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 boundary (in more technical language) The ray is refracted parallel to the edge of the block

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)

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)

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

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