The basic function of any optical fiber is to guide light, i., to act as a dielectric waveguide: light injected into one end should stay guided in the fiber. by reaching the outer surface and escaping there. To understand how light signals travel along an optical fiber, this chapter first describes the fundamental nature of light and discusses how light propagates in a dielectric medium such. An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Light entering the fiber at angles greater than the critical angle reflects off the fiber walls, bouncing along the fiber without escaping.