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```					IGCSE Physics

Waves

1
Lesson 7 – Refraction
Aims:
•To describe experiments to investigate the refraction
of light, using rectangular blocks, semicircular blocks
and triangular prisms.
•To recall and use the relationship between refractive
index, angle of incidence and angle of refraction n =
(sin i) / (sin r).
•To describe an experiment to determine the refractive
index of glass, using a glass block.
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Introducing refraction

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Refraction of light

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Real and apparent depth
The rays of light from a stone get bent (refracted) as they
leave the water.

these rays of light
have travelled in
straight lines.
image

at the place where it thinks              actual location
the rays have come from –
the stone appears to be
higher than it really is.
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The Archer fish
The Archer fish allows
for the refraction of            image
light at the surface of          of prey
the water when aiming            prey
at the prey.                     location

The fish does not aim
at the refracted image
it sees but at a
location where it
knows the prey to be.
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Refraction diagrams and law of
refraction

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What do you notice about the light
rays either side of the glass block?

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Air to glass
When light is refracted as it travels from air to glass:
angle of incidence > angle of refraction
i > r             i > r

As the light ray travels from
air into glass it moves                                air
towards the normal.                                   glass

In general, when light rays
move from a less dense
medium (air) to a more dense
medium (glass) they ‘bend’
towards the normal.                                        15
Glass to air
When light is refracted as it travels from air to glass:
angle of incidence < angle of refraction
i < r
As the light ray travels from
glass into air it moves away
from the normal.
glass
In general, when light rays travel         air
from a more dense medium (glass)
to a less dense medium (air) they
‘bend’ away from the normal.                  i    <      r
If the two surfaces of the block are
parallel, then the ray at the start is
parallel to the ray at the end.                                 16
Refraction – angle of incidence
What happens to light
travelling from air      i = 0°
through a glass block
when the angle of                     air
incidence is 0°?                     glass

When the angle of
incidence is 0 the
light ray is not
deviated from its        undeviated light ray
path.
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Refraction of light

Conclusion
When light enters glass or water it bends towards the
normal line. When light leaves glass or water it
bends away from the normal line.                     18
Refraction in water waves
When waves in water travel through water of different depths
they change speed. In shallow water the waves slow down; in
deeper water they speed up.
We can investigate this by changing the depth of the water in
a ripple tank.
As the water waves slow
down, their direction
changes due to the
change of speed. This is
called refraction.

Perspex sheet used to
change depth of water
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Imagine a car driving from the road into a muddy field.

 In the muddy field it slows
down as there is more friction.      road
 If it enters the field at an angle
then the front tyres hit the mud
at different times.                  tyre 1        tyre 2
 Tyre 1 hits the mud first and
will move more slowly than
tyre 2. This causes the car to
turn towards the normal.
 When the car leaves the mud
for the road, tyre 1 hits the        mud
road before tyre 2 and this
causes the car to turn away
from the normal.                                        21
If the car approached the muddy field at an angle
of incidence of 0° then both front tyres would hit
the mud at the same time.

The tyres would have the same speed relative to
each other so the direction of the car would not
change, it would just slow down.
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Formula and examples

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Travelling through different materials
If you were running along a beach and then ran
into the water when would you be moving
slower – in the water or on the sand?
In the water.

In a similar way, as light moves from one medium
to another of different density, the speed of light
changes.
Do you think light moves faster or slower in a
more dense medium?
Light moves slower through a more dense
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medium.
The speed of light waves depends on the material they
are travelling through.

air = fastest        glass = slower       diamond = slowest

If light waves enter a different material (e.g. travelling
from glass into air) the speed changes.
This causes the light to bend or refract.

air

glass

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The speed of light
Light travels at
300
300,000 km/s in a            270                            Speed of
240                            light
vacuum.                      210                            (thousands
180                            km/s)
As light enters              150
denser media, the            120
90
speed of light                60
decreases.                    30
0

Vacuum

Glass
Water
From this bar chart, which
material do you think is
denser, glass or water?
Glass must be denser than water because light
travels more slowly through glass than water. 26
Glass and water

Glass is denser than water. Light travelling through
glass will be refracted more than light travelling through
water.                                                 27
The speed of light
We can study refraction of light by comparing its speed in
air to that in a different material.

A number called the refractive index is the ratio of
these two speeds:

Refractive index       =    speed of light in air
speed of air in material
Example:
The speed of light in air is 300,000,000 m/s, and
the speed of light in water is 225,000,000 m/s.
What is the refractive index of water? 1.33
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Calculating refractive index
The speed of light in air is 300,000,000 m/s.
The speed of light in crystal is 150,000,000 m/s.
What is the refractive index of crystal?

Refractive index = speed of light in air
speed of light in crystal

Refractive index = 300,000,000
150,000,000

Refractive index of crystal = 2.0
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Snell’s law
Snell’s law can be used in experiments and calculations
to find out the refractive index of a material.

Refractive index = sin i
sin r

n = Refractive Index,
i = Angle of incidence,
r = Angle of refraction.
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Snell’s law – Example 1
A light ray incident upon a glass block at 45º is
refracted to 28º, calculate the refractive index of the
glass.
n = sin i  sin r
n = sin (45)  sin (28)
n = 0.71  0.47
n = 1.51

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Snell’s law – Example 2
A light ray incident upon a glass block at 48º is
refracted to 32º, calculate the refractive index of the
glass.
n = sin i  sin r
n = sin (48)  sin (32)
n = 0.74  0.53
n = 1.40

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Snell’s law – Example 3
A light ray incident upon a glass block is refracted to
25º by a block of refractive index 1.55, calculate the
angle of incidence.
n = sin i  sin r
sin i = n × sin r
sin i = 1.55 × sin (25)
sin i = 1.55 × 0.42 = 0.66
i = 41º
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Snell’s law – Example 4
A light ray is incident at 60º to a material with refractive
index of 2.4 Calculate the angle of refraction of the light
ray.
n = sin i  sin r
sin r = sin i  n
sin r = sin (60)  2.4
sin r = 0.87  2.4 = 0.36
r = 21º
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Experiment

You need to know this experiment for

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Determining refractive index
Aim:
• To determine the refractive index of a perspex block.
Method:
• Use a slit and raybox to illuminate a glass block at a
range of angles.
• Carefully record the angle of incidence and angle of
refraction each time.
• Complete the following table:
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Refractive index table
Angle of   Sin (i)    Angle of    Sin (r)
incidence             refraction

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•Draw a graph of sin (i) against sin (r).
•Calculate the gradient of the graph.
•The refractive index of the glass is equal to
Remember
n = sin (i)  sin (r)

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Animated refraction

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Summary – Refraction
•When light changes the material it is travelling through it
is refracted. The change in material causes a change in
speed.
•Light entering a glass block from air is refracted towards
the normal line and light leaving glass is refracted away
from the normal line.
•Snell’s law is given by
n = sin i  sin r
•Where: n = refractive index, i = angle of incidence in the
air and r = angle of refraction in the medium.         41

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