Total internal reflection

Total internal reflection is the optical equivalent of a perfect mirror that costs nothing — and it's why fibres, prisms and even the sparkle of a diamond work. When light travelling inside a dense material (like glass) hits the boundary with a thinner one (like air) at a glancing angle, it doesn't pass through; it bounces back entirely, losing essentially none of its energy. Tilt the angle steeper than a certain 'critical angle' and refraction switches off completely — the surface becomes a flawless reflector.

The cause is refraction taken to its limit: as light tries to bend away from the surface normal on entering a lower-index medium, there's a maximum angle beyond which it physically cannot escape, so it reflects instead. The critical angle depends only on the two materials' refractive indices. This is the whole basis of light guiding: a fibre's core keeps light inside not by walls but by always presenting the boundary at a shallow-enough angle that the photon is forever turned back.