Optical fibers are composed of a central core, and a surrounding layer known as the cladding. The core is provided with a high refractive index, while a lower refractive index is used for the cladding. This difference in refractive indices is incorporated to ensure smooth travel of light along the core. Light travels across the boundary between mediums with the same refractive index, but from a medium with a high refractive index to a medium with a low refractive index, light is totally reflected when the incident angle becomes rather large.

Optical signals travel along the cores of optical fibers. Optical fibers are made in such a way that optical signals undergo total internal reflection at the boundary between the core and the cladding, owing to the difference in the refractive indices of the two media. This principle of total internal reflection is used to trap optical signals within the core.