1. 12.3 Refraction of light Invisible Crystals Introduction
Laws of refraction
Refractive index 1 ? 2
12.3 Refraction of light

2. 12.3 Refraction of light Refractive index and speed of light 3
Check-point 3
Examples of refraction of light
Check-point 4 ? 3 4 ?
12.3 Refraction of light

3. Invisible Crystals Here are the invisible crystals.
They become invisible when put in water!
12.3 Refraction of light

4. Invisible Crystals
Video
12.3 Refraction of light

5. Introduction
If light waves hit the boundary at an angle with the normal, the direction of light waves changes.
normal
Relationship between the angle of incidence
and the angle of refraction?
incident ray
reflected ray
refracted ray
12.3 Refraction of light

6. Experiment 12c
Laws of refraction
1 Direct a ray to enter the semicircular glass block with different angle of incidence i.
2 Measure the corresponding angle of refraction r. A B
90 0 i 0 r
3 Plot a graph of sin i against sin r.
90
12.3 Refraction of light

7. Experiment 12c Video Simulation Laws of refraction
12.3 Refraction of light

8. 1 Laws of refraction The incident ray, the refracted ray,
and the normal
all lie in the same plane.
incident ray
normal
air
glass
glass
refracted ray
refracted ray
12.3 Refraction of light

9. 1 Laws of refraction The ratio of the sin i to sin r is constant.
i.e.
= constant
sin r
– This is called Snell’s law.
The constant depends on the media across which light ray travels.
Also see Simulation program 3.1 in Physics at Work Multimedia CD-ROM 1.
12.3 Refraction of light

10. 2 Refractive index sin i sin r refractive index n = sin qa sin qg
e.g. for glass,
ng=
where
a : air
g : glass
12.3 Refraction of light

11. 2 Refractive index
Simulation
12.3 Refraction of light

12. Refractive indices of some materials
2 Refractive index
Refractive indices of some materials
Material
Refractive index
Glass
1.5 – 1.7
Water
1.33
Perspex
1.5
Diamond
2.42
12.3 Refraction of light

13. 2 Refractive index In general, the Snell’s law can be expressed as
n1 sinθ1 = n2 sinθ2
12.3 Refraction of light

14. 3 Refractive index and speed of light
Refractive index of a medium is related to the speed of light.
Refractive index
Speed of light in the medium
Materials with larger refractive indices:
Optically denser media
Materials with smaller refractive indices:
Optically less dense media
12.3 Refraction of light

15. 3 Refractive index and speed of light
Light travels to a denser medium:
Light travels to a less dense medium:
air
glass
Refracted ray bent towards normal
Refracted ray bent away from normal
air
glass
12.3 Refraction of light

16. 3 Refractive index and speed of light
12.3 Refraction of light

17. Check-point 3 1 A boy shines a torch under...
2 A light rays is incident on a...
3 A ray of light passes from...
12.3 Refraction of light

18. A boy shines a torch under water as shown.
Check-point 3 - Q1
A boy shines a torch under water as shown.
Which one shows the correct path of the light ray?
A Path X.
B Path Y.
C Path Z.
D All of them.
12.3 Refraction of light

19. A light ray is incident on a glass prism.
Check-point 3 - Q2
A light ray is incident on a glass prism.
Which equation can be used to find  ?
30  A =
sin 30
sin 20
sin 
sin 40
20
40 B =
sin 30
sin 20
sin 40
sin  C
= 1.5
sin 
sin 40
D sin 40 = 1.5 sin 
12.3 Refraction of light

20. Check-point 3 - Q3
A ray of light passes from air to water. If the angle of incident in air is 45, what is the angle of refraction in water?
Refractive index of water = 1.33
By Snell’s law, na sinθa = nw sinθw
1.00 × sin 45 =
1.33 × sinθw
sinθw =
0.532 θw =
32.1
12.3 Refraction of light

21. 4 Examples of refraction of light
a Bent chopstick
The chopstick appears bent because of refraction
12.3 Refraction of light

22. The pool looks shallower.
b Shallower in water
The pool looks shallower.
The depth as seen by the observer is called the apparent depth. O I
real depth
apparent
depth
Simulation
12.3 Refraction of light

