Refractive Index

Excerpt

The index of refraction (n) is an essential parameter in optics that determines the speed by which light travels through a medium other than a vacuum. A vacuum, like is present in outer space, is the only medium in which electromagnetic waves experience no dispersion and travel at the speed of light c. Traveling through all other media slows the sine propagation wave of light, and thus the resultant speed is given by the index of refraction: n = speed of light (c) / speed of light in material (v).