; Refraction
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  • pg 1

             Noadswood Science, 2011
                                 Tuesday, October 16, 2012

To explain refraction of waves
• 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

                     • 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
• Light can bend at the boundary between two materials with different
  densities - this is called refraction
•   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 = fastest                 glass = slower              diamond = slowest
 • 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
• 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 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…
• 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)
• 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
• The fish does not aim at the                                     of prey
  refracted image it sees, but at a                                prey
  location where it knows the prey to
Archer Fish
• The Archer fish allows for the refraction of light at the surface of the
  water when aiming at the prey
• 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

• 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

• 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

                               1.33 = sin50o

                             sinƟ = 0.5759732

                                  Ɵ = 35o

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