1. The document summarizes key concepts about the refraction of light, including the definition of refraction as the change in light's direction when passing from one medium to another. It discusses how the refractive index relates to this, and laws of refraction. 2. Phenomena like mirages, twinkling stars, and colorful sunrises and sunsets are explained by atmospheric refraction and the varying refractive indices of hot and cool air. 3. Dispersion of light by prisms and raindrops is described, forming rainbows. Scattering of light by particles explains the blue sky and reddish sunrises and sunsets.

1. Gurudatta K Wagh
Standard/ Class/ Grade - X SSC, CBSE; - VIII ICSE
Light – Part 2

2. Summary
Refraction of light
Definition, Why does the direction of light rays change?
Appreciation of refraction
Refraction of light through a glass slab
Laws of refraction
Refractive index (n)
Rare to dense, Dense to rare, Special effect, Mirage, Hot and cool air
Twinkling of stars
Planets, Sunrise and sunset,
Dispersion of light
Rainbow
Scattering of light
Tyndall effect, Scattering of light through atmosphere, Blue sky and red
signal, Blue sky and dark signal, Sunrise and sunset

3. Refraction of light
Light travels along a straight line path in a
transparent medium.
Light does not travel in the same direction in all
media.
Refraction When light travels from one medium to
another obliquely, the direction of propagation of light
in the second medium changes.
Definition The phenomenon of change in the
direction of light when it passes from one transparent
medium to another.

5. Why does the direction of light rays change?
The direction of light rays changes when it enters from
one medium to another as the velocity of light is
different in different media.
Appreciation of refraction
Refraction of light through a glass slab
Refraction of light takes place twice in the glass slab.
Due to this the ray emerges parallel to incident ray.
But the light ray is shifted slightly to the left side.
Following are the reasons.

6. Laws of refraction
1. The incident ray and the refracted ray are on the
opposite sides of the normal at the point of incidence
and all three lie in the same plane.
2. For a given pair of media, the ratio of the sine of
the angle of incidence (i) to the sine of the angle of
refraction (r) is constant.
sin i/ sin r = constant (n)
constant (n) = refractive index of the second medium
with respect to the first medium

7. Refractive index (n)
Values of ‘n’ are given with respect to vacuum
‘n’ depends upon the relative speed of propagation of
light in different media
If v1 is the velocity of light in medium 1 and v2 in
medium 2 then ‘n’ of the 2nd medium with respect to
the 1st is given by the ratio of the magnitude of velocity
of light in medium 1st to that in medium 2nd and is
represented as 1n2 = v1/v2

9. Similarly n of 1st medium with respect to 2nd is
2n1 = v2/v1
If medium 1st is vacuum or air then ‘n’ of medium 2nd is
considered with respect to vacuum. It is the absolute
‘n’ of medium and represented as n2
When the ray of light moves from an optically rarer
medium to an optically denser medium, the ray bends
towards the normal, e.g. from air to glass.

10. Rare to dense
‘n’ of the 2nd medium is greater than the ‘n’ of the 1st.
Greater the value of ‘n’, the more is the bending of
refracted ray towards normal. ‘i’ > ‘r’
Dense to rare
‘n’ of the 1st medium is greater than the ‘n’ of the 2nd.
Greater the value of ‘n’, the lesser is the bending of
refracted ray away from normal. ‘i’ < ‘r’
Special effect
When the ray is incident normal to the interface
between any two media in any way i.e. rarer to
denser or denser to rarer, then ‘i’ = 0 and ray of light
propagates undeviated from one medium to another.

13. Mirage
It is observed on a very hot summer afternoon. It is
the apparent random wavering of the hot air rising
above the heated roads.

14. The air just above the fire becomes hotter than the air
further up. The hot air is lighter and rarer than the cool
air, which is denser, above it. ‘n’ of hot air is less than
cool air.
As the physical conditions of air are not stationary, the
apparent position of object fluctuates. It is the effect of
atmospheric refraction on a small scale in our local
environment.
Hot and cool air
At the time of holi festival, we see
flickering of an object through a
turbulent stream of hot air rising
above the holi fire.

15. During refraction of star light from atmosphere as star
light bends towards normal, the apparent position of
stars is slightly higher than its actual position.
This apparent position of the stars isn't stationary but
changes slightly.
Twinkling of stars
The large scale effect is of twinkling of
stars. The stars emit their own light at
night. The stars are point sources of
light as they are very far away.

