresponds to ϕ=0 (here ϕ is the angle between Planes of
transmission of polarizer and analyzer.)
4. The analyzer A is rotated through a small angle, say 10o and then the steady micro-ammeter
deflection is noted.
5. The experiment is repeated by rotating the analyzer through 10o degree each time and noting
down the corresponding micro-ammeter deflection till it become practically zero.

1.6 OBSERVATION

S. No. Angles through Micro-ammeter reading cos ϕ cos2ϕ θ/cos2ϕ

which analyzer
is rotated ϕ µA θ

1. 10o

2. 20 o

3. 30 o

4. 40 o

5. 50 o

6. 60 o

7. 70 o

8. 80 o

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1.7 CALCULATION
Find the value of θ/cos2ϕ from each observation and plot the graph for cos2ϕ on X-Axis and θ on Y-
Axis.

1.8 RESULT
The graph for cos2ϕ on X-Axis and θ on Y-Axis is a straight line and hence, Malus law is verified.

1.9 PRECAUTIONS AND SOURCES OF ERROR

1. The position of the polarizer should not be disturbed throughout the experiment.

2. The source of light, lens, polarizer, analyzer, and the solar cell should be adjusted to the same
height.

3. The voltage applied to the light source should be constant throughout the experiment.

4. The experiment should be performed in the dark room to avoid any external light inside the
photovoltaic cell.

5. Care should be taken while performing the experiment as the bulb becomes very hot.

1.10 SUMMARY
Malus law is the law stating that the intensity of a beam of plane-polarized light after passing through
a rotatable polarizer varies as the square of the cosine of the angle through which the polarizer is
rotated from the position that gives maximum intensity.

This law is named after E. L. Malus (1775-1812), a French physicist.

1.11 GLOSSARY
Convex Lens – Convex lenses are thicker at the middle. Rays of light that pass through the lens are
brought closer together (they converge). A convex lens is a converging lens.

A double convex lens is symmetrical across both its horizontal and vertical axis.

Halogen Lamp – A halogen lamp, also known as a tungsten halogen, quartz-halogen or quartz iodine
lamp, is an incandescent lamp consisting of a tungsten filament sealed into a compact transparent
envelope that is filled with a mixture of an inert gas and a small amount of a halogen such as iodine or
bromine.

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Intensity – In physics, intensity is the power transferred per unit area, where the area is measured on
the plane perpendicular to the direction of propagation of the energy. In the SI system, it has units
watts per square metre.

Microammeter – an instrument for measuring extremely small electric currents, calibrated in

microamperes.

Photovoltaic Cell – A solar cell, or photovoltaic cell, is an electrical device that converts the energy
of light directly into electricity by the photovoltaic effect, which is a physical and chemical
phenomenon.

Polarizer – A polarizer or polarizer is an optical filter that lets light waves of a specific polarization
pass through while blocking light waves of other polarizations. It can convert a beam of light of
undefined or mixed polarization into a beam of well-defined polarization, i.e. polarized light.

1.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017

2. University Practical Physics, D. C. Tayal – Himalaya Publishing House, 2000

1.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. Can we interchange polarizer and analyzer in this experiment?

Ans: Yes.

2. When the polarizer and analyzer are crossed, will any light reach the photocell?

Ans: No light will reach the photocell.

3. What is the meaning of polarization?

Ans: Polarization is a property applying to transverse waves that specifies the geometrical orientation
of the oscillations.

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EXPERIMENT 2: FOCAL LENGTH BY NODAL SLIDE

CONTENTS
2.1 Objectives

2.2 Introduction

2.3 Apparatus

2.4 Theory and Formula Used

2.5 Procedure

2.6 Observation

2.7 Calculation

2.8 Result

2.9 Precautions and Sources of Error

2.10 Summary

2.11 Glossary

2.12 References

2.13 Viva-voce Questions and Answers

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2.1 OBJECTIVES
To determine the focal length of the combination of two lenses separated by a distance with
the help of a nodal slide and then to verify the formula

1 1 1 𝑑
= + −
𝐹 𝑓1 𝑓2 𝑓1 𝑓2

where F is the focal length of the combination of two convex lenses of focal lengths 𝑓1 and 𝑓2, when
they are separated by a distance d.

2.2 INTRODUCTION
The focal length of the combination of two lenses separated by a distance d is given by the
formula

1 1 1 𝑑
= + −
𝐹 𝑓1 𝑓2 𝑓1 𝑓2

The focal point for the combined combination of lenses is a distance f from the secondary principal
point of the second lens. If you are approximating the lenses as thin then the answer you're looking for
is the distance from the second lens to the final focal point.

2.3 APPARATUS
Nodal slide assembly consisting of an optical bench comprising of four uprights – a bulb in a metallic
container, cross-slits screen, nodal slide and plane mirror, and two convex lenses of nearly the same
short focal lengths.

The function of the nodal slide is based on one property of nodal points of an optical system.
According to this property, if an incident ray passes through first nodal point N 1 of an optical system,
then after refraction through the system, the refracted ray necessarily emerges through its second
nodal point N2 in a direction parallel to the original direction.

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Figure 2: Experimental Setup

Figure 2: Nodal Slide

2.4 THEORY AND FORMULA USED

If 𝑓1 and 𝑓2 are the focal lengths of the two given convex lenses, separated by a distance ‘d’, then the
focal length of the combined system is given by the relation

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1 1 1 𝑑
= + −
𝐹 𝑓1 𝑓2 𝑓1 𝑓2

Thus, the determination of the focal length of the combination of the two lenses involves the separate
measurements of focal lengths of the two lenses 𝑓1 and 𝑓2. Moreover, the verification of the focal
length formula for the combination of two converging lenses system involves the measurement of F.

2.5 PROCEDURE
1. Mount a plane mirror, the nodal slide, the cross‐slit screen holder and the lamp on the optical
bench in such a way that their axis lies along the same horizontal line as illustrated in Figure
3.
2. Clamp one convex lens on the lens carriage at the centre of the nodal slide assembly. Adjust
the position of the nodal slide until the distance between the lens and screen is approximately
the focal length of the lens. Orient the mirror until the light from the object O on the screen,
rendered parallel by the lens, is reflected back normally and forms an image I of the object O
on the same screen. Move the nodal slide along the bench until the image I is sharply
focused.
3. The lens carriage is now rotated through a small angle and it will be found that the image
shifts sideward's to the right or the left. The lens carriage and the nodal slide upright are then
adjusted such that the image remains stationary for a slight rotation of the lens carriage.
4. The distance between the screen and the axis of the rotation of nodal slide for no shift in the
image measures the focal length of one face of the lens. The focal length of other face can be
determined by the turning the nodal slide through 180° and repeating the experiment. The
mean of focal length of both the faces is the focal length f1 of one lens.
5. Repeat the above procedure with the other lens and determine the focal length f2.
6. Now clamp both the lenses on the lens carriage at a known separation (d) in such a way that
both the lenses are at equidistance from the centre of the nodal slide assembly, shown in
Figure 4. Repeat the procedure with the combination of two lenses and determine the focal
length F.
7. The procedure is repeated at least three times with changing the distance between the lenses.

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Figure 3: Experimental arrangement to observe focal length f1 or f2 of any one convex lens.

Figure 4: Experimental arrangement to observe focal length F of combination of two lenses.

2.6 OBSERVATION

(i) Table for determination of focal length f1 and f2 of two convex lenses:

Position of Lamp = ……... cm

(ii) Table for determination of focal length F of combination of two convex lenses:

Position of Lamp = ……... cm

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2.7 CALCULATION
Theoretically, the focal length of a combination of two convex lenses is given by,

1 1 1 𝑑
= + −
𝐹 𝑓1 𝑓2 𝑓1 𝑓2

where F is the focal length of the combination of two convex lenses of focal lengths 𝑓1 and 𝑓2, when
they are separated by a distance d. Hence, calculate the value of focal length for different distances
using above formula.

2.8 RESULT
The values of calculated and observed focal lengths of combination of two lenses for each separation
are matching nearly. Hence the expression for the focal length of a combination of two convex lenses
is verified.

2.9 PRECAUTIONS AND SOURCES OF ERROR

1. The parallax should be removed very carefully and the stationary point is obtained.
2. All the uprights should be exactly at same height and at same horizontal axis.
3. The cross‐slit must be properly illuminated by the intense light coming from the lamp.
4. The rotation of the nodal slide carriage about the vertical axis while testing stationary point of
the image should not exceed by 5° or so.

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5. Lenses should be of small aperture to get well defined and sharp image on the screen.
6. The mirror employed must be truly plane mirror.

