Total Internal Reflection

When (1) light enter from an optically dense medium to rare one, and

(2) the angle of incidence is larger than the critical angle,

there's no refract ray at all, all the light is reflected.

Prisms

The critical angle for glass is 42?

So prisms with a 45?angle can

reflect light to 90?or 180?

Fiber-Optic Lines

In fiber-optic lines, a beam of light can carry a great amount of voices and data. The angle of light striking the interior boundary of the fiber is larger than the critical angle. Light is therefore always completely reflected at the boundary, and can be transmitted along the line with almost no loss.