Transcript 11.4 Refraction of light

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.
Introduction
Refraction is the bending of light
when the light passes from one medium
to another.
air
glass
Introduction
Useful words to describe refraction of light
angle of
incidence
air
glass
normal
angle of
refraction
Introduction
From a less dense to a denser medium
• e.g. from air to glass
normal
air
glass
• Light is bent towards the normal.
Introduction
From a denser to a less dense medium
• e.g. from water to air
normal
water
air
• Light is bent away from the normal.
Relation between angle of incidence
and angle of refraction
sin i
i = angle of incidence
r = angle of refraction
O
sin r
straight line passing through the origin
 sin i is directly proportional to sin r.
Laws of refraction
Video
Simulation
Laws of refraction
• The incident ray, the refracted ray,
and the normalall lie in the same plane.
normal
air
glass
glass
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 1 = n2 sin 2
Refractive index
sin
i
• refractive index n =
sin r
sin

a
• e.g. for glass, ang=
sin g
where a: air
g: glass
Refractive index
Refractive indices of some materials
Material
Refractive index
Glass
1.5 – 1.7
Water
1.33
Perspex
1.5
Diamond
2.42
Refractive index and speed of light
vacuum (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
3
Refractive index and speed of
light
vacuum (or air)
3  108 m
water (n = 1.33)
2.25  108 m
fastest
glass (n = 1.5)
2  108 m
Refractive index
Speed of light
diamond (slowest
n = 2.42)
1.25  108 m
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
Example 3
Applying Snell’s law, nw sin w= na sin a
 1.33  sin w = 1  sin 30
 sin w = sin 30/1.33 = 0.376
30
air
water

22.1
w = 22.1
Example 4
A ray of laser hits a rectangular block.
The table gives a set of results.
Notes: a = angles of incidence,
g = angles of refraction.
a 15 30 45 60 75
g 11 18 28 34 40
(a)
Is the refracted ray bent towards or
away from the normal?
Towards
Example 4
(b)
Find out how sin a and sin g are
related by plotting a graph of sin a
against sin g.
a 15 30 45 60 75
g 11 18 28 34 40
sina 0.259 0.500 0.707 0.866 0.966
sin g 0.191 0.309 0.470 0.559 0.643
Example 4
sin a 0.259 0.500 0.707 0.866 0.966
sin g 0.191 0.309 0.470 0.559 0.643
sin a
1.0
0.8
0.6
0.4
0.2
0
0.94  0.30
= 0.64
0.94  0.30
slope =
= 1.52
0.62  0.20
0.62  0.20 = 0.42
0.2 0.4 0.6 0.8 sin g
Example 4
(c)
Find the refractive index ng of the glass
block in used in the experiment.
sin a
Refractive index = sin  = slope of graph
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
B
Path X.
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 ?
normal
A
B
C
D
nwater : 1
nair : 1
1 : nwater
1 : nair
air
water
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  ?
A
sin 30
sin 
=
sin 20
sin 40
B
sin 30
sin 40
=
sin 20
sin 
C
sin 
= 1.5
sin 40
D
sin 40 = 1.5 sin 
30
20 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
Light is bent
towards the normal
when it passes from
air to glass.
true?
Yes
2nd statement
Glass has a greater
refractive index than
air.
true?
Yes
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.
I
O
Simulation
real
depth
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.
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.