Physics Department

Faculty of Science
An-Najah National University

Experimental Lab Manual

10221213

Dr. Muna Hajj Yahea

Mohammed Bashar
 1. Determination of the apex angle of the given prism.
2. Determination of the deflection angle of the (yellow) light beam as a function
of the incident angle.
3. Determination of the angle of minimum deviation of the yellow line and all
other lines appearing in the spectra.
4. Determination of the refractive index of all appearing color lines.
5. Determining the relation between the refractive index and the wavelength of
light.

The set-up of the experiment is shown in the figure below. It is mainly composed of:
1. Spectrometer with two independent scales.
2. Equilateral prism.
3. Power supply and Sodium lamp having the following wavelengths:

Red 613nm Yellow(orange 590nm

)
Green 567.4nm Bluish Green 516nm
Light Blue 498.4nm Violet 464nm
 Spectrometer:
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an
instrument used to measure properties of light over a specific portion of the
electromagnetic spectrum, typically used in spectroscopic analysis to identify
materials.[1] The variable measured is most often the light's intensity but could also,
for instance, be the polarization state. The independent variable is usually the
wavelength of the light or a unit directly proportional to the photon energy, such as
reciprocal centimeters or electron volts, which has a reciprocal relationship to
wavelength.

A spectrometer is used in spectroscopy for producing spectral lines and measuring

their wavelengths and intensities. Spectrometers may also operate over a wide range
of non-optical wavelengths, from gamma rays and X-rays into the far infrared. If the
instrument is designed to measure the spectrum in absolute units rather than relative
units, then it is typically called a spectrophotometer. The majority of
spectrophotometers are used in spectral regions near the visible spectrum.

In general, any particular instrument will operate over a small portion of this total
range because of the different techniques used to measure different portions of the
spectrum. Below optical frequencies (that is, at microwave and radio frequencies), the
spectrum analyzer is a closely related electronic device.

Spectrometers are used in many fields. For example, they are used in astronomy to
analyze the radiation from astronomical objects and deduce chemical composition.
The spectrometer uses a prism or a grating to spread the light from a distant object
into a spectrum. This allows astronomers to detect many of the chemical elements by
their characteristic spectral fingerprints. If the object is glowing by itself, it will show
spectral lines caused by the glowing gas itself. These lines are named for the elements
which cause them, such as the hydrogen alpha, beta, and gamma lines. Chemical
compounds may also be identified by absorption. Typically these are dark bands in
specific locations in the spectrum caused by energy being absorbed as light from other
objects passes through a gas cloud. Much of our knowledge of the chemical makeup
of the universe comes from spectra.
 Reflection, Refraction and the Prism:
Huygens’ principle can be used to determine various experimentally verifiable and
predictable behavior of the path of light through any optical system. However, as seen
in the chapter on Light, the Huygens wavefront construction can be become
complicated, especially in systems with a large number of optical components. A
simpler approach to track the behavior of light is based on the propagation of light
rays based on some well-defined optics laws.
1. Law of reflection: When light travelling in medium with index n1 is incident at an
interface and some of it is scattered into medium n1 then the phenomenon is known as
reflection. Two forms of reflection are generally recognized. The first is diffuse
scattering or reflection in which the reflected light direction is unpredictable or
random with respect to the incident direction. The second is specular reflection in
which the incident and scattered light have a well-established relation with respect to
direction. Referring to Fig. 1, when light is incident on a surface making an angle θi
with the surface normal, it is reflected at an angle θr with respect to the normal. In the
case of specular reflection, θi = θr. In addition, another important property of this
reflection is that the incident and reflected rays lie on the same plane known as the
plane of reflection. These two properties define the law of reflection.

Specular reflection of light at a smooth interface separating mediums n1 and n2. I and R represent the
incident and reflected beams while T is the transmitted or refracted beam. According to the laws of
reflection I, R and the surface normal, lie on a single plane, the plane of reflection. and the incident and
reflected angles are identical.