23. c Flickering objects in hot air
The object you see through the unstable hot air appears blurred and flickering.
12.3 Refraction of light

24. 4 Examples of refraction of light

25. Check-point 4 1 True or false: Light slows…
2 True or false: The refractive index…
3 True or false: If light travelled…
4 True or false: If the speed of…
5 Sketch a ray diagram for...
12.3 Refraction of light

26. True or false: Light slows down when it enters a material from air.
Check-point 4 - Q1
True or false: Light slows down when it enters a material from air.
(T / F)
12.3 Refraction of light

27. True or false: The refractive index cannot be smaller than 1.
Check-point 4 - Q2
True or false: The refractive index cannot be smaller than 1.
(T / F)
12.3 Refraction of light

28. Check-point 4 - Q3
True or false: If light travelled at the same speed in all materials, refraction would still occur when it passes from air to water.
(T / F)
12.3 Refraction of light

29. Check-point 4 - Q4
True or false: If the speed of coloured lights in raindrops were equal to that in air, there would be no rainbows.
(T / F)
12.3 Refraction of light

30. You can see it when water is added into the bucket.
Check-point 4 - Q5
In the following figure, a pebble is placed in a bucket and you cannot see it.
You can see it when water is added into the bucket.
12.3 Refraction of light

31. Check-point 4 - Q5
Draw light rays to show how you see point P on the pebble, and show its position as seen by you. P’
water
12.3 Refraction of light

32. The End
12.3 Refraction of light

33. What is angle of refraction in water?
Example 4
Finding angle of refraction
A ray of light passes from air into water.
Angle of incidence = 30
What is angle of refraction in water?
nwater = 1.33
30
air
water
12.3 Refraction of light

34. Example 4 Applying Snell’s law, nw sinθw = na sinθa
Finding angle of refraction
Applying Snell’s law,
nw sinθw = na sinθa
 1.33  sinθw = 1  sin 30
 sinθw = sin 30/1.33
= 0.376
air
water
30
θw = 22.1
22.1
12.3 Refraction of light

35. Return
12.3 Refraction of light

36. Example 5 A ray of laser hits a rectangular block.
Finding refractive index
A ray of laser hits a rectangular block.
The table gives a set of results.
Notes: θa = angles of incidence,
θg = angles of refraction. θa
15
30
45
60
75 θg
11
18
28
34
40
(a) Is the refracted ray bent towards or away from the normal?
Towards
12.3 Refraction of light

37. Example 5
Finding refractive index
(b) Find out how sinθa and sinθg are related by plotting a graph of sin a against sin g. θa
15
30
45
60
75 θg
11
18
28
34
40
sinθa
0.259
0.500
0.707
0.866
0.966
sinθg
0.191
0.309
0.469
0.559
0.643
12.3 Refraction of light

38. Example 5
Finding refractive index
sinθa
0.2
0.4
0.6
0.8
1.0
0.94  0.30
= 0.64
0.62  0.20 = 0.42
slope = = 1.52
0.94  0.30
0.62  0.20
0.2
0.4
0.6
0.8
sinθg
12.3 Refraction of light

39. Example 5
Finding refractive index
(c) Find the refractive index ng of the glass block in used in the experiment.
Refractive index = = slope of graph
sinθa
sinθg
= 1.52
 The refractive index of the glass block used is 1.52.
12.3 Refraction of light

40. Return
12.3 Refraction of light

41. Example 6 A kingfisher sees a fish in the water.
Apparent depth of the fish
A kingfisher sees a fish in the water.
(a) What is its actual position of the fish? Draw a ray diagram to locate its position.
12.3 Refraction of light

42. Example 6
Apparent depth of the fish
(a) As shown in the ray diagram, the fish is at O, somewhere below the position as seen by the kingfisher above the water.
12.3 Refraction of light

43. Example 6
Apparent depth of the fish
(b) If you want to shine a laser beam on the fish, where should you direct the beam?
The beam should direct at the apparent position of the fish, as the beam also undergoes refraction as it passes into water from air.
12.3 Refraction of light

44. Return
12.3 Refraction of light