Total Internal Reflection

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Total Internal Reflection
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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

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At the Critical Angle
The refracted ray skims along the boundary (in
more technical language) The ray is refracted
parallel to the edge of the block
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As we increase the angle of incidence

• 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)

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But.

• Total Internal Reflection only happens when
• 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)

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Uses of Total Internal Reflection

• 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

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Advantages of Optic Fibres

• Less susceptible to noise than analogue signals
• 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