2.10 SUMMARY
In many optical instruments there may be compound lenses, that is, two or more lenses in contact.

Figure 5

In the first case, when the two lenses are in contact, the combined focal length F is given by the
formula

1 1 1
= +
𝐹 𝑓1 𝑓2

When the two lenses are separated by a distance d, the combined focal length of the combination of
lenses is given by the following formula

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1 1 1 𝑑
= + −
𝐹 𝑓1 𝑓2 𝑓1 𝑓2

2.11 GLOSSARY
Concave Lens – Concave lenses are thinner at the middle. Rays of light that pass through the lens are
spread out (they diverge). A concave lens is a diverging lens. When parallel rays of light pass through
a concave lens the refracted rays diverge so that they appear to come from one point called the
principal focus.

A bi-concave lens is symmetrical across both its horizontal and vertical axis.

Convex Lens – Convex lenses are thicker at the middle. Rays of light that pass through the lens are
brought closer together (they converge). A convex lens is a converging lens.

A bi-convex lens is symmetrical across both its horizontal and vertical axis.

Focal Length – The focal length of an optical system is a measure of how strongly the system
converges or diverges light. For an optical system in air, it is the distance over which initially
collimated (parallel) rays are brought to a focus. A system with a shorter focal length has greater
optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them
to a focus in a shorter distance.

Nodal Slide – The nodal slide is an instrument used for locating and measuring the cardinal points of
a lens or a system of lenses.

Principal Axis – a line passing through the centre of curvature of a lens or spherical mirror and
parallel to the axis of symmetry.

Reflection – Reflection is the change in direction of a wavefront at an interface between two different
media so that the wavefront returns into the medium from which it originated. Common examples
include the reflection of light, sound and water waves.

Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

2.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017

2. Engineering Physics Practical, S. K. Gupta – Krishna Prakashan Media, Meerut, 2010

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2.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. A bi-convex lens of focal length 10 cm is fixed to a plano-concave lens of focal length 20 cm made
of glass of the same refractive index. What is the focal length of the combination?

Ans: F = 10x(-20)/[10-20] = +20 cm

The focal length of the combination is positive and so it acts as a convex lens.

2. Why do you call it nodal slide?

Ans: It is so called because it is used to locate the nodal points of a lens system.

3. What do you mean by coaxial lens system?

Ans: A system of two or more lenses having common principal axis is called a coaxial lens system.

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EXPERIMENT 3: LOCATION OF CARDINAL POINTS

BY NODAL SLIDE METHOD
CONTENTS
3.1 Objectives

3.2 Introduction

3.3 Apparatus

3.4 Theory and Formula Used

3.5 Procedure

3.6 Observation

3.7 Calculation

3.8 Result

3.9 Precautions and Sources of Error

3.10 Summary

3.11 Glossary

3.12 References

3.13 Viva-voce Questions and Answers

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3.1 OBJECT IVES

To locate the positions of cardinal points of a coaxial optical system of two thin convergent
lenses separated by a distance, with the help of nodal slide and then to verify the formulae
𝑥𝐹 𝑥𝐹
𝐿1 𝐻1 = + 𝑎𝑛𝑑 𝐿2 𝐻2 = −
𝑓2 𝑓1

where F is the focal length of the coaxial optical system of two thin convex lenses of focal lengths 𝑓1
and 𝑓2, when they are separated by a distance x.

3.2 INTRODUCTION
The cardinal points lie on the optical axis of the optical system. Each point is defined by the effect the
optical system has on rays that pass through that point, in the paraxial approximation. The paraxial
approximation assumes that rays travel at shallow angles with respect to the optical axis, so that
sin 𝜃 ≈ 𝜃and cos 𝜃 ≈ 1.

Principal planes and points

The two principal planes have the property that a ray emerging from the lens appears to have crossed
the rear principal plane at the same distance from the axis that the ray appeared to cross the front
principal plane, as viewed from the front of the lens. This means that the lens can be treated as if all of
the refraction happened at the principal planes. The principal planes are crucial in defining the optical
properties of the system, since it is the distance of the object and image from the front and rear
principal planes that determine the magnification of the system. The principal points are the points
where the principal planes cross the optical axis.

If the medium surrounding the optical system has a refractive index of 1 (e.g., air or vacuum), then the
distance from the principal planes to their corresponding focal points is just the focal length of the
system. In the more general case, the distance to the foci is the focal length multiplied by the index of
refraction of the medium.

For a thin lens in air, the principal planes both lie at the location of the lens. The point where they
cross the optical axis is sometimes misleadingly called the optical centre of the lens. Note, however,
that for a real lens the principal planes do not necessarily pass through the centre of the lens, and in
general may not lie inside the lens at all.

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Nodal points

The front and rear nodal points have the property that a ray aimed at one of them will be refracted by
the lens such that it appears to have come from the other, and with the same angle with respect to the
optical axis. The nodal points therefore do for angles what the principal planes do for transverse
distance. If the medium on both sides of the optical system is the same (e.g., air), then the front and
rear nodal points coincide with the front and rear principal points, respectively.

3.3 APPARATUS
Nodal slide assembly consisting of an optical bench comprising of four uprights – a bulb in a metallic
container, cross-slits screen, nodal slide and plane mirror, and two convex lenses of nearly the same
short focal lengths.

The function of the nodal slide is based on one property of nodal points of an optical system.
According to this property, if an incident ray passes through first nodal point N 1 of an optical system,
then after refraction through the system, the refracted ray necessarily emerges through its second
nodal point N2 in a direction parallel to the original direction.

Figure 3: Experimental Setup

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Figure 2: Nodal Slide

3.4 THEORY AND FORMULA USED

In a coaxial optical system of two thin convergent lenses, the medium on either side of the lenses and
also between them is air; therefore, the nodal points coincide with the principal points. Hence, such an
optical system has 4 cardinal points.

 Two principal points (H1, H2) or nodal points (N1, N2), and
 Two focal points (F1, F2)

1. The distance of the first principal point H1 from the first lens L1 is given by

𝑥𝐹
𝐿1 𝐻1 = +
𝑓2

2. The distance of the first principal point H2 from the first lens L2 is given by

3.5 PROCEDURE

1. Make all the adjustments in the apparatus and measure the focal lengths 𝑓1 and 𝑓2 of the two
convergent lenses separately in a similar way as described in experiment 4.
2. Mount the convex lenses on the lens holders of the nodal slide such that the lens L 1 is
towards the plane mirror and lens L2 faces the cross-slits as shown in Figure 3.
3. Proceed as described in the previous experiment to measure the focal length of the
combination of lenses. When the position of the nodal slide carriage and nodal slide upright
are adjusted for no lateral shift, the cross-slit screen lies in the second focal plane and the
nodal slide upright represents the second nodal plane of the lens system. Since the nodal

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points coincide with the principal points, the distance between the lens L2 and the principal
point H2 or axis of rotation H2 L2. Note this distance between lens L2 and axis of rotation of
nodal slide or nodal slide upright. The distance H2 F2 between the nodal slide upright (or axis
of rotation of nodal slide) and cross-slit upright are recorded from their index marks on the
optical bench. This is equal to the focal length F of the combination. By convention L 2H2 is
negative as it lies left to the lens L2.
4. Now rotate the nodal slide carriage through 1800 so that the positions of lenses L1 and L2
interchange. In this stage, L1 faces the cross-slit and L2 is towards the plane mirror. Again,
obtain the position of no lateral shift of the image. Note the distance of the first principal
point H1 from the first lens L1 by recording readings of their respective uprights. Also note
the distance H1F1, that is the focal length of combination of two lenses when separated by
distance x.
5. Calculate the bench correction. The distance measured between index marks of the cross-
slits and nodal slide uprights may not be the same as the actual distance between pole of the
lens or lens system and the center of the cross-slit. Thus, there is an error in the measurement
of the focal length. To find the correct focal length, index correction or bench error is to be
applied. To find the bench error, a glass rod of about 1 m is kept parallel to the bench such
that its one end touches the center of the slit and the other to the center of the lens of optical
system. Note the position index marks of the uprights on the optical bench. Let their
difference be y, then

Bench error = Actual distance or actual length of the rod – observed distance
= (y – x) [+ve or –ve]

Figure 3: Experimental setup

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3.6 OBSERVATION

(i) Table for the measurement of focal lengths of the two lenses

Length of glass rod, y = ……... cm

Observed distance between the cross-slits and nodal slide upright, x = ……….. cm

Bench correction, y – x = ………….. cm

(ii) Table for determination of focal length F of the combination and distance L1H1 and L2H2:

Separation between the two lenses, x = ……... cm

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3.7 CALCULATION
1. The mean focal length of the combination of two coaxial thin convex lenses F = ……cm
2. The theoretical value of 𝐿1 𝐻1 as obtained by the relation

𝑥𝐹
𝐿1 𝐻1 = + = +____𝑐𝑚
𝑓2
3. The theoretical value of 𝐿2 𝐻2 as obtained by the relation

𝑥𝐹
𝐿2 𝐻2 = − = +____𝑐𝑚
𝑓1

3.8 RESULT
The calculated and experimentally determined values of 𝐿1 𝐻1 and 𝐿2 𝐻2 are approximately the same
and hence the formulae

𝑥𝐹 𝑥𝐹
𝐿1 𝐻1 = + 𝑎𝑛𝑑 𝐿2 𝐻2 = −
𝑓2 𝑓1

are verified.