17. Because of the mobility of the air and in the
temperature, the atmosphere is not steady. Hence ‘n’
of air in the given region goes on changing
continuously and randomly.
When the atmosphere refracts more light towards us,
the star is seen bright. When the atmosphere refracts
less lights towards us the star is seen dim.
Thus due to change in ‘n’ of atmosphere, stars appear
twinkling at night.

18. Planets
The rays of light from a planet also pass through the
atmosphere of earth. But planets do not twinkle.
As compared to stars, planets are nearer to the earth.
So a planet can be considered as a collection of a
large number of point sources of light.
If the intensity of light from one point source
decreases it increases from the other source. Hence
average intensity remains the same. Therefore
planets do not twinkle.

19. Sunrise and sunset
Advanced sunrise and delayed sunset can also be
explained in the basis of atmospheric refraction.
The observer should see the sun when it reaches the
horizon; but it is seen two minutes before that. As a
ray of light from the sun enters the earths
atmosphere it follows a curved path due to refraction
before reaching the observer.

20. Hence advanced sunrise and delayed sunset
increases duration of day by 4 minutes.
It appears to the observer
as if the rays are coming
from the position where the
sun is seen by the
observer.
Hence the sun is seen
earlier before the sun
reaches the horizon.

21. Dispersion of light
Sir Isaac Newton was the first to use a glass prism to
obtain the spectrum of sunlight.
A prism is a transparent medium bound by two plane
surfaces inclined at an angle. When white light is
dispersed into seven colours by a prism, different
colours of light bends through different angles with
respect to incident ray.

22. Out of these seven colours, red light bends the least
while violet light bends the most. So the rays of each
colour emerge along different paths and become
distinct. Hence we get a spectrum of seven different
colours.

24. Rainbow
The beautiful phenomenon of
the rainbow is a combination
of different phenomena -
dispersion, refraction and
reflection of light.
The water droplets act as
small prisms.
When sunlight enters the
water droplets, they refract
and disperse the incident
sunlight. Then they reflect it
internally inside the droplet
and finally again refract it.

27. Scattering of light
Tyndall effect
When light incident on very small particles in the
solution in a medium is scattered, we can see the
path of light.
Scattering of light
When a ray of light is incident on extremely small
particles, the particles deflect the light in different
directions.
When a parallel beam of light is incident on a plane
polished surface like a mirror, the angle of incidence
is the same for all the rays and so we get a parallel
beam after reflection of this light.

29. But when the same parallel beam of light is incident
on a rough surface like bricks, walks or ground, the
angles of incidence are different for different rays of
the beam. Hence the reflected rays travel in different
directions leading to scattering of light.
Scattering of light through the atmosphere
The atmosphere is a homogeneous mixture of minute
particles like dust, smoke, tiny water droplets and
molecules of air. When the sunlight passes through a
canopy of dense forest, scattering of light can be
observed.

30. Particles of larger size can scatter white light also. In
the visible range of light, maximum scattering of blue
light and least scattering of red light take place.
As red light is scattered the least by atmosphere, it
can travel larger distance. Hence danger signals are
red coloured.
Blue sky and red signal
The size of scattering particles plays an
important role related to colours of
scattered light. Very fine particles mainly
scatter blue light.

31. Blue sky and Dark sky
Blue colour of sky is related to scattering of light
through atmosphere. The molecules of air and other
particles of atmosphere have very fine size as
compared to the wavelength of visible light. When
sunlight passes through the atmosphere, these fine
particles scatter light of different wavelengths in the
same proportion of wavelengths of respective
colours. The scattered resultant light enters or eyes
and the sky appears blue.
When astronauts fly at higher altitude, the same sky
appears dark to them. This is because there is no
atmosphere at high altitude and so the scattering of
light does not take place.

32. Sunrise and sunset
Even though the sky looks blue, it appears reddish or
orange at sunrise or sunset. This is related to
scattering of light.
At the time of sunrise or sunset, the sun is very close
to the horizon. Sunlight has to travel a longer path
through the atmosphere to reach the observer.

33. The blue and violet colours are scattered in a greater
amount than red colour. They are scattered away
from the path of light as thickness of the atmosphere
is more between the horizon and the observer. The
light that reaches the observer is mostly red and
yellow. Hence the colour of the sky is reddish orange.

34. THANK YOU
SSC Std 10th Textbook
CBSE Std 10th Textbook
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Wikipedia
Suggestions and Appreciations welcome
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