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# Refraction

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```									Refraction

Tuesday, October 16, 2012

Refraction
To explain refraction of waves
Refraction
• Light travels in straight lines

• Light can bend at the boundary between two materials with different
densities - this is called refraction

• The light ray bends towards the normal as it enters

• The light ray bends away from the normal as it
leaves

• The ray entering the block is parallel to the ray
leaving the block, if the block has parallel faces

• A ray entering the block at 90° is not refracted
Refraction
• Light can bend at the boundary between two materials with different
densities - this is called refraction
Speed
•   The speed of light waves depends on the material they are travelling through

•   If light waves enter a different material (e.g. travel from glass into air) the
speed changes, causing the light to bend or refract

air
glass

air = fastest                 glass = slower              diamond = slowest
Refraction
Refraction
• Why does the light ray bend towards the normal when it enters the
glass block, and then bend away from normal as it leaves?

• In the muddy field it slows down as there is
more friction

• If it enters the field at an angle then the front
tyres hit the mud at different times
Tyre 1          Tyre 2
• First to hit the mud is tyre 1, 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 road before tyre 2 and this
causes the car to turn away from the normal
Refraction
• 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
Water
• Water is denser than air, so light is refracted when it travels through
the surface of the water.

• This is why sticks seem to be bent if they are partly underwater, and
why swimming pools look shallower than they really are

• Refraction lets you see objects
that are normally hidden…
Experiment
• Which of these would you expect to refract more: -
– Light through Perspex
– Light through glycerol
– Light through water

• What is your prediction? Explain why you think this is so…

• Using the protractors measure and record the
incident and refracted ray angles in a table

• Then work out the refractive index and plot
your results for the three materials on a
suitable graph (RI = sin i ÷ sin r)
Experiment
• Refractive index: -
– Perspex = 1.48
– Glycerol = 1.47
– Water = 1.33
Archer Fish
• The Archer fish is a predator that shoots jets of water at insects near
the surface of the water, e.g. on a leaf

• The Archer fish allows for the refraction of light at the surface of the
water when aiming at the prey
image
• The fish does not aim at the                                     of prey
refracted image it sees, but at a                                prey
location where it knows the prey to
be
location
Archer Fish
• The Archer fish allows for the refraction of light at the surface of the
water when aiming at the prey
Refraction
• The Sun also appears to have set below the horizon later than it
actually has – due to refraction…
Using Refraction
• A forensic scientist may sometimes be asked to match pieces of
glass, e.g. from a crime scene and from the clothes of a suspect…
Using Refraction
• To do this the forensic scientist will try to match the refractive index –
a small piece of glass will be immersed in oil and then looked at
under a microscope

• The oil is then slowly heated and cooled – this changes the oil’s
refractive index…

• When the refractive index of the oil and glass match the glass will
‘disappear’ – a computer then converts the temperature of the oil to a
refractive index value
Refractive Index
• When a ray of light is incident at normal incidence, (at right angles), to
the surface between two optical materials, the ray travels in a straight
line

• When the ray is incident at any other angle, the ray changes direction
as it refracts

• The change in direction of a ray depends on the change in speed of
the light and can be used to calculate refractive index
Refractive Index
• Refractive index n of the glass is given by

n = sinƟ1
sinƟ2

• Angle Ɵ1 must always be the angle in the vacuum (or air)
Refractive Index – Question
• A ray of red light is incident on the surface water as shown – state
weather the light travels faster in air or water (refractive index of water
is 1.33 for red light). Calculate the angle of refraction
Refractive Index – Question
• The light travels faster in air as the angle of incidence (50o) is greater
than the angle of refraction (Ɵ)

Ɵ1 = 50o
nwater = 1.33

n = sinƟ1
sinƟ2

1.33 = sin50o
sinƟ

sinƟ = 0.5759732

Ɵ = 35o

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