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Location of Cardinal Points

To locate the cardinal points, sketch the two thin converging lenses at a known distance x as shown in
Figure 4. Mark the positions of first and second nodal points H1 and H2 at a distance 𝐿1 𝐻1 from lens
𝐿1 and 𝐿2 𝐻2 from lens 𝐿2 on the common principal axis of the two lenses with proper sign. As the
medium on both sides of the lenses are air, the principal points coincide with the nodal points,
therefore, mark N1 and N2 on H1 and H2, respectively. Measure the distance H1F1 from H1 and H2F2
from H2 and locate focal points F1 and F2 with proper sign.

Figure 4

3.9 PRECAUTIONS AND SOURCES OF ERROR

1. All uprights arranged on the optical bench should be adjusted to the same height.
2. The cross-slits must be properly and intensely illuminated.
3. For obtaining well defined and sharp image of the cross-slit on the cross-slit screen, the
aperture of the lens or lenses should be taken small.
4. For searching nodal points on the principal axis of the lens system or lens, the rotation of the
nodal slide about the vertical axis should not exceed 50 or so.
5. The mirror used in the experiment should be truly plane.
6. The position of no shift should be precisely determined.
7. Bench error should be properly accounted for.
8. To avoid false images, the plane mirror is slightly turned, which turns the genuine image
while the false image will remain stationary.

3.10 SUMMARY
The points on the optical axis OO’ (see Figure 4) of a centered optical system that can be used to
construct the image of an arbitrary point in space for objects in the paraxial region, which is the region
around the axis of symmetry of the system where a point is represented by a point, a straight line by a
straight line, and a plane by a plane.

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Figure 5: Position of the image A’ of an arbitrary point A projected by an optical system S can be
found if the cardinal points F, F’, H, and H’ of the system are known: a ray passing through the front
focus F is directed by the system parallel to its optical axis OO’, and a ray that is incident upon the
system parallel to the axis OO’ is directed through the back focus F’ after refraction

There are four cardinal points in an optical system: the front and back foci F and F’ and the
front and back principal points H and H’. The back focus is the image of an infinitely remote
point located on the optical axis in the object space, and the front focus is the image in the
object space of an infinitely remote point in the image space. The principal points are the
points of intersection with the optical axis of the principal planes, which are the planes for
which the optical system S produces full-size mutual images (every point H1 located in the
principal plane HH1 at a distance h from the axis OO’ appears in the other principal plane H’
H’1 as the point H’1 at the same distance h from the axis as point H1).

The distance from point H to point F is called the front focal distance (negative in the figure),
and the distance from point H’ to point F’ is called the back focal distance (positive in the
figure).

The construction of an image A’ of an arbitrary point A for a centered optical system using
the points F, H, H’, and F’ is shown in Figure 4.

3.11 GLOSSARY
Concave Lens – Concave lenses are thinner at the middle. Rays of light that pass through the lens are
spread out (they diverge). A concave lens is a diverging lens. When parallel rays of light pass through
a concave lens the refracted rays diverge so that they appear to come from one point called the
principal focus.

A bi-concave lens is symmetrical across both its horizontal and vertical axis.

Convex Lens – Convex lenses are thicker at the middle. Rays of light that pass through the lens are
brought closer together (they converge). A convex lens is a converging lens.

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A bi-convex lens is symmetrical across both its horizontal and vertical axis.

Focal Length – The focal length of an optical system is a measure of how strongly the system
converges or diverges light. For an optical system in air, it is the distance over which initially
collimated (parallel) rays are brought to a focus. A system with a shorter focal length has greater
optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them
to a focus in a shorter distance.

Nodal Slide – The nodal slide is an instrument used for locating and measuring the cardinal points of
a lens or a system of lenses.

Principal Axis – a line passing through the centre of curvature of a lens or spherical mirror and
parallel to the axis of symmetry.

Reflection – Reflection is the change in direction of a wavefront at an interface between two different
media so that the wavefront returns into the medium from which it originated. Common examples
include the reflection of light, sound and water waves.

Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

3.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017

2. Engineering Physics Practical, S. K. Gupta – Krishna Prakashan Media, Meerut, 2010

3.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. What are cardinal points of an optical system?

Ans: There are six cardinal points of an optical system, namely:

a) Two focal points

b) Two principal points
c) And two nodal points

2. What is the importance of cardinal points of the coaxial optical system?

Ans: By the knowledge of cardinal points, one can treat the optical system of coaxial lenses as a
single lens and the position and size of the image of an object may directly be obtained by using
simple formulae developed for thin lenses without considering refraction through each component of
the system separately.

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3. Under what condition, the six cardinal points of an optical system reduces to four?

Ans: If the medium on either side of the optical system and also between the lenses is same (or air),
the nodal points coincide with the principal points. Hence, the six cardinal points reduces to four.

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EXPERIMENT 4: HARTMANN’S FORMULA USING

PRISM SPECTROMETER

CONTENTS
4.1 Objectives

4.2 Introduction

4.3 Apparatus

4.4 Theory and Formula Used

4.5 Procedure

4.6 Observation

4.7 Calculation

4.8 Result

4.9 Precautions and Sources of Error

4.10 Summary

4.11 Glossary

4.12 References

4.13 Viva-voce Questions and Answers

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4.1 OBJECTIVES
To verify Hartmann’s dispersion formula using a constant deviation spectrometer.

4.2 INTRODUCTION
Hartmann’s dispersion formula is a semi-empirical formula relating the index of refraction n and
wavelengths λ. It is also referred to as Cornu-Hartmann formula.

4.3 APPARATUS
Constant deviation spectrometer, mercury lamp, sodium lamp, reading lamp and reading lens.

Figure 1: (a) Constant Deviation Spectrometer; (b) Constant Deviation Prism.

4.4 THEORY AND FORMULA USED

Hartmann suggested an equation for the wavelength of the spectral line which is at a distance x from a
fixed mark in the spectrum and is known as Hartmann’s dispersion formula.

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It is given by

𝐵
=𝐴+
𝑥−𝐶

where A, B and C are constants.

4.5 PROCEDURE
8. Place the sodium lamp before the collimator. Open the slit widely and see it through the
telescope tube (without eye piece).
9. Adjust the position and height of the lamp to get the image in the center of he prism face.
10. Remove the dust particles if any and narrow the slit to have a fine pencil of rays.
11. Set the wavelength drum at 5890 A0 (D1 or D2 marked on the drum). Focus the eye piece. If
the image of the spectral line viewed in the eye piece is far removed from the center of the
field of view of the eye piece, the constant deviation prism requires setting. If the image of
the spectral line is near the center, do not move the prism. A minor adjustment by means of
screws provided with the eye piece is sufficient. The spectrometer is now set up.
12. Replace the sodium lamp by the mercury lamp, which provides a number of lines of
accurately known wavelengths in the visible spectrum. Note these wavelengths from the
standard tables.
13. Rotate the wavelength drum to bring the spectral lines in turn to coincide with the cross-wire
or the pointer in the field of view of the eye piece. Read and record the wavelengths of the
visible spectral lines on the drum. Now compare these wavelengths with the standard
wavelengths noted from the table.
14. With the help of the drum, adjust the spectrum in the field of view (if whole of the spectrum
does not come in the field of view, divide it into two parts). Note down the position of the
lines on the micrometer scale attached with the eye piece.

4.6 OBSERVATION
Table for the measurement of wavelength of spectral lines and their positions

Wavelength of spectral lines Positions of spectral lines

1 = 𝑥1 =

2 = 𝑥2 =

3 = 𝑥3 =

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4.7 CALCULATION
Consider three conveniently spaced lines of known wavelengths 1 , 2 and 3 . Their positions 𝑥1 , 𝑥2
and 𝑥3 are noted on the micrometer scale attached with the eye piece (or a travelling microscope).

Using the relation

𝐵 𝐵 𝐵
1 = 𝐴 + , 2 = 𝐴 + , 3 = 𝐴 +
𝑥1 − 𝐶 𝑥2 − 𝐶 𝑥2 − 𝐶

A, B and C are calculated.

Now using the values of A, B and C, the wavelengths of other spectral lines are calculated with the
Hartmann’s dispersion formula by substituting the observed values of x for these lines.

4.8 RESULT
The wavelengths so obtained are compared with the drum’s readings and the values given in the
standard tables. Since, the values are very close to the required or expected values, hence the
Hartmann’s formula is verified.