2. Snell’s law of refraction: In the chapter on light-matter interaction we saw that when light
entering a medium with a different refractive index, it bends. This refraction or the amount
of bending can be quantitatively estimated. by the Snell’s law. With reference to Fig. 2 a ray
travelling in medium n1 is incident at an angle θi and is transmitted into medium n2 and at
an angle θt with respect to the normal. Snell’s law defines the relation between the two
angles for any pair of media as:
n1sinθi = n2sinθt
 Refraction of light and Snell’s law. Light incident at an angle θi at the interface separating
two media n1 and n2 is transmitted into medium n2 at angle θt determined by Eq. 1.

By the use of these simple laws and ray diagrams in combination with the laws of dispersion,
numerous problems involving the passage of light through matter and through various
optical components, like prisms, lenses, microscopes, telescopes, etc can be evaluated. We
first discuss the interesting refractive and dispersive properties of a prism.

\
Notice: Before starting measurements in this experiment, wou ave to adjust the
spectrometer by its telescope system so that you see the clearest picture. Attertus ou
better keep it it is as:
Part (1): Determination of the apex angle of an equilateral prism:
1. Put the light ON and look through the eyepiece of the telescope in order to see
the yellow light through the opposite slit. Make the image as clear as possible,
and make the slit as narrow as possible.
2. Place the prism on the top table of the spectrometer.
3. while watching through the eyepiece of the spectrometer, turn the telescope to
the right until you reach an image of the yellow slit formed by reflection on
one side of the prism, and register the angle e, at this position. Note that you
will pass an image formed by refraction characterized by the presence of
several colored lines, skip this position.
4. Turn the telescope to the left until you reach another image of the yellow slit
object formed by reflection on the other side of the prism, rand register this
angle ∅ 2
5. Use the eq. to determine the apex angle of the prism.

Part (2): Determining the deflection angle as a function of the incident angle:
 1. Adjust the table of the spectrometer so that the zero indicates the position of
the yellow slit. squeeze the proper screw to fix this position.
2. Place the prism on the top table of the spectrometer with its side
perpendicular to the incident light. The incident angle in this case is zero
3. Start increasing the incident angle while watching the refracted beam from
the other side of the prism through the eyepiece. You might not see anything
from 0 to 35.
4. When you see the first refraction position, register the Figure incident angle
and the corresponding deviation angle.
5. Keep increasing the incident angle in steps of 5 until you can't see anything
through the eyepiece (about 800)

In the first part of determining the Apex angle

θ 2−θ1
A=
2
By the exp we got that
θ1=120 0 & θ2=2400
So
240−120
A= = 600
2

In the second part of determining the min angle of deviation :

From the exp we got that the min angle of deviation =370

Dmin + A
sin ⁡( )
n 2
=
n0 A
sin ⁡( )
2

n0 =1
A =600
Dmin =370
So
37+60
sin ⁡( )
n 2
=
1 60
sin ⁡( )
2
n of prism =1.5
Source of errors :
1. Fixing the prism in a good position .
2. Error in human.
3. Error in reading the angle .

1- What are the major parts of the given spectrometer?.

Prism make reflection and reflection the incident light
2- Prove equation (1).
θ=i due to
A
Parallel θ 1≤ θ 2 is half A and θ=
2
θ 2−θ 1
Then the Apex angle =
2
3- Prove eq(3).

4- Does the refractive index of the prism differ for different colors? If yes,
write down the formula of n as a function of the wavelength explaining all
variables.
Yes, it depended on wavelength different colors have different wavelength

5- Redraw fig. (2) in a manner that shows the reflected beams on the two
sides of the prism and the refracted beams through its base. (Use colors).
Light incident on the Apex , it's angle with perpendicular equal zero , it's pass
without refracted , and it pass using the base due to it's angle with
perpendicular equal zero
6- Make a comparison between the spectra obtained by prism spectrometer
and that obtained by diffraction grating spectrometer.
Prism spectrometer speate light with diff wavelength , resolving power