Std : 10 th Sub :Sci (Sec A)

6.Refraction of Light

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Can you recall:

1.Reflection of light : When light rays fall on any opaque surface they turn back.This is called as
reflection of light .

2.Laws of reflection of light : i) The angle of reflection is equal to angle of incidence .

ii)The incident ray , the reflected ray and normal all lie in a same plane .

iii)The incident ray and reflected ray are always on opposite sides of the normal.

Generally , light always travels in straight line .

Refraction of light : Light changes its direction when going from one transparent medium to
another transparent medium this is called refraction of light .

Refraction of light passing through a glass slab :

(During refraction of light through the glass slab , incident ray and emergent ray are parallel to
each other )

1.Refraction of light takes place twice in a glass slab.

Diagram 6.1
2.The first refraction occurs when light enters the glass from air .The second refraction occurs
when light enters air from slab .

3.The angle of incident is equal to angle of emergence .

4.On both the parallel sides of glass slab , the change in direction of light ray is equal but in
opposite directions.

5.Thus , light ray emerging from the glass slab is parallel to the incident ray .

Laws of refraction : (State and explain the laws of reflection )

1)The incident ray and reflected ray are on the opposite side of normal at the point of incidence
and all three lies in same plane .
Diagram 6.2

2) a. For a given pair of media , the ratio of the sine of angle of incidence to the sine of angle of
refraction is constant .

b.The second law is also called as Snell’s law .

c.If ‘ I ‘ is angle of incidence and ‘ r’ is angle of refraction , then sin i / sin r = constant .
d.This constant is called the refractive index of second medium with respect to first medium .It is
denoted by ‘ n ‘.

Therefore , sin i / sin r = n

Refractive index :The ratio of sin i to sin r is called refractive index .

Absolute refractive index : The refractive index of a medium with respect to vaccum is called its
absolute refractive index .

* Refractive index depends on the velocity of light in that medium.

*For different medium , refractive index is different .

Relation between refractive index and velocity of light :

Let the velocity of light in first medium is V1 and in second medium is V2 .
Diagram 6.3
Then , refractive index of second medium with respect to first medium is ,

Refractive index 2 n 1 = velocity of light in first medium / velocity of light in second medium.

Ie 2 n 1 = V1 / V2

Simillarly , refractive index of first medium w.r.t second medium is ,

Refractive index 1 n 2 = velocity of second medium / velocity of first medium

Ie 1 n 2 = V2 / V1

Refraction of light in different media :

Rule 1 : When a light ray passes from a rare medium to denser medium , it bends towards normal
.( angle i > angle r)

Rule 2 : When a light ray passes from a denser medium to a rare medium , it bends away from the
normal ( anglr < angle r )

Rule 3 : When a light ray is incident normally at boundary between two media , it does not
changes its direction and hence do not get refracted .( angle i = angle r)

Applications of Refraction of light :

Small scale effect( local effects )

1)Formation of mirage :

1) In summer the air near hot road or desert surface has very high
temperature and hence it becomes lighter than cool air above it .

2) As density of air goes on decreasing with increase in height above the surface , the refractive
index of air increases.
3) hence , the direction of light rays coming from distant objets keeps changing according to laws
of refraction .

4)This makes the light rays coming from distant objects appear to come from an image of object
inside the ground as shown in fig .This is called as mirage .

2)Holi fire : 1) At the time of holi, objects beyond and above the holi fire appear to be shaking .

2) The air just above the fire becomes hotter than the air further up .The hot air is lighter(rarer)
than cool air .

3) Hence , the refractive index of hot air is less than that of cool air .

4) A s physical conditions of air are not stationary , the apparent position of object changes sue to
atmospheric refraction of light .

Atmospheric effect :
1)Twinkling of star

1) Stars are point sources of light as they are very far away from us .

2) The apparent position of star is slightly higher than its actual position due to refraction of
starlight from the atmosphere.The starlight travels from rarer medium to denser medium and
continuously bends towards the normal .

3) Due to the motion of atmospheric air and change in density and temperature,the atmosphere
is not steady .As a result refractive index of air in given region changes continuously and randomly

4) Hence , the apparent position of star and its brightness is not stationary but changes slightly .

