Total internal reflection, animation
Animation of a beam of light in a medium, changing its angle to show total internal reflection. At any interface between two materials of different refractive indices, there exists a critical angle that depends on the ratio of their refractive indices. Light striking the interface at a high angle, as at the start of the clip, is refracted (bent) as it passes out of the first medium, with some of the light reflected internally. Below this angle, no refraction occurs and all of the light is reflected internally, bouncing from one boundary to the other. This effect is the principle behind optical fibres.