4.9 PRECAUTIONS AND SOURCES OF ERROR

7. The mechanical and optical adjustments of the telescope must be made carefully and
correctly.
8. While taking observations, the prism table and telescope must keep clamped.
9. The prism should be placed properly and correctly on the prism table.
10. Telescope should be rotated in the same direction.
11. While taking observations, the prism table and the telescope should never be unclamped
together.
4.10 SUMMARY
A simple interpolation formula for the prismatic spectrum was proposed by J. Hartmann in
Astrophysical Journal, vol. 8, p.218. In this experiment, we compare the wavelengths obtained with
the drum’s readings and the values given in the standard tables. Because, the values are very close to
the required or expected values, the Hartmann’s formula is deemed to be true.

4.11 GLOSSARY
Collimator – A collimator is a device that narrows a beam of particles or waves. To narrow can mean
either to cause the directions of motion to become more aligned in a specific direction (i.e., make

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collimated light or parallel rays), or to cause the spatial cross section of the beam to become smaller
(beam limiting device).

Dispersion – the separation of white light into colours or of any radiation according to wavelength.

Prism – Prisms can be made from any material that is transparent to the wavelengths for which they
are designed. Typical materials include glass, plastic, and fluorite. A dispersive prism can be used to
break light up into its constituent spectral colors (the colors of the rainbow).

Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

Semi-empirical – partly empirical; especially involving assumptions, approximations, or

generalizations designed to simplify calculation or to yield a result in accord with observation.

Spectral Line – A spectral line is a dark or bright line in an otherwise uniform and continuous
spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with
the nearby frequencies. Spectral lines are often used to identify atoms and molecules.

Spectrometer – an apparatus used for recording and measuring spectra, especially as a method of
analysis.

Spectrum – Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and
infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an
instrument that photographs or maps spectra is a spectrograph.

Wavelength – the distance between successive crests of a wave, especially points in a sound wave or
electromagnetic wave.

4.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017

2. University Practical Physics, D. C. Tayal – Himalaya Publishing House, 2000

4.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. Can we identify each element with a constant deviation spectrometer?

Ans: An unknown element can be identified with its characteristic spectral lines.

2. What is the use of a spectrometer and what are its main components?

Ans: The spectrometer is an instrument used to obtain a pure spectrum. It has three components:

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(a) Collimator
(b) Telescope
(c) Prism Table

3. What is a travelling microscope?

Ans: A travelling microscope is an instrument for measuring length with a resolution typically in the
order of 0.01mm. The precision is such that better-quality instruments have measuring scales made
from Invar to avoid misreadings due to thermal effects. The instrument comprises a microscope
mounted on two rails fixed to, or part of a very rigid bed. The position of the microscope can be
varied coarsely by sliding along the rails, or finely by turning a screw. The eyepiece is fitted with fine
cross-hairs to fix a precise position, which is then read off the vernier scale.

4. How are spectra classified?

Ans: Spectra may be classified according to the nature of their origin, i.e., emission or absorption.

An emission spectrum consists of all the radiations emitted by atoms or molecules, whereas in an
absorption spectrum, portions of a continuous spectrum (light containing all wavelengths) are missing
because they have been absorbed by the medium through which the light has passed; the missing
wavelengths appear as dark lines or gaps.

The spectrum of incandescent solids is said to be continuous because all wavelengths are present. The
spectrum of incandescent gases, on the other hand, is called a line spectrum because only a few
wavelengths are emitted. These wavelengths appear to be a series of parallel lines because a slit is
used as the light-imaging device. Line spectra are characteristic of the elements that emit the
radiation. Line spectra are also called atomic spectra because the lines represent wavelengths radiated
from atoms when electrons change from one energy level to another. Band spectra is the name given
to groups of lines so closely spaced that each group appears to be a band, e.g., nitrogen spectrum.
Band spectra, or molecular spectra, are produced by molecules radiating their rotational or vibrational
energies, or both simultaneously.

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EXPERIMENT 5: VERIFICATION OF CAUCHY’S

DISPERSION FORMULA

CONTENTS
5.1 Objectives

5.2 Introduction

5.3 Apparatus

5.4 Theory and Formula Used

5.5 Procedure

5.6 Observation

5.7 Calculation

5.8 Result

5.9 Precautions and Sources of Error

5.10 Summary

5.11 Glossary

5.12 References

5.13 Viva-voce Questions and Answers

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5.1 OBJECTIVES
To find the refractive index and Cauchy’s constants of a prism using spectrometer.

5.2 INTRODUCTION
Cauchy's equation is an empirical relationship between the refractive index and wavelength of light
for a particular transparent material. It is named for the mathematician Augustin-Louis Cauchy, who
defined it in 1836.

Principle: It is based on the phenomenon of dispersion of light, is split or dispersed into different
colors, when it passes through a prism. Shorter wavelengths (like blue) bend the most while longer
wavelengths (like red) bend the least.

5.3 APPARATUS
Spectrometer, prism, mercury lamp, spirit level.

Spectrometer Design:

The Collimator: The collimator C consists of two hollow concentric metal tubes, one being longer
than other. The longer tube carries an achromatic lens L at one end and the smaller tube at the other
end. The smaller tube is provided with a slit at the outer end and can be moved in our out the longer
tube with the help of rack and pinion arrangement. The slit is adjusted in the focal plane of the lens L
to obtain a pencil of parallel rays from the collimator when light is allowed to be incident upon the
slit. The collimator is also provided with two screws for adjusting inclination of the axis of the
collimator. This is rigidly fixed to the main part of the apparatus.

The Prism Table: It is a circular table supported horizontally in the center of the instrument and the
position can be read with the help of two verniers attached to it and moving over a graduated circular
scale carried by the telescope. The leveling of the prism table is made with the help of three screws
provided at the lower surface. The table can be raised or lowered and clamped in any desired position
with the help of a screw. The prism table is also provided with tangent screws for a slow motion.
There are concentric circles and straight lines parallel to the line joining two of the leveling screws on
the prism table.

The Telescope: The telescope consists of similar tubes as in case of collimator carrying achromatic
objective lens O at one end and eyepiece E on the another side end. The eyepiece tube can be taken in
or out with the help of rack and pinion arrangement. Two cross wire are focused on the focus of the
eyepiece. The telescope can be clamped to the main body of the instrument and can be moved slightly

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by tangent screws. The telescope is attached to the main scale and when it rotates, the graduated scale
rotates with it. The inclination of telescope is adjusted by two screws provided at the lower surface.

5.4 THEORY AND FORMULA USED

(i) The refractive index of the material of the prism is given by the formula.

 A m 
sin 
  2 
 A
sin 
2

(ii) Variation of refractive index with wavelength may be represented by the Cauchy’s relation

B
  A
2

Where A and B are the Cauchy’s constant and can be determined as

1 1 B B
B  1   2 /  .......... cm 2 , A  1   2  ....cm 2

2
1  2
2 
2
1  2
2

5.5 PROCEDURE
1. Adjust the spectrometer for parallel rays.
2. Determine the least count of the spectrometer.
3. Place the prism on the prism table with its refracting edge at the centre and towards the
collimator as shown in Fig. 1.
4. The light reflected from each of the two polished faces is observed through the telescope. The
image of the slit so formed is focused on the cross-wire and the two positions of the telescope
are noted. The difference of the two readings gives twice the angle of the prism i.e. 2α.
5. Now place the prism such that its centre coincides with the centre of the prism table and the
light falls on one of the polished face (Fig.2).
6. The spectrum obtained out of the other face is observed through the telescope. Set the
telescope at particular color. Rotate the prism table in one direction adjusting the telescope
simultaneously to keep the spectral line in view. On continuing this rotation in the same
direction, a position will come where the spectral lines recede in the opposite direction. This
position where the spectrum turns away is the minimum deviation position for this color.
Lock the prism table and note the readings of the verniers.
7. Set the telescope crosswire on another color and again note the vernier readings. Take this
observation for various colors.
8. Removal the prism and see the slit directly through the telescope. Set the slit on the crosswire
and note the readings of the verniers.

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9. The difference in minimum deviation positions of various colors and direct positions of the
slit give the angles of minimum deviation for corresponding colors.

Figure 4

Figure 5

5.6 OBSERVATION
(a) Determination of the least count of the Spectrometer.

Value of one Main Scale Division x = Degree

Total number of divisions in circular scale, n =

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Hence, least count of the microscope screw = x/n = Degree

(b) Table for the angle of Prism (A)

Verniers Telescope Readings for Reflection from Difference Angle of Mean

Prism(A) A
First face (a) Degree Second face (b) Degree 2A=b-a Degree
Degree
M.S. V.S. Total M.S. V.S. Total Degree

Degree Degree

V1

V2

(c) Table for angle of minimum deviation (δm)

Telescope Readings for Difference Mean

Color Vernier Dispersed image Direct image δm = b-a δm

M.S. V.S. Total M.S. V.S. Total Degree Degree

Degree Degree

V1

Blue

V2

V1

Green V2

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5.7 CALCULATIONS
Using formula from equation (3) we can calculate the values of refractive index, μ, for all the colors.
Refractive index for the material of the prism for different wavelengths is given in the following table

Sr. No. Color Standard Wavelength Refractive index, 

 in Å

1 Blue 4693

2 Green 5461

Using any two values of wavelengths and μ for two colors, we get the values of B and A by using
equations (2a) and (2b) respectively.

