The speed of light through optical glass is slower than the speed of light through a vacuum. The speed of light in optical fiber is expressed as the “index of refraction”. Index of refraction values for optical fiber vary between 1.40 and 1.59. The lower the index of refraction, the faster the speed of light in that particular composition of glass. Both the core and the cladding are made of optical glass, however, the “index of refraction” of the core is lower than that of the cladding. It is this difference in refractive indexes which causes the light signals to stay within the core, providing that the fiber strand is not bent severely.