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Physics > Total Internal Reflection
When waves (including light) travel from a slower medium to a faster one, for example from glass to air, they speed up and bend where the two surfaces meet.
The pictures below show the light beam as the red line.
Light bends where surfaces meet
Beyond a certain angle – called the Critical Angle, all the waves bounce back. In this case they are Totally Internally Reflected.
Total internal reflection
The critical angle for most glass is about 42° and it is this critical angle that allows light to travel through optical fibres without escaping (see fig 3 below), even if the fibre does not have a plastic shroud covering. Optical fibres can be made of glass or plastic and are very effective at enabling digital data to travel through them with minimal data loss. The glass or plastic absorbs almost none of the light passing through because of Total Internal Reflection.
Total Internal Reflection can only occur when light passes from a medium with a higher refractive index to a lower refractive index. For example it can occur when light passes from glass to air but not from air to glass. In simple terms, the refractive index is a measurement of how radiation (in this case light) is slowed.