5.8 RESULT
(i) The refractive indices of the material of prism for various colors are in table above.

(ii) The Cauchy’s constants are A = and B =

5.9 PRECAUTIONS AND SOURCES OF ERROR

(i) Spectrometer leveling and adjustments should be properly done.

(ii) The slit should be sharp and vertical

(iii) The position of angle of minimum deviation should be accurately determined.

(iv) The refracting surfaces of the prism should not be touched with fingers.

5.10 SUMMARY
When an electromagnetic wave is incident on an atom or a molecule, the periodic electric force of the
wave sets the bound charges into vibratory motion. The frequency with which these charges are
forced to vibrate is equal to the frequency of the wave. The phase of this motion as compared to the

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impressed electric force will depend on the impressed frequency. It will vary with the difference
between the impressed frequency and the natural frequency of the charges.

Dispersion can be explained with the concept of secondary waves that are produced by the induced
oscillations of the bound charges. When a beam of light propagates through a transparent medium
(solid or liquid), the amount of lateral scattering is extremely small. The scattered waves travelling in
a lateral direction produce destructive interference.

However, the secondary waves travelling in the same direction as the incident beam superimpose on
one another. The resultant vibration will depend on the phase difference between the primary and the
secondary waves. This superimposition, changes the phase of the primary waves and this is equivalent
to a change in the wave velocity. Wave velocity is defined as the speed at which a condition of equal
phases is propagated. Hence the variation in phase due to interference, changes the velocity of the
wave through the medium. The phase of the oscillations and hence that of the secondary waves
depends upon the impressed frequency. It is clear, therefore, that the velocity of light in the medium
varies with the frequency of light. Also refractive index depends upon the velocity of light in the
medium. Therefore the refractive index of the medium varies with the frequency (wavelength) of
light.

Cauchy's equation is an empirical relationship between the refractive index and wavelength of light
for a particular transparent material. It is named for the mathematician Augustin-Louis Cauchy, who
defined it in 1836. The theory of light-matter interaction on which Cauchy based this equation was
later found to be incorrect. In particular, the equation is only valid for regions of normal dispersion in
the visible wavelength region. In the infrared, the equation becomes inaccurate, and it cannot
represent regions of anomalous dispersion. Despite this, its mathematical simplicity makes it useful in
some applications.

The Sellmeier equation is a later development of Cauchy's work that handles anomalously dispersive
regions, and more accurately models a material's refractive index across the ultraviolet, visible, and
infrared spectrum.

5.11 GLOSSARY
Dispersion – the separation of white light into colors or of any radiation according to wavelength. It
is the phenomenon in which the phase velocity of a wave depends on its frequency.

Prism – Prisms can be made from any material that is transparent to the wavelengths for which they
are designed. Typical materials include glass, plastic, and fluorite. A dispersive prism can be used to
break light up into its constituent spectral colors (the colors of the rainbow).

Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

Spectrometer – an apparatus used for recording and measuring spectra, especially as a method of
analysis.

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Spectrum – Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and
infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an
instrument that photographs or maps spectra is a spectrograph.

Wavelength – the distance between successive crests of a wave, especially points in a sound wave or
electromagnetic wave.

Wave Interference – In physics, interference is a phenomenon in which two waves superpose to

form a resultant wave of greater, lower, or the same amplitude.

5.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017s

2. University Practical Physics, D. C. Tayal – Himalaya Publishing House, 2000

5.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. What are the values of A and B in the Cauchy’s dispersion formula depend on?

Ans: The values of A and B, the Cauchy’s constants, depend on the medium and can be determined
for a material by fitting the equation to measured refractive indices at known wavelengths.

2. Does the refractive index of a medium decreases or increases with an increase in the wavelength of
light?

Ans: It is evident that the refractive index of the medium decreases with increase in wavelength of
light.

3. What are some of the desirable and undesirable effects of dispersion vis-à-vis optical applications?

Ans: Material dispersion can be a desirable or undesirable effect in optical applications.

The dispersion of light by glass prisms is used to construct spectrometers and spectroradiometers.
Holographic gratings are also used, as they allow more accurate discrimination of wavelengths.

However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade
images in microscopes, telescopes, and photographic objectives. In optics, chromatic aberration
(abbreviated CA; also called chromatic distortion and spherochromatism) is an effect resulting from
dispersion in which there is a failure of a lens to focus all colors to the same convergence point. It
occurs because lenses have different refractive indices for different wavelengths of light. The
refractive index of transparent materials decreases with increasing wavelength in degrees unique to
each.

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EXPERIMENT 6: DISPERSIVE POWER OF A PRISM

CONTENTS
6.1 Objectives

6.2 Introduction

6.3 Apparatus

6.4 Theory and Formula Used

6.5 Procedure

6.6 Observation

6.7 Calculation

6.8 Result

6.9 Precautions and Sources of Error

6.10 Summary

6.11 Glossary

6.12 References

6.13 Viva-voce Questions and Answers

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6.1 OBJECT IVES

To determine the dispersive power of the material of the prism with the help of a
spectrometer.

6.2 INTRODUCTION
Dispersive power is basically a measure of the amount of difference in the refraction of the highest
and lowest wavelengths that enter the prism. This is expressed in the angle between the two extreme
wavelengths (i.e. Red and Violet). The greater the dispersive power, the greater the angle between
them, and vice-versa.

6.3 APPARATUS
Spectrometer, mercury lamp (white light source), prism, spirit level, electric lamp and reading lamp.

6.4 THEORY AND FORMULA USED

If 𝜇𝑣 , 𝜇𝑟 and 𝜇𝑦 be the refractive indices of the material of the prism for violet, red and yellow colors,
respectively, then the dispersive power 𝜔 of the material of prism is given by

𝜇𝑣 − 𝜇𝑟
𝜔=
𝜇𝑦 − 1

The refractive index of the material of prism for a particular wavelength is given by

 A m 
sin 
  2 
 A
sin 
2

where A is the refracting angle of the prism and  m is the angle of minimum deviation for the ray of
given wavelength.

Therefore, the determination of dispersive power of a material of prism involves the measurements of
refracting angle of the prism A and the angles of minimum deviation  m for violet, yellow and red
colors.

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6.5 PROCEDURE
1. DO NOT PLACE THE PRISM ON THE SPECTROMETER YET.

2. First check leveling of the spectrometer base, prism table, collimator and telescope. If needed, level
them using the adjustment screws and a spirit level.

3. The collimator is adjusted for parallel beam of light and the telescope for focusing the parallel
beam by Schuster’s method (details of which is given in another experiment). But the present set up
may not require it.

4. Adjusting the telescope: While looking through the telescope, slide the eyepiece in and out until
the crosswire comes into sharp focus. Point the telescope at some distant object and view it through
the telescope. Turn the focus knob of telescope until the image is sharp. The telescope is now focused
for parallel light rays. DO NOT change the focus of the telescope henceforth.

5. Ensure the Hg lamp is fully illuminated and placed close to the slit of the collimator. Check that the
slit is partially open.

6. Adjusting the collimator: Align the telescope directly opposite the collimator and look through
the telescope, to see a focused image of the slit. If necessary, adjust the slit width until the image of
the slit as seen through the telescope is sharply focused on the crosswire. The collimator is then set to
produce parallel light from the slit.

7. Determine the vernier constant of the spectrometer. Report all the angles in degree unit. Details
about reading angles in spectrometer are given in the manual for finding angle of minimum deviation.

Figure 6

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8. Angle of prism: (Refer Fig. 1)

Place the prism such that its vertex is at the center of the prism table, directly in line with the
illuminated slit.

The opaque face (AC) should face towards you so that light from the collimator is reflected at the
two faces AB and AC.

Rotate and adjust the telescope to position I where the image of the slit reflected at AB is centered
on the crosswire. Record the angular positions on each vernier.

Now, turn the telescope to position II for the image reflected at AC and record again the angular
positions on each vernier.

Take three independent sets of readings for telescope position I and II on each vernier. Let the

mean of these three sets of readings of the two verniers V1 and V2 are respectively,

telescope position I: α1 , α2

telescope position II: β1 , β2

Then the mean angle of the prism A is obtained using 2A = (A1 + A2)/2, where A1 = α1 ~ β1 and A2
= α2 ~ β2

9. Direct ray reading: Remove the prism from the spectrometer and align the telescope so that the
direct image of the slit is seen through the telescope centered on the crosswire. Record the angular
position of the telescope on the two verniers as D1 and D2. This will be the reference angular position
for any measurements later.

