Refraction is the change in direction of light when passing from one medium to another of different density. Snell's law states that the ratio of sines of the angle of incidence and refraction is equal to the ratio of refractive indices of the two media. Due to refraction, the sun appears above the horizon before actual sunrise and remains visible after sunset. Stars twinkle because their light refracts through our atmosphere, causing their apparent position to fluctuate. Planets do not twinkle as their extended disks average out the effects of refraction. The sun's disk appears flattened at sunrise and sunset due to refraction and mirages caused by variations in the atmosphere's density.

1. GAURAV GUPTA(22)
GUNISH GROVER(25)

2. LIST OF CONTENTS
 What is refraction
 Refraction through glass slab
 Laws of refraction
 Advanced sunrise and delayed sunset
 Twinkling of stars
 Why don’t planets twinkle..
 Apparent flattening of sun’s disc at sunrise and sunset
 Mirage
 References

3. WHAT is REFRACTION of light ?
 REFRACTION of light is the phenomenon of change
in the path of light, when it travels from one medium
to another.
 The basic cause of refraction is change in velocity in
going from one medium to the other.

7. Incident ray

8. Incident ray

9. Incident ray

10. Interface of
two media

11. Angle of incidence
i

12. i
Refracted ray

13. i
Angle of refraction
r

14. i
r
Emergent ray

15. i
r
e
Angle of emergence

16. i
r
e

17. i
r
e
Lateral displacement

18. laws of REFRACTION
 The incident ray, refracted ray and the normal to the
interface at the point of incidence, all lie in the same plane.
 The product of refractive index and sine of angle of
incidence/refraction at a point in a medium is constant
i.e., u * sin i= constant.
therfore, u1 sin i1 = u2 sin i2
i1 = i and i2 = r
hence, u2 / u1 = sin i / sin r
and it is also known as SNELL’s LAW.

20. ADVANCED SUNRISE & DELAYED
SUNSET  The sun is visible to us before the
actual sunrise and after the sunset.
 The density of air near the surface of
earth is slightly higher. The rays
from the sun coming through space
enter at the top of earth’s
atmosphere and travel from rarer to
denser medium.
 They bend slightly towards normal
at each refraction and appear to
come from apparent position of sun.
Hence, the sun appears to rise a few
minutes before the actual rise and
for the same reason, it continues to
be seen a few minutes after it has
actually set.

21. HORIZON

22. HORIZON

23. HORIZON

24. HORIZON
Good morning...!

25. HORIZON
Good morning...!

26. HORIZON

27. HORIZON

28. Twinkling of stars
 The starlight on entering earth’s
atmosphere, undergoes refraction
continuously before it reaches
observer.
 Since the refractive index of earth’s
atmosphere is gradually changing, the
atmosphere bends the starlight
towards the normal, and the apparent
position of star is slightly higher than
its actual position.
 As the path of rays of light coming
from the star goes on varying slightly,
the apparent position of the star
fluctuates and the amount of light
entering the eye flickers..!!

29. Actual star
position
Observer

30. Actual star
position
Observer

31. Actual star
position
Refraction in
atmosphere..
Observer

32. Actual star
position
Refraction in
atmosphere..
Observer

33. Actual star
position
Refraction in
atmosphere..
Observer

34. Apparent star
position
Actual star
position
Refraction in
atmosphere..
Observer

35. Apparent star
position

36. Actual star
position

37. Apparent star
position

38. Actual star
position

39. Apparent star
position

40. Why don’t planets twinkle..??
The planets are much closer to
the earth, and thus are seen as
extended sources. If we consider a
planet as a collection of large
number of point sized sources of
light, the total variation in the
amount of light entering our eye
from all the individual point
sized sources will average out to
zero, thereby nullifying the
twinkling …effect..!!

41. APPARENT FLATTENING OF SUN’S
DISC AT SUNRISE AND SUNSET
 If the sun (or moon) is low
above the horizon, the optical
path of the light through the
atmosphere is very long, and the
atmosphere usually has a
layered structure: different
temperature gradients and
pressure.
 Refraction of the light by these
layers can cause the sun's disk to
be deformed, flattened or
distorted. Inferior and superior
mirages are responsible for the
distortions.

42. MIRAGE:MOST NOTICEABLE
 A highway mirage is an inferior
mirage which can be seen very
commonly on roadways by day or
by night.
 Warm air is less dense than cool
air, and the variation between the
hot air at the surface of the road
and the denser cool air above it
creates a gradient in the refractive
index of the air. Light from the sky
at a shallow angle to the road
is refracted by the index gradient,
making it appear as if the sky is
reflected by the road's surface.
 The mind interprets this as a pool
of water on the road, since water
also reflects the sky.

43. REFRENCES
 Pradeep vol.2 edition 2006
 http://en.wikipedia.org/wiki/Atmospheric_refraction
 PHYSICS textbook NCERT EDITION MARCH 2007.

44. THANK
YOU!!!