5) In this way .due to change in refractive index of atmosphere , the stars appear twinkling at
night

2) Planets do not twinkle.

1) the rays of light from a planet pass through the atmosphere of earth .

2)As compared to stars planets are nearer to earth.

3) So a planet can be considered as a collection of large number of point sources of light .

4) If intensity of light from one point source decreases , it increases from other source .Thus , the
average intensity remains the same .

Hence , planets do not twinkle .

4)Occurance advanced sunrise and delayed sunset

1) in morning , the sun should be seen by observer when it reaches the horizon , but it is seen two
minutes before that .
2) As ray of light from sun enters the earth’s atmosphere , it follows a curved path due to
refraction before reaching the observer .

3) This makes apparent position of sun slightly higher than actual position for the observer .Hence
, the sun is seen earlier before sun reaches horizon .

4) Atmospheric refraction of sunlight also occurs at sunset .In this case , the observer on earth
continues to see the setting sun for two min . after the sun has dipped below horizon . thus
delaying the sunset

Dispersion of light :

1) The process of separation of light into its component colours while passing through a medium
is called dispersion of light .

2) Sun light is made up of seven colours .It is a spectrum of sevem colours as Violet, Indigo ,Blue
,Green , Yellow ,Orange and Red.

3)*Wavelength is property of radiation .

4) Red light has maximum wavelength ie close 700 nm

Violet has smallest wavelength close to 400 nm .

5)Red bends least while violet bends most .

6) Sir Issac Newton was the first person to use a glass prism to obtain sun’s spectrum.

Partial and total internal reflection

1)When light ray travels from denser medium to rare medium, reflection takes Diagram 6.9

place partially at the interface this is called partial reflection .

2) Angle of incidence ( i) is smaller than angle of refraction ( r) .If ‘i’ increases gradually. Then ‘ r’
also increases as refractive index is constant .

3) the angle of incident for which the angle of refraction is 900 , , is called critical angle.

4) When the angle of incidence in a denser medium is more than critical angle , the light reflects
back into the denser medium .This is called total internal reflection.

Formation of Rainbow : (diagram 6.10)

1) The rainbow appears in the sky after rainfall.

2)Water droplets present in atmosphere act as small prism .

3)When sunlight enters these water droplet it gets refracted and dispersed .

4)This dispersed light gets totally reflected inside the droplet and again is reflected while coming
out of droplet .

5)As combined effect of all these phenomena , the seven coloured rainbow is observed
STD : 10 th Sub :Science

7.Lenses

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Lenses : A lens is transparent medium bound by two surfaces.

Types : Convex lens and Concave lens

*Convex lens / Biconvex lens / double convex lens / Converging lens :

The lens which have two spherical surfaces puffed up outwards is called convex lens .

*Concave lens / Biconcave lens / double concave lens / diverging lens

Difference :

Convex lens Concave lens

1. It is thicker at centre than at the edges. 1.It is thinner at centre than at the edges.
2.It is also called as converging lens 2.It is also called as diverging lens
3.The nature of image formed depends upon 3.the nature of image formed is always diminished
The position of object. and erect .
4.The focal length is always positive 4.The focal length is always negative .
5.It can form real and virtual image 5.It forms only virtual image.
6.It is used to correct hypermetropia or 6.It is used to correct myopia or nearsightedness
farsightedness

*Terms related to lens :

From exercise (Q.2. Draw a figure explaining various terms related to lens )

1)Centre of curvature(C) :The centre of sphere whose part forms the surface of the lens is called 7.4
Diagram

Centres of curvature (C1 & C2)

2)Radius of curvature (R): The radius of the sphere whose part forms the surface of the lens is
called radius of curvature .(R1 & R2)

3)Principal axis : The imaginary line passing through both centres of curvature is called principal
axis of lens

4)Optical centre ( O): the point inside the lens on the principle axis , through which light rays pass
without changing their path is called optical center of lens.

5) Principal focus(F) :a. When light rays parallel to principal axis are incident on convex lens ,
they converges to point on principal axis .This point is called the principal focus of lens .
b. Ray travelling parallel to principal axis of a concave lens diverges after refraction in such a way
that they appear to be coming out of a point on principal axis .this point is called as principal
focus of concave lens.