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Figure 7

10. Angle of minimum deviation: (Refer Fig. 2)

Replace the prism on the spectrometer table so that it is oriented as shown in Fig. 2.

Locate the image of the spectrum with naked eye. Then rotate the telescope to bring the spectrum in
the field of view.

Gently turn the prism table back and forth. As you do so, the spectrum should appear to migrate in
one direction until a point at which it reverses its direction.

Lock the prism table. Now, using fine adjustment screw of the telescope fix the crosswire on one of
the spectral lines of wavelength λ1 at an extreme end.

. Then move the prism table using fine adjustment screw so that the angle where the line starts
reversing its direction is precisely located. Take three such independent readings. Let the mean of
these readings on the two verniers V1 and V2 for λ1 are θ1 and θ'1. Calculate the mean value of δm(λ1)
as follows:

Similarly, note down the angles of minimum deviation for all the spectral lines, whose wavelengths
and colors are given in the chart.

11. Calculate the refractive index for each wavelength and then determine the dispersive power.

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6.6 OBSERVATION

Table 1: Determination of vernier constant (VC) of the spectrometer

Value of 1 small main scale division (MSD) = ………

……. vernier scale divisions = ….. main scale divisions

Hence, 1 vernier scale division = ………… main scale division (VSD)

Vernier Constant (VC) = (1 – VSD) x MSD = ………

Table-1. Determination of the angle of the prism

Table-2. Direct ray reading

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Table-3. Angle of minimum deviation for various λ

6.7 CALCULATIONS

Dispersive power of the prism =

𝜇𝑣 − 𝜇𝑟
𝜔=
𝜇𝑦 − 1

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6.8 RESULT
(i) The dispersive power of the prism is _________.

6.9 PRECAUTIONS AND SOURCES OF ERROR

1. Do not touch the refracting surfaces by hand. Place the prism on the prism table or remove it from
the prism table by holding it with fingers at the top and bottom faces. The reflecting surfaces of the
prism should be cleaned with a piece of cloth soaked in alcohol.

2. Rotate the adjustment screws slowly. Do not force any movement. If something is not moving
check the clamping screw. Use fine adjustment screw after locking the clamping screw.

3. Take the readings without any parallax errors.

6.10 SUMMARY
The difference between the refractive indices of a transparent material for a specific blue light and a
specific red light is known as the dispersion of the material. The usual choices of blue and red lights
are the so-called “F” and “C” lines of hydrogen in the solar spectrum, named by Fraunhofer, with
wavelengths 4861 and 6563 angstroms (the angstrom unit, abbreviated Å, is 10−8 centimeter),
respectively.

It is generally more significant, however, to compare the dispersion with the mean refractive index of
the material for some intermediate color such as the sodium “D” Fraunhofer line of wavelength 5893
angstroms. The dispersive power (𝜔) of the material is then defined as the ratio of the difference
between the “F” and “C” indices and the “D” index reduced by 1.

6.11 GLOSSARY
Dispersion – the separation of white light into colors or of any radiation according to wavelength. It
is the phenomenon in which the phase velocity of a wave depends on its frequency.

Dispersive Power – a measure of the amount of difference in the refraction of the highest and lowest
wavelengths that enter the prism. This is expressed in the angle between the two extreme wavelengths
(i.e. rED and Violet). The greater the dispersive power, the greater the angle between them, and vice-
versa.

Prism – Prisms can be made from any material that is transparent to the wavelengths for which they
are designed. Typical materials include glass, plastic, and fluorite. A dispersive prism can be used to
break light up into its constituent spectral colors (the colors of the rainbow).

Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

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Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

Spectrometer – an apparatus used for recording and measuring spectra, especially as a method of
analysis.

Spectrum – Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and
infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an
instrument that photographs or maps spectra is a spectrograph.

Wavelength – the distance between successive crests of a wave, especially points in a sound wave or
electromagnetic wave.

6.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017

2. University Practical Physics, D. C. Tayal – Himalaya Publishing House, 2000

6.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. What is total internal reflection? How is it useful?

Ans: When a ray of light emerges obliquely from glass into air, the angle of refraction between ray
and normal is greater than the angle of incidence inside the glass, and at a sufficiently high obliquity
the angle of refraction can actually reach 90°. In this case the emerging ray travels along the glass
surface, and the sine of the angle of incidence inside the glass, known as the critical angle, is then
equal to the reciprocal of the refractive index of the material.

At angles of incidence greater than the critical angle, the ray never emerges, and total internal
reflection occurs.

Light is totally internally reflected in many types of reflecting prism and in fiber optics, in which long
fibers of high-index glass clad with a thin layer of lower index glass are assembled side-by-side in
precise order. The light admitted into one end of each fiber is transmitted along it without loss by
thousands of successive internal reflections at the interlayer between the glass and the cladding.
Hence, an image projected upon one end of the bundle will be dissected and transmitted to the other
end, where it can be examined through a magnifier or photographed. Many modern medical
instruments, such as cystoscopes and bronchoscopes, depend for their action on this principle. Single
thick fibers (actually glass rods) are sometimes used to transmit light around corners to an otherwise
inaccessible location.

2. What happens to the dispersive power of a prism immersed in water?

Ans: By immersing the prism in water, you effectively reduce the difference in the refractive indices
of the two media (because refractive index of glass > refractive index of water > refractive index of
air) which results in a smaller Δθ (or smaller refraction).

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EXPERIMENT 7: DISPERSIVE POWER OF A

GRATING
CONTENTS
7.1 Objectives

7.2 Introduction

7.3 Apparatus

7.4 Theory and Formula Used

7.5 Procedure

7.6 Observation

7.7 Calculation

7.8 Result

7.9 Precautions and Sources of Error

7.10 Summary

7.11 Glossary

7.12 References

7.13 Viva-voce Questions and Answers

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7.1 OBJECTIVES
Measurement of the wavelength separation of sodium D-lines using a diffraction grating and to
calculate the angular dispersive power of the grating.

7.2 INTRODUCTION
In optics, a diffraction grating is an optical component with a periodic structure that splits and
diffracts light into several beams travelling in different directions. The emerging coloration is a form
of structural coloration. The directions of these beams depend on the spacing of the grating and the
wavelength of the light so that the grating acts as the dispersive element. Because of this, gratings are
commonly used in monochromators and spectrometers.

For practical applications, gratings generally have ridges or rulings on their surface rather than dark
lines. Such gratings can be either transmissive or reflective. Gratings that modulate the phase rather
than the amplitude of the incident light are also produced, frequently using holography.

The principles of diffraction gratings were discovered by James Gregory, about a year after Newton's
prism experiments, initially with items such as bird feathers. The first man-made diffraction grating
was made around 1785 by Philadelphia inventor David Rittenhouse, who strung hairs between two
finely threaded screws. This was similar to notable German physicist Joseph von Fraunhofer's wire
diffraction grating in 1821.

Diffraction can create "rainbow" colors when illuminated by a wide spectrum (e.g., continuous) light
source. The sparkling effects from the closely spaced narrow tracks on optical storage disks such as
CDs or DVDs are an example, while the similar rainbow effects caused by thin layers of oil (or
gasoline, etc.) on water are not caused by a grating, but rather by interference effects in reflections
from the closely spaced transmissive layers. A grating has parallel lines, while a CD has a spiral of
finely-spaced data tracks. Diffraction colors also appear when one looks at a bright point source
through a translucent fine-pitch umbrella-fabric covering. Decorative patterned plastic films based on
reflective grating patches are very inexpensive, and are commonplace.

7.3 APPARATUS
Spectrometer, prism, a diffraction grating of known grating element, spirit level, electric lamp, electric
lamp and reading lens.

7.4 THEORY AND FORMULA USED

The sodium spectrum is dominated by the bright doublet known as the sodium D-lines at 589.0 and
589.6 nanometers. Using an appropriate diffraction grating the wavelength separation of these two
lines can be determined. A schematic for diffraction of sodium light (Na-D lines) with a plane
transmission grating is shown in Fig. 1.

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Diffraction Grating:

An arrangement consisting of a large number of parallel slits of the same width and separated by
equal opaque spaces is known as diffraction grating. It is usually made by ruling equidistant,
extremely close tine grooves with a diamond point on an optically plane glass plate. A photographic
replica of a plate made in this way is often used as a commercial transmission grating.

