Refraction Guide

REFRACTION
2Dr. Pius Augustine, Sacred Heart College

Refraction
Refraction is the change in direction of a
wave due to a change in its speed.
Bending of light at the surface of
separation of two optical media.
Observed when a wave passes from one
medium to another at any angle other than
90° or 0°.

Refraction
Is a surface phenomena.
Intensity (amplitude) of
refracted wave is less than
incident wave (a portion of
light is reflected.

Velocity and wavelength
changes in refraction, but
frequency is unaffected
in refraction

Laws of refraction
1. Snell’s law.
Ratio of sine of angle of incidence
to sine of angle of refraction is a
constant for a given pair of
media and is called refractive
index of first medium w.r.to
second medium.

Laws of refraction
2. Incident ray , refracted
ray and the normal at the
point of incidence lie on
the same plane.

Refractive index or index of
refraction (of a medium)
Defined as the ratio of
the speed of light in
vacuum to that in the
medium.

Velocity of light in vacuum
is a physical constant, and
the fastest speed at which
energy or information can
be transferred.

It is a measure of the
bending of a ray of light
when passing from one
medium into another.

Refractive index or index of
refraction
Relative refractive index
n21= sini/ sinr
= v1/v2
= λ1/ λ2 = n2/n1.
Absolute refractive index
n=sini/sinr=c/v= λ0/ λm

n21 = v1/v2
= (c/v2 ) / ( c/v1)
= n2/n1.
n21 = 1/ n12 15Dr. Pius Augustine, Sacred Heart College

Note: Absolute refractive
index is always greater
than one .
Relative refractive index
can be < , = or > one.

Denser and Rarer
1. n – high
2. Vel - less
3. Angle b/w
ray and
normal is
less
4. Wavelength
– less
 You may add)

Conditions for no change in
direction on refraction
1. Angle of incidence = zero
2. Refractive indices of both
the media are equal.

Factors affecting μ of a
medium
i. Temperature: speed of light in
a medium increases with
increase in temp.
ie. μ decreases.
(Medium becomes more and more rarified – ie.
towards vacuum.)

medium
ii. Wavelength λ : in a
transparent medium (other
than vacuum) red light travels
faster and violet slower.
μviolet > μred.

All colours travel
with same velocity
in vacuum

medium
iii Optical density :
higher the optical
density lesser is
velocity of light inside.

Deviation produced in refraction
i. Rarer to denser
d = i–r
ii. Denser to rarer
d = r-i
iii. For normal incidence
d = 0

n21 = n2/n1
n21 = 1/ n12
 n12 x n23 x n34 x n41 = 1
 n1sinθ1 = n2sinθ2
Snell’s law 26Dr. Pius Augustine, Sacred Heart College

Note: if the boundaries of the media are parallel
the emergent ray although laterally displaced,
is parallel to the incident ray (glass slab)
n1sinθ1 = n5sinθ5
If any of the boudary is not parallel , we
cannot use this law directly by jumping
the intervening media.
1 2 3 4 5 Snell’s law
nsinθ = constant

Name one factor on which the direction
of bending of a ray of light depends.
Speed of light in two
media
Refractive indices of the
medium 28Dr. Pius Augustine, Sacred Heart College

Refractive index of water
w.r.t air is 4/3 while that of
glass is 3/2. Find the
refractive index of glass w.r.t
water?
9/8= 1.125 29
Dr. Pius Augustine, Sacred Heart College

Draw labeled diagrams to
showing refraction of light
i) air to glass
ii) glass to water.

Glass is transparent in nature. Why does
glass powder look opaque? When water is
poured over it, it again becomes
transparent. Explain
Irregular reflections in glass powder sent
light back to same med, makes it
opaque.
On pouring water, light is refracted through
water in the gap and make it transparent

Sun is seen even after actual sunset.
Explain.
Due to refraction of light.
Atmosphere bends sunlight towards
the normal.
Apparent position of sun is raised and
comes above horizon even after
sunset (before surise) 32

Why does the rising sun appear bigger?
Due to refraction of light.
Rising (setting) sun is at horizon –light comes
obliquely.
Rays from different parts of the sun travel
through different parts of earth’s atmosphere.
unequal refractions - horizontal diameter appears
bigger while the vertical diameter appears
shorter.(oval) 33Dr. Pius Augustine, Sacred Heart College

A glass slab is placed over a page in
which letters are printed in different
colours. Will the image of all the
letters lie in the same plane?
Refractive index of glass is
different for different colours.
Different colours are raised
differently (blue raised max
and red least). 34

For which color of white light
is the refractive index of a
transparent medium
(i) max and ii) min. ?
Maximum – viloet
Minimum – red. 35

Does the apparent depth of
a tank of water change,
normally ?
No. when viewed
normally,
i= 0 and r = 0

37

39

40

42

Try this please
With the help of a well-labeled
diagram show that the apparent
depth of an object such as a coin
in water is less than its real
depth.

How is refractive index of
water related to the real
depth and the apparent
depth of a column of water?

Draw a ray diagram to
illustrate the bending of
a stick in water.

The adjacent diagram shows
the path of a ray of light
through a rectangular glass
block placed in a liquid of
uniform density. i (
) r
Liquid
Glass
Try to answer the following question 47

a)Does the light speed
up or slow down in
glass?
The speed of the light
slows down.

b)Give reasons for your answer.
In the diagram, the ray of light
traveling from liquid to glass bends
towards the normal, which implies
that glass is optically denser medium
than liquid. Hence, the speed of light
slows down.

What is the angular
deviation of the
emergent ray from the
glass block with respect
to the incident ray? 50

Water in a pond appears to be only
three quarters of its actual depth.
i)What property of light is
responsible for the observation?
ii)Illustrate your answer with the help
of a ray diagram.