6.Focal length (f) :The distance between the optical centre and principal focus of a lens is called
its focal length .

Ray diagram for refraction : We can obtain the position , size and nature of images with the help
of ray diagram .

Images formed by convex lenses :

Rules to draw ray diagram of convex lens :

1)When the incident ray is parallel to the principal axis , the refracted ray passes through the
principal focus .

2) When the incident ray passes through the principal focus , the refracted ray is parallel to
principal axis .

3) When the incident ray passes through the optical centre of lens , it passes without changing its
direction.

Cartensian sign convention : (fig 7.10)

According to Cartesian sign convention ,

1)The optical centre O is taken as origin .

2) The object is always kept on left hand side of the lens.

3)All distances parallel to the principle axis are measured from the optical centre O .

4)The distances measured to right of the O are taken to be positive while those measured to the
left are taken to be negative .

5)Distances perpendicular to principle axis and above it are taken to be positive .

6) Distances perpendicular to principle axis and below it are taken to be negative .

7)The focal length of convex lens is positive while that of a concave lens is negative .

Lens formula : The formula showing the relation between object distance (u), image distance (v)
and the focal length (f) is called as lens formula .It is given by ,

- =

Magnification formula : The magnification due to lens is ratio of height of the image (h2) to the
height of image (h1) and is given by ,

Magnification = i.e . M =
Magnification is also related to the distance of object (u) and that the image (v) from lens and is
given by ,

Magnification = i.e. M=

Power of the lens : The capacity of the lens to converge or diverge the incident ray is called its
power (P).It depends upon it’s focal length .

Power is the inverse of its focal length (f) and f is expresses in meters .

Unit : unit of power is Dioptre (D)

Formula : P =
( )

Combination of lenses : If two lenses with focal length f1 and f2 are kept in contact with each
other , the combination has an effective focal length given by ,

= +

P = P1 + P2

Human eye and working of its lens :

Structure and function: Human eye is nearly spherical in shape with a diameter of about 2.4 cm

1)Cornea : Light enters our eye through the thin membrane known as cornea .The maximum
refraction of light rays enetering the eye takes place from cornea .

2)Iris : Behind the cornea there is a dark fleshy screen called as Iris .The colour of the iris is
different for different people .

33)Pupil : At cenre of iris ,there is a small opening of variable diameter called pupil .The pupil
controls and regulates the amount of light enetering the eye .

*for bright light pupil contracts while for dim light it widens .

4)Lens : There is transparent biconvex crystalline lens located just behind the pupil called as lens.

It provides fine adjustment of focal length . for normal eye relaxed position,focal length of eye
lens is about 2 cm .

5)Retina : it is a delicate membrane consisting of large number of light sensitive cells .When light
falls on retina, cells get activated and produces electrical signals .

6)Optic nerve : these signals are sent to brain via optic nerve.

7)Ciliary muscles : these are surrounded by lens .These muscles hold the eye lens in place.When
the muscles relax, they increase the focal length of the lens, allowing the eye to see the objects
that are far away.To see closer objects clearly ciliary muscles must contract in order to decrease
the focal length of the lens.

*For normal eye , the minimum distance of distict vision is about 25 cm .

Change in shape of the lend while seeing distant and nearby objects(fig 7.12)

* while seeing objects at large or infinite distances : a. eye lens becomes flat

b. its focal length increases

*while seeing nearby objects : a.lens becomes more rounded

b. its focal length decreases

Power of accommodation : The capacity of the lens to change its focal length as per need is
called its power of accommodation .

Minimum distance of distinct vision: The minimum distance of an object from a normal eye , at
which it is clearly visible without stress on the eye is called as minimum distance of distinct vision.

Near point : The position of object at this distance is called near point .For normal eye it is at 25
cm

Farthest distance of distinct vision : The farthest distance of an object from human eye ,at ehich
it is clearly visible without stress on eye is called as farthest distance of distinct vision .

Farther point : The position of the object at this distance is called the far point of the eye .For
normal human eye , it is at infinity .

