4.5 What causes Total Internal Reflection?

Total Internal Reflection … … is the condition in which light cannot escape the medium because the angle of incidence is larger than the critical angle. So all the light stays in the medium When the angle of incidence is greater than 90o, total internal reflection results.

Total Internal Reflection normal air refracted ray R water i r incident ray reflected ray When light moves from a more dense to less dense medium it speeds up which increase the angle of refraction (compared to the angle of incidence)

Total Internal Reflection normal air refracted ray R water r i incident ray reflected ray As the angle of incidence increases the angle of reflection continues to increase at a greater rate

Total Internal Reflection normal air refracted ray R water r i incident ray reflected ray As the angle of incidence increases the angle of reflection continues to increase at a greater rate

Total Internal Reflection normal air refracted ray R water i r incident ray reflected ray Eventually the angle of incidence reaches 90o (the critical angle) and all light is reflected internally

Total Internal Reflection normal air water i r incident ray reflected ray At 90o and beyond, all light is internally reflected, none escapes.

Fibre Optics Optical fibres are made of tiny glass fibre cores that are coated with a less dense material (called the cladding) When light enters the core it is internally reflected

Fibre Optics The core and cladding are protected by more material (usually plastic) Optical fibres are more efficient at carrying electrical signals that copper wire Not affected by electrical storms Carry signals over longer distances Carry more signals that copper wire Smaller and lighter (and glass is plentiful and cheap!)