Figure 8: Schematic for diffraction of sodium Na-D lines setup

For N parallel slits, each with a width e, separated by an opaque space of width b. the diffraction
pattern consists of diffraction modulated interference fringes. The quantity (e+b) is called the grating
element and N (= 1/ (e+b)) is the number of slits per unit length, which could typically be 300 to
12000 lines per inch. For a large number of slits, the diffraction pattern consists of extremely sharp
(practically narrow lines) principal maxima, together with weak secondary maxima in between the
principal maxima. The various principal maxima are called orders.

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For polychromatic incident light falling normally on a plane transmission grating the principal
maxima for each spectral color are given by

Where m is the order of principal maximum and θ is the angle of diffraction.

Angular dispersive power:

The angular dispersive power of the grating is defined as the rate of change of angle of diffraction
with the change in wavelength. It is obtained by differentiating Eqn. 1 and is given by

7.5 PROCEDURE
1. Adjust the spectrometer. Determine the vernier constant of the spectrometer.

2. Now remove the prism from the turntable. The next step is to adjust the grating on

the turntable so that its lines are vertical, i.e. parallel to the axis of rotation or the turntable. Moreover,
the light from the collimator should fall normally on the grating. To achieve this, the telescope is
brought directly in line with the collimator so that the centre of the direct image of the slit falls on the
intersection of the cross-wires. In this setting of the telescope, its vernier reading is taken; let it be 𝜑.

3. The telescope is now turned through 90° from this position in either direction so that the reading of
the vernier becomes (𝜑 + 900 ) or (𝜑 − 900 ). Now the axis of telescope is at right angles to the
direction of rays of light emerging from the collimator. The telescope is clamped in this position.

4. The grating of known grating element is then mounted on the grating holder, which is fixed on the
turntable in such a way that the ruled surface of the grating is perpendicular to the line joining two of
the leveling screws (say Q and R).

5. The table is now rotated in the proper direction till the reflected image of the slit from the grating
surface coincides with the intersection of the cross-wires of the telescope.

6. With the help of two leveling screws (Q and R), perpendicular to which grating is fixed on the
table, the image is adjusted to be symmetrical on the horizontal cross- wires. The plane of the grating,
in this setting, makes an angle of 45° with the incident rays as well as with the telescope axis.

7. The reading of vernier is now taken and with its help, the turntable is rotated through 450 from this
position so that the ruled surface becomes exactly normal to the incident rays. The turntable is now
firmly clamped.

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8. The final adjustment is to set the lines of the grating exactly parallel to the axis of rotation of the
telescope. The telescope is rotated and adjusted to view the first order diffraction pattern. The third
leveling screw (P) of the prism table is now worked to get the fringes (spectral lines) symmetrically
positioned with respect to the horizontal cross-wire.

9. If this adjustment is perfect, the centers of all the spectral lines on either side of the direct one will
be found to lie on the intersection of the cross-wires as the telescope is turned to view them one after
another. The rulings on the grating are now parallel to the axis rotation of the telescope. The grating
spectrometer is now fully to make the measurements. Do not disturb any of the setting of the
spectrometer henceforth throughout the experiment.

10. Look through the telescope to notice the first or second order (whichever you see is completely
resolved) D lines of sodium. That means you will see two yellow lines on both sides of the direct
image (which is a single line) of the slit at the center. Note down the positions of the cross wire for
each line on one side using the two verniers on the spectrometer. Use a torch, if needed, to read the
verniers. Repeat the above step by turning the telescope to the other side too. Determine the
diffraction angle, α, for all the two spectral lines.

11. Take two sets of reading for each D-line and calculate the corresponding wavelength λ1 and λ2
using Eq. 1.

7.6 OBSERVATION

Number of lines on grating = --------

Grating element = ----------

Order, m=----

7.7 CALCULATIONS
1. Calculate λ1 and λ2 and the uncertainty of the result.

2. Calculate the difference λ2~ λ1 and compare with the literature value.

3. Calculate the angular dispersive power.

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7.8 RESULT
(i) The dispersive power of the grating is _________.

7.9 PRECAUTIONS AND SOURCES OF ERROR

1. Once the collimator and the telescope are adjusted for parallel rays, their focusing should not be
disturbed throughout the experiment.

2. Once the grating is properly adjusted on the turntable it should be locked.

3. While taking measurements at different positions of the telescope. It must always be in locked
condition.

4. While rotating the telescope arm if the vernier crosses over 0º (360º) on the circular main scale take
the angular difference appropriately.

7.10 SUMMARY
Dispersive power of a grating is defined as the ratio of the difference in the angle of diffraction of any
two neighboring spectral lines to the difference in the wavelength between the two spectral lines. It
can also be defined as the diffraction in the angle of diffraction per unit change in wavelength.

In grating spectrum, red color is deviated (diffracted) most and violet least. The sequence of the colors
in grating spectrum is reversed than that of prism spectrum.

7.11 GLOSSARY
Diffraction Grating – an optical component with a periodic structure that splits and diffracts light
into several beams travelling in different directions.

Dispersion – the separation of white light into colors or of any radiation according to wavelength. It
is the phenomenon in which the phase velocity of a wave depends on its frequency.

Dispersive Power – a measure of the amount of difference in the refraction of the highest and lowest
wavelengths that enter the prism. This is expressed in the angle between the two extreme wavelengths
(i.e. rED and Violet). The greater the dispersive power, the greater the angle between them, and vice-
versa.

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Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

Spectrum – Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and
infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an
instrument that photographs or maps spectra is a spectrograph.

Wavelength – the distance between successive crests of a wave, especially points in a sound wave or
electromagnetic wave.

7.12 REFERENCES
1. Introduction to Physics, Cutnell, Johnson, Young, Stadler – Wiley India, 2017

2. Engineering Physics Practical, S. K. Gupta – Krishna Prakashan Media, Meerut, 2003

7.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. What is the main difference between the spectrum obtained by grating and that due to a prism?

Ans: In grating spectrum, red color is deviated (diffracted) most and violet least. The sequence of the
colors in grating spectrum is reversed than that of prism spectrum.

2. How many types of gratings are known to you?

Ans: There are two types of gratings:

(a) Transmission grating

(b) Reflection grating

3. Why are transmission gratings less angle sensitive than reflection gratings?

Ans: The main difference can be seen in Figure 2. All the angles are measured relative to the normal
of the grating. First we will consider the 0th order diffraction order for both types of gratings while
tilting the grating slightly. For the 0th order diffraction the diffraction angle (β) equals the angle of
incidence (α).

For a reflective grating, the 0th order is obviously reflected back from the surface just as if the grating
was a plane mirror so, when the grating is tilted the 0th order diffraction shifts by twice the angular
tilt. The higher order diffractions basically follow the 0th order diffraction so, the mth order
diffraction also shifts angularly by twice the tilt angle of the grating.

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For a transmission grating however, the 0th order goes straight through the grating and is not affected
by tilting the grating. Again, since the higher order diffractions follow the 0th order diffraction, the
mth order diffraction is almost unaffected by tilting the grating.

Figure 9

4. On what factors does the dispersive power depend?

Ans: The dispersive power of a grating depends on the following factors:

(a) The dispersive power of a grating is directly proportional to the order of spectrum (n, i.e.
higher orders are dispersed more than the lower orders).
(b) The dispersive power of a grating is inversely proportional to the grating element, i.e. smaller
the grating element (a + b), more is the dispersive power.
(c) The dispersive power of a grating is inversely proportional to the cosine of the angle of
diffraction, i.e. the larger the angle of diffraction, more is the dispersive power, i.e. the
dispersion is more in the red region than the violet region.

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EXPERIMENT 8: ZONE PLATE EXPERIMENT

CONTENTS
8.1 Objectives

8.2 Introduction

8.3 Apparatus

8.4 Theory and Formula Used

8.5 Procedure

8.6 Observation

8.7 Calculation

8.8 Result

8.9 Precautions and Sources of Error

8.10 Summary

8.11 Glossary

8.12 References

8.13 Viva-voce Questions and Answers

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8.1 OBJECTIVES
To construct Fresnel’s zone and to determine the focal length of zone plates.

8.2 INTRODUCTION
A zone plate is a device used to focus light or other things exhibiting wave character. Unlike lenses or
curved mirrors however, zone plates use diffraction instead of refraction or reflection. Based on
analysis by Augustin-Jean Fresnel, they are sometimes called Fresnel zone plates in his honor. The
zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from
an opaque disc.

A zone plate consists of a set of radially symmetric rings, known as Fresnel zones, which alternate
between opaque and transparent. Light hitting the zone plate will diffract around the opaque zones.
The zones can be spaced so that the diffracted light constructively interferes at the desired focus,
creating an image there.

8.3 APPARATUS
He-Ne Laser, zone plate, lens holder, lens, object holder, ground glass screen, polarizing filter, optical
bench, slide mount.

8.4 THEORY AND FORMULA USED

A zone plate is illuminated with parallel laser light. The focal points of several orders of the zone
plate are projected on a ground glass screen.

