Total Internal Reflection

When a boundary acts as a perfect mirror.

When light travels from a denser to a rarer medium, the refracted ray bends away from the normal. Increase the angle of incidence far enough and the refracted ray skims along the boundary. That angle of incidence is the critical angle. Beyond it, no light escapes and all of it reflects.

Optical fibres use this effect to guide light along thin glass strands with very little loss. It also explains the sparkle of cut diamonds and the behaviour of prisms in binoculars.

• The light must travel from a denser to a rarer medium.
• The angle of incidence must exceed the critical angle.
• Fibre-optic cables carry data this way.

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