What do you mean by
reversibility of light?
Plot relevant figure

According to the principle of
reversibility of light, the final path of a
ray of light after suffering a number of
reflections and refractions is reversed,
ie., retraces the same path in the
opposite direction as shown in the
diagram. The refractive index of the
denser medium (d) with respect to a
rarer medium (r)
rµd = sin i
sin r …………………….(i) 53

The refracted ray then strikes on a
mirror so it reflects back through the
same path and refraction takes place
in which MO is the incident ray in
denser medium (d) and OL is the
refracted ray in the rarer medium (r).
Now the refractive index of rarer
medium (r) w.r.t. denser medium (d)
is
dµr = sin r
sin i …………………….(ii) 54

rµd x rµd = sin i sin r
X
sin r sin I
rµd x rµd = 1

Write four advantages in
using right angled prisms
instead of plane mirrors
in optical instruments 56

Right angled prism
 The image is very bright
almost as bright as the
object
 Total internal reflection
takes place only when a
ray of light travels from
a denser medium to rare
medium with angle of
incidence greater than
critical angle
Plane Mirror
 The image formed
is much less bright
compared to the
object
 Reflection takes
place for all angles
of incidence
 57

Right angled
prism
 Reflection is
total with no
refraction into
the second
medium
 The presence of
moisture and
dust does not
affect the quality
of image
Plane Mirror
 Reflection from a
plane mirror is
only partial
because part of
the incident light
is refracted
 In the presence
of moisture and
dust, the
functioning of the
mirror stops.
58

What do you mean by
phenomenon of mirage?
Explain briefly with the
help of a ray diagram.

What is periscope?
Draw a ray diagram
using right angled
prisms.

During sunset and
sunrise, the sun is seen
even when it is slightly
below the horizon.
Explain

Light from the sun towards earth suffers
refraction from rarer to denser layers upto
the earth.
it bends towards the normal at successive
refractions.
For observer on the earth - it appears to be
coming from a straight line path above the
horizon.
So it is seen above the horizon while
actually it is below the horizon.
At sunset, sun goes down below the
horizon but appears above the horizon.
62

Why do stars show
the twinkling effect
where as planet do
not?

Can a glass slab
disperse light? If
not, why?

No, when the light is incident on
the glass slab, it disperses the
white light into different colours
but these different colours
combine to form white light on
emerging from the other parallel
face.

How does the speed of
light in glass change
on increasing the
wavelength of light?

Speed of light in
glass increases on
increasing the
wavelength of light.

Can light be piped like sound
in a doctors’ stethoscope?
Yes, Through Total Internal
Reflection in optical fibre,
Light can be piped without
much loss in energy. 68

Does the apparent depth of an object
below water depend on the angle of
view of the observer in air?
Yes,
As the angle increases, the depth
appears to decrease gradually
and is limited by TIR. 69

Why do diamonds dazzle in light ?
n = 2.42, angle C = 24o.
This property combined with
proper faceting causes
diamond sparkle.
Light entered inside undergoes
no. of TIR before comes out.
70

Images formed by totally
reflecting prisms are brighter than
the images formed by ordinary
reflected light. why?
TIR – 100% of incident light
gets reflected.
Ordinary reflection 20% -
30% of incident light is lost.71

An empty TT is kept in a beaker
filled with water shines. Why?
Ray of light incident on the
surface of the tube, moves
from denser to rarer medium
and at a certain tilting
position, the ray suffers TIR
and shines .

Can a prism transmit rays at all
angles of incidence?
No.
There is a lower bound of the
angle of incidence at which
prism can start transmission
of light. 73

How does the angle of
deviation produced by
prism depend on the angle
of incidence of light at the
prism surface?
Illustrate graphically. 74

What are the special
properties attributed to
a prism when it is in the
minimum deviation
position?

i. Refracted ray inside the prism
is parallel to base.
ii. Angle i = angle e
iii.Angle of refraction at both the
refracting surfaces are equal .
iv.Incident and emergent rays
are symmetrically placed with
respect to prism 76Dr. Pius Augustine, Sacre Heart College

State one factor on
which critical angle of
given pair of media
depends? Explain.

What is a total
reflecting prism?
Name two instruments
in which it is used?

State three actions that a
total reflecting prism
can produce? Attach
diagrams

Does a beam of white
light give a spectrum on
passing through a
hollow prism?
No. air inside does not produce dispersion . 80

Explain briefly what
you know about an
optical fibre.

How do you account
for the wavy effect
seen in the air above
a hot radiator?

Temperature affects the refractive
index. The waviness observed
when light passes through air
rising above a hot object is caused
by the differing of refractive
indices of the various layers of hot
and cold air. 83

In camp fire face of
person sitting at the
other side of fire
appears shimmering.
Why? 84Dr. Pius Augustine, Sacred Heart College

A glass slab 2.5 cm thick is
placed over a coin . If the
refractive index of glass is
3/2, find the height through
which coin is raised.

State two advantages of
using a right angled
prism as a reflector,
rather than a plane
mirror.

i. No loss of intensity of the image
when formed by the prism. So
image will be brighter.
ii. Image is real and can be
deviated through 90o or 180o
w.r.to incident ray. Plane mirror
image is always virtual. 87

A glass slab is placed over a page on
which the word ‘VIBGYOR’ is printed
with each letter in its corresponding
colour. i. will the image of all the
letters be in the same place? If not,
state which letter will be raised to
the maximum? Give reason for your
answer. 88Dr. Pius Augustine, Sacred Heart Collge

Apparent depth = Real depth /n
n is maximum for violet
So apparent depth will be
least for violet, and violet will
be raised to the maximum.

Refraction Guide

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