According to Fresnel, interference of waves diffracted by obstacles may be treated simply by splitting
the primary wave front into so called zones. The optical path difference from the common boundaries
of a zone pair up to a point of observation P is always /2. Secondary waves originating from
neighboring zones impinge in P with opposed phases, thus extinguishing each other except for a part
coming from the first zone.

Using a so called zone plate, which consists of alternating transparent and opaque circles, it is
possible to let either the odd or the even zones exert an influence at a point of observation P. If the
number of zones is 2k, the amplitude A at point P is (under the justified assumption that the secondary
waves have the same amplitude, due to the fact that the areas of the single zones are equal):

A = A1 + A3 + A5 + ... + A2k-1; A ~ kA1 (1)

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At the point of observation P, the amplitude A without zone plate is 1/2 A1 (contribution of half of the
first zone). Using a zone plate, it is thus possible to increase light intensity at P by a factor of 4k2. This
means that the zone plate acts as a focusing lens.

Figure 10: Geometry of the Zone Plate

In Figure 1, the first rings of a zone plate illuminated by a plane wave (parallel beam) are shown.

Assuming the distance between P and the centre of the zone plate to be f1, in case of constructive
interference at P, the following holds for radii rn (n=1,2,3 ...):

For the radii rn of the zone plate and the focal length f we thus have:

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If the point of observation P is shifted along OP towards the zone plate, alternating brightness and
darkness are observed, which means that the zone plate has several focal points.

The existence of these focal points of higher order is due to the difference in the optical path of the
zone rays of 3/2, 5/2, 7/2, 9/2 ...

The zone plate used for the experiment has 20 zones, the radius of the first dark central circle is r 1 =
0.6 mm. The following radii are found to be:

rn = n1/2 · 0.6 mm (5)

8.5 PROCEDURE
Figure 2 shows the complete experimental set-up.

1. The slide mount for the laser is placed at the head of the optical bench.
2. To start with, the laser beam is widened with lenses L1, L2 and L3 to a diameter of approx. 5 mm
(cf. Figure 3).
3. Careful shifting of lenses L2 and L3 allows making the laser beam parallel over a length of several
meters (maximum 10 m). The correct values for the different focal lengths of the zone plate can
only be obtained under this condition. For this purpose, a piece of black cardboard, into which a
hole is punched with a desk punch and which is used as a test diaphragm, proves useful.
4. The other components should then be mounted; making sure the zone plate is well illuminated.
5. The image of the zone plate is observed on the ground glass plate, which is located nearly at the
end of the optical bench at the beginning, with magnifying lens L4.
6. Moving the ground glass screen and L4 in the direction of the zone plate simultaneously, the
different focal points of the zone plate are searched for and the corresponding focal lengths are
determined.
7. The polarizing filter, which is used to reduce the brightness of the image, is set together with the
ground glass screen in the same mounting frame.

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Figure 11: Experimental set-up to determine the different focal lengths of a zone plate

Figure 3: Position of the optical components

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8.6 OBSERVATION

In Table 1, the averaged experimental values are compared to the values calculated according to (3),
(4) and (5) and with  = 632.8 nm.

Table 1

m f(theor.)/cm f(exp.)/cm n r(theor.)/mm r(exp.)/mm

1 1

3 2

5 3

7 4

9 5

Figure 4: Focal length of first and higher order of the zone plate as a function of the reciprocal value of the order.

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8.7 CALCULATIONS
Figure 4 shows the empirical focal lengths as a function of the inverse value of the order of the focal
points.

8.8 RESULT
Since, the graph follows a straight line; we can infer that focal lengths are inversely proportional to
the order of focal points.

8.9 PRECAUTIONS AND SOURCES OF ERROR

1. Never look directly into a non-attenuated laser beam.
2. The zone plate should be well illuminated.

8.10 SUMMARY

A zone plate is a diffractive optic that consists of several radially symmetric rings called
zones. Zones alternate between opaque and transparent, and are spaced so that light
transmitted by the transparent zones constructively interferes at the desired focus. One
common use of a zone plate is to bring light from a distant source to focus. In this scenario,
the incident wave can be approximated as a plane wave.

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Figure 5

8.11 GLOSSARY
Diffraction – It refers to various phenomena that occur when a wave encounters an obstacle or a slit.
It is defined as the bending of light around the corners of an obstacle or aperture into the region of
geometrical shadow of the obstacle.

In classical physics, the diffraction phenomenon is described as the interference of waves according to
the Huygens–Fresnel principle. These characteristic behaviors are exhibited when a wave encounters
an obstacle or a slit that is comparable in size to its wavelength. Similar effects occur when a light
wave travels through a medium with a varying refractive index, or when a sound wave travels through
a medium with varying acoustic impedance. Diffraction occurs with all waves, including sound
waves, water waves, and electromagnetic waves such as visible light, X-rays and radio waves.

Focal Length – The focal length of an optical system is a measure of how strongly the system
converges or diverges light. For an optical system in air, it is the distance over which initially
collimated (parallel) rays are brought to a focus. A system with a shorter focal length has greater
optical power than one with a long focal length; that is, it bends the rays more sharply, bringing them
to a focus in a shorter distance.

Refraction – Refraction is the change in direction of wave propagation due to a change in its
transmission medium. The phenomenon is explained by the conservation of energy and the
conservation of momentum.

Refractive Index – In optics, the refractive index or index of refraction of a material is a

dimensionless number that describes how light propagates through that medium.

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Spectrum – Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and
infrared light. An instrument designed for visual observation of spectra is called a spectroscope; an
instrument that photographs or maps spectra is a spectrograph.

Wavelength – the distance between successive crests of a wave, especially points in a sound wave or
electromagnetic wave.

Zone Plate – A zone plate is a device used to focus light or other things exhibiting wave character.
Unlike lenses or curved mirrors however, zone plates use diffraction instead of refraction or
reflection.

8.12 REFERENCES

1. A. Fresnel: Calcul de l'intensite de la lumiere au centre de l'ombre d'un ecran, Oeuvres

completes d' Augustin Fresnel, Vol.1, Note 1, 365 (1866)

2. J.L. Soret: Ueber die von Kreisgittern erzeugten Diffraktionsphaenomene, Ann.Phys.Chem

156, 99 (1875)

3. D. Attwood, Soft X-rays and Extreme Ultaviolet Radiation: Principles and Applications.
Cambridge University Press, 1999.

8.13 VIVA-VOCE QUESTIONS AND ANSWERS

1. What is meant by Fresnel zones?

Ans: A zone plate consists of a set of radially symmetric rings (example shown in Figure 6), known as
Fresnel zones, which alternate between opaque and transparent.

Figure 5

2. What are some of the applications of zone plates?

Ans: Following are some of the applications of zone plates:

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PHY (N)-220 Optics &
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(a) Physics: There are many wavelengths of light outside of the visible area of the
electromagnetic spectrum where traditional lens materials like glass are not transparent, and
so lenses are more difficult to manufacture. Likewise, there are many wavelengths for which
there are no materials with a refractive index significantly larger than one. X-rays, for
example, are only weakly refracted by glass or other materials, and so require a different
technique for focusing. Zone plates eliminate the need for finding transparent, refractive,
easy-to-manufacture materials for every region of the spectrum. The same zone plate will
focus light of many wavelengths to different foci, which means they can also be used to filter
out unwanted wavelengths while focusing the light of interest.

Other waves such as sound waves and, due to quantum mechanics, matter waves can be
focused in the same way. Wave plates have been used to focus beams of neutrons and helium
atoms.

(b) Photography: Zone plates are also used in photography in place of a lens or pinhole for a
glowing, soft-focus image. One advantage over pinholes (aside from the unique, fuzzy look
achieved with zone plates) is that the transparent area is larger than that of a comparable
pinhole. The result is that the effective f-number of a zone plate is lower than for the
corresponding pinhole and the exposure time can be decreased. Common f-numbers for a
pinhole camera range from f/150 to f/200 or higher, whereas zone plates are frequently f/40
and lower. This makes hand held shots feasible at the higher ISO settings available with
newer DSLR cameras.

(c) Gun sights: Zone plates have been proposed as a cheap alternative to more expensive optical
sights or targeting lasers.

(d) Lenses: Zone plates may be used as imaging lenses with a single focus as long as the type of
grating used is sinusoidal in nature.

(e) Reflection: A zone plate used as a reflector will allow radio waves to be focused as if by a
parabolic reflector. This allows the reflector to be flat, and so easier to make. It also allows an
appropriately patterned Fresnel reflector to be mounted flush to the side of a building,
avoiding the wind loading that a parabaloid would be subject to.

(f) Software testing: A bitmap representation of a zone plate image may be used for testing
various image processing algorithms, such as:

- Image interpolation and image re-sampling;

- Image filtering.

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