In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how light propagates through that medium. It is defined as n = c/v where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times faster in a vacuum than in the water. The refractive index determines how much the path of light is bent or refracted when entering a material. This is the first documented use of refractive indices and is described by Snell’s law of refraction.