Here is a ray diagram showing why the chopstick looks bent when dipped into water: air water incident ray refracted ray normal The diagram shows a ray of light traveling from air into water at the tip of the chopstick. Due to the change in refractive index, the light ray bends towards the normal as it enters the water. This makes the tip of the chopstick appear higher and causes the illusion that the chopstick is bent.

1. Refraction of light

2. 1 John says,
‘When hunting a fish under water, you
should aim your spear directly at the
fish.’
Do you agree?
Yes, of course.
 No, because the fish is actually located
somewhere else.
No, because size of objects changes when
they are put under water.

3. 2 When sunlight falls on the water surface,
which of the following occur(s)?
 It is reflected back to the air.
 It refracts into the water.
 It is absorbed by water and
turned into heat.

4. Introduction
Refraction is the bending of light
when the light passes from one medium
to another.
air
glass

5. Introduction
Useful words to describe refraction of light
angle of
incidence
incid
ent
ray
normal
air
glass re
f ra
c te
d
ra
angle of y
refraction

6. Introduction
From a less dense to a denser medium
• e.g. from air to glass
incid normal
ent
ray
air
glass re
f ra
c te
d
ra
y
• Light is bent towards the normal.

7. Introduction
From a denser to a less dense medium
• e.g. from water to air
in
cid normal
en
tr
ay
water
air refra
cted
ray
• Light is bent away from the normal.

8. Relation between angle of incidence
and angle of refraction
sin i
i = angle of incidence
r = angle of refraction
sin r
O
straight line passing through the origin
⇒ sin i is directly proportional to sin r.

9. Laws of refraction
Video Simulation

10. Laws of refraction
• The incident ray, the refracted ray,
and the normal all lie in the same plane.
incid
ent normal
ray
air
glass re
glass fra
ct
ed
ra
y

11. Laws of refraction
• The ratio of the sin i to sin r is constant.
i.e. sin i = constant
sin r
– This is called Snell’s law.
In general,
n1 sin θ1 = n2 sin θ2

12. Refractive index
• refractive index n = sin i
sin r
• e.g. for glass, ang=sin θa
sin θg
where a: air
g: glass

13. Refractive index
Refractive indices of some materials
Material Refractive index
Glass 1.5 – 1.7
Water 1.33
Perspex 1.5
Diamond 2.42

14. Refractive index and speed of light
distan
ce tha
travel t light
vacuum (or air) in 1 s
3 × 108 m
water (n = 1.33)
2.25 × 108 m
glass (n = 1.5)
2 × 108 m
diamond (n = 2.42)
1.25 × 108 m

15. 3 Refractive index and speed of
light distan
ce tha
travel t light
vacuum (or air) in 1 s
3 × 108 m
water (n = 1.33) fastest
2.25 × 108 m
glass (n = 1.5)
2 × 108 m
Refractive index
Speed of light n = 2.42)
diamond (slowest
1.25 × 108 m

16. 4 Examples of refraction of light
a Bent chopstick
• The chopstick appears bent because
of refraction

17. 4 Examples of refraction of light
b Shallower in water
• The depth that the object is
actually at is called the real
depth.
real
I depth
O
Simulation

18. 4 Examples of refraction of light
c Flickering objects in hot air
• The object you see through the
unstable hot air appears blurred and
flickering.

19. Example 3
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

20. Example 3
Applying Snell’s law, nw sin θw= na sin θa
⇒ 1.33 × sin θw = 1 × sin 30°
⇒ sin θw = sin 30°/1.33 = 0.376
30° ∴ θw = 22.1°
air
water
22.1°

21. Example 4
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

22. Example 4
(b) Find out how sin θa and sin θg are
related by plotting a graph of sin θa
against sin θg.
θa 15° 30° 45° 60° 75°
θg 11° 18° 28° 34° 40°
sinθa 0.259 0.500 0.707 0.866 0.966
sin θg 0.191 0.309 0.470 0.559 0.643

23. Example 4
sin θa 0.259 0.500 0.707 0.866 0.966
sin θg 0.191 0.309 0.470 0.559 0.643
sin θa
1.0
0.8 0.94 − 0.30
= 0.64
0.6
0.94 − 0.30
0.4 slope = = 1.52
0.62 − 0.20
0.2 0.62 − 0.20 = 0.42
0 0.2 0.4 0.6 0.8 sin θg

24. Example 4
(c) Find the refractive index ng of the glass
block in used in the experiment.
sin θa
Refractive index = = slope of graph
sin θg
= 1.52
∴ The refractive index of the glass block
used is 1.52.

25. Q1 A boy shines a torch under…
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.

26. Q2 A beam of light travels air to…
A beam of light travels from air to water.
Which of the following is equal to the ratio
sin r : sin i ?
normal
A nwater : 1
i
B nair : 1 air
C 1 : nwater water
D 1 : nair r

27. Q3 A light rays is incident on a…
A light ray is incident on a glass prism.
Which equation can be
used to find θ ?
30°
sin 30° sin θ 20° 40°
θ
A =
sin 20° sin 40°
B sin 30° sin 40°
=
sin 20° sin θ
C sin θ
= 1.5
sin 40°
D sin 40° = 1.5 sin θ

28. Q4 True or false: When light is…
True or false: When light is incident on a
surface along the normal, only refraction
occurs; there is no reflection.
(T/F)

29. Q5 Statements:...
1st statement 2nd statement
Light is bent Glass has a greater
towards the normal refractive index than
when it passes from air.
air to glass.
true? true?
Yes Yes

30. Q1 True or false: Light slows…
True or false: Light slows down when it enters
a material from air.
(T/F)

31. Q2 True or false: The refractive…
True or false: The refractive index cannot be
smaller than 1.
(T/F)

32. Q3 True or false: If light travel…
True or false: If light travel at the same speed
in all materials, refraction would still occur
when it passes from air to water.
(T/F)

33. Q4 True or false: If the speed of…
True or false: If the speed of light in raindrop
is equal to that in air, there would be no
rainbows.
(T/F)

34. Q5 Sketch a ray diagram for the...
Sketch a ray diagram for the tip of the
chopstick to show why the chopstick looks
bent when dipped into water.