Defects of vision and their corrections : Defetcs occur due to ……

a.loss of power of accommodation

b.defective refraction

Types of vision :

1.Nearsightedness / Myopia

2.Farsightdness / hypermetropia

3.Presbyopia

Difference :

Points Nearsightdness Farsightedness

Definition The defect of vision in which eye can see The defect of vision in which eye
nearby objects clearly but the distanct objects can see distant objects clearly but
appea indistinct is called as narsightdness. the near objects appear indistinct
is called as farsightedness.
Formation In thus defect ,image is formed in front of In this defect , image is formed
of image retina behind the retina
Reasons 1.The curvature of cornea and eye lens 1.The curvature of cornea and eye
decreases so that the converging power lens increases so that the
becomes less converging power becomes more
2.The distance between eye lens and retina 2.The distance between eye lens
decreases and retina increases
Correction This defect can be corrected by spectacles with This defect can be corrected by
 concave lens of proper focal length spectacles with convex lens of
proper focal length .
Power The power of lens is negative The power of lens is positive

Presbyopia : It has two reasons : i) focusing power decreases with age

ii)The muscles near the lens lose their ability to change length of
the lens

iii)The near point of the lens shifts farther from eye

Correction : This defect can be corrected by using bifocal lens.

In this lenses, The upper part is concave lens to correct nearsightedness and lower part is convex
lens which correct farsightedness.

*Use of concave lenses :

1)Medical equipment ,scanner,CD player :

2)Peep hole in door

3)Spectacles

4)Torch

5)Camera, telescope and microscope

*Use of convex lens :

1.Simple microscope

2)Compound microscope

3)Telescope

4)Optical instrument

5)Spectacles

1.Simple microscope : Uses : for watch repair , testing precious gems, finding their defects

A convex lens with small focal length produces a virtual , erect and bigger image of an object such
lens is called simple microscope .

We can get 20 times larger image of an object .

2.Compound microscope : (fig. 7.17)

Construction : 1) compound microscope consists of two convex lenses called as objective lens (O)
and eyepiece(E)

2)The objective has smaller aperture and smaller focal length as compared to eyepiece.
3)The two lenses are placed inside a hollow metallic tube such that the distance between can be
changed .

Working : 1) Any object , which is to be magnified is placed in front of objective lens.

2)The image formed by objective lens acts as an object for the eyepiece .

3)The image formed by objective is real , inverted and formed at a distance within the focal
length of eyepiece .

4)Hence , the eyepiece forms a virtual , inverted and highly magnified image of object .

5)As a combined effect of two lenses , higher magnification is obtained in compound microscope
.This is used to observe minute object or minute living organisms.

Telescope : Astronomical telescopes are of two types : i)Refracting telescope

ii)Reflecting telescope

Refracting telescope : i) The telescope which works on refraction of light is refracting telescope .

ii)It consists of two convex lenses called objective (O) and eyepiece (E).The objective has large
diameter and larger focal length .The euepiece has smaller focal length and small diameter .These
two lenses are mounted at ends of hollow metallic tube such that , the distance between them
can be adjusted .

iii)Light rays from distant object is incident on objective and forms an image AB

iv)This image AB acts as an object for eyepiece, it is adjusted in such a way that final image is
formed at infinity.

v) To change the magnification , eyepieces of different focal lengths can be used by keeping the
objective same .

Persistence of vision : The image formed on retina disappears as soon as object is taken away
.However it is not instantaneous and image remains imprinted on our retina for 1/16 th second ,
the sensation on retina persist for a while this is called persistence of vision .

Eg. Roataing fan blades appear continuous due to persistence of vision ,Newton’s disc uses
persistence of vision to show white colour when disc is spinning rapidly .

*Perceive different colour :

i)Our eye retina consists of light sensitive cells.

ii)These cells are of two types : a.Rod like cells which respond to intensity of light and gives
information about brightness or dimness of object to brain

b.The conical cells respond to colour and give information about colour of object to brain .

Colour blindness:Some people lack conical cells responding to certain colour and these persons
can not recognize colour or cannot distinguish between different colours and are called colour
blind .