Refraction of light
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.
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.
airglass
Refraction is the bending of light
when the light passes from one medium to another.
Introduction
airglass
Useful words to describe refraction of light
normal
incident ray
angle of refraction
angle of incidence
refracted ray
Introduction
airglass
• e.g. from air to glass
• Light is bent towards the normal.
incident raynormal
refracted ray
From a less dense to a denser medium
Introduction
airwater
From a denser to a less dense medium
• e.g. from water to air
• Light is bent away from the normal.
incident rayrefracted ray
normal
Introduction
sin i is directly proportional to sin r.
sin i
sin rO
Relation between angle of incidence and angle of refraction
i = angle of incidencer = angle of refraction
straight line passing through the origin
Laws of refraction
Video Simulation
airglass
incident ray
refracted ray
normal
airglass refracted ray
normal
• The incident ray,the refracted ray,and the normalall lie in the same plane.
Laws of refraction
i.e. sin isin r
= constant
• The ratio of the sin i to sin r is constant.
– This is called Snell’s law.
Laws of refraction
In general,
n1 sin 1 = n2 sin 2n1 sin 1 = n2 sin 2
sin isin r
• refractive index n =
sin asin g
ang=
a: air g: glass
• e.g. for glass,
where
Refractive index
Material Refractive index
Glass 1.5 – 1.7
Water 1.33
Perspex 1.5
Diamond 2.42
Refractive indices of some materialsRefractive index
Refractive index and speed of light distance that light travel in 1 s
distance that light travel in 1 svacuum (or air)
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
diamond (n = 2.42)1.25 108 m
3 Refractive index and speed of light distance that light travel in 1 s
distance that light travel in 1 svacuum (or air)
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
diamond (n = 2.42)1.25 108 m
Refractive index
Speed of light
fastestfastest
slowestslowest
4 Examples of refraction of light
a Bent chopstick
• The chopstick appears bent because of refraction
4 Examples of refraction of light
b Shallower in water• The depth that the object is
actually at is called the real depth.
O
Ireal dept
h
Simulation
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.
airwater
Example 3A ray of light passes from air into water.Angle of incidence = 30
What is angle of refraction in water?nwater = 1.33
30
Example 3
Applying Snell’s law, 1.33 sin w = 1 sin 30 sin w = sin 30/1.33
w = 22.1
airwater
30
nw sin w= na sin a
= 0.376
22.1
Example 4A ray of laser hits a rectangular block.The table gives a set of results.
Notes: a = angles of incidence,g = angles of refraction.
(a) Is the refracted ray bent towards or away from the normal?
a
g 15 30 45 60 7511 18 28 34 40
TowardsTowards
Example 4(b) Find out how sin a and sin g are
related by plotting a graph of sin a against sin g.
a
g 15 30 45 60 7511 18 28 34 40
sina
sin g 0.259 0.500 0.707 0.866 0.9660.191 0.309 0.470 0.559 0.643
Example 4sin a
sin g 0.259 0.500 0.707 0.866 0.9660.191 0.309 0.470 0.559 0.643
0.2 0.4 0.6 0.8 sin g
sin a
0
0.20.40.60.81.0
0.62 0.20 = 0.42
0.94 0.30 = 0.64
slope = = 1.52 0.94 0.300.62 0.20
Example 4(c) Find the refractive index ng of the
glass block in used in the experiment.
Refractive index = = slope of graph sin a
sin g
= 1.52
The refractive index of the glass block used is 1.52.
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.
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 ?
A nwater : 1
B nair : 1
C 1 : nwater
D 1 : nair
waterair
normal
i
r
Q3 A light rays is incident on a…A light ray is incident on a glass prism.
Which equation can be used to find ? 30
20 40
D sin 40 = 1.5 sin
A =sin 30sin 20
sin sin 40
B =sin 30sin 20
sin 40sin
C = 1.5sin sin 40
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)
Q5 Statements:...
1st statement 2nd statementLight is bent towards the normal when it passes from air to glass.
Glass has a greater refractive index than air.
true? true? true? true?
YesYes YesYes
Q1 True or false: Light slows…
True or false: Light slows down when it enters a material from air.
(T/F)
Q2 True or false: The refractive…
True or false: The refractive index cannot be smaller than 1.
(T/F)
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)
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)
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.