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					Total Internal Reflection
      Total Internal Reflection
 Can occur when light inside a glass block hits the
  edge at certain angles
 Remember – if light hits the boundary between
  two media it will pass straight through
 As you increase the angle of incidence, the ray
  gets bent
 However – at a particular angle called the Critical
  Angle, something strange happens
At the Critical Angle
                 The refracted ray
                 skims along the
                 (in more technical
                 The ray is refracted
                 parallel to the edge
                 of the block
    As we increase the angle of
 As the angle of incidence increases, the ray
  is now only reflected back inside the block
 The edge of the block acts like a mirror
 The ray does not pass out of the block
 The is total internal reflection
 (See Sang page 60)
 Total Internal Reflection only happens
 The ray is travelling through a medium of
  higher refractive index and meets a
  boundary with a medium of lower refractive
  index (ie the light is slow moving and meets
  a medium where it would be fast moving)
           Uses of Total Internal
 A ray of light can be made to travel along the inside of
  a glass or plastic fibre which is coated with a material
  of lower refractive index
 Rays travel through the fibre taking slightly different
  routes, so they might take different times to travel the
  full length
 The ray of light is switched on and off as a way of
  signalling (called modulation). If the rays are travelling
  at different speeds, this causes problems known as
  multipath dispersion
 To avoid this, the fibres have a very narrow core so
  that most rays pass straight down them
    Advantages of Optic Fibres
 Less susceptible to ‘noise’ than analogue
 Can carry huge amounts of data
 Can travel long distances before the signal
  needs to be boosted
Optic fibres have made it possible for us to
  have cable TV and High speed links
  between computers