Total internal reflection

Total internal reflection is the mechanism by which an optical fiber guides light.  At the interface between the core and cladding (which have different indices of refraction), there exists a critical angle such that light incident at any smaller angle will be entirely reflected (none is transmitted into the cladding where it is lost).  The critical angle depends on both the index of refraction in the core and in the cladding.

See also: index of refraction, core, cladding, fiber