refractive index, index of refraction(noun)

the ratio of the velocity of light in a vacuum to that in a medium

refractive index(Noun)

The ratio of the speed of light in air or vacuum to that in another medium.

Refractive index

In optics the refractive index or index of refraction n of a substance is a dimensionless number that describes how light, or any other radiation, propagates through that medium. It is defined as where c is the speed of light in vacuum and v is the speed of light in the substance. For example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water. The historically first occurrence of the refractive index was in Snell's law of refraction, n1sinθ1= n2sinθ2, where θ1 and θ2 are the angles of incidence of a ray crossing the interface between two media with refractive indices n1 and n2. Brewster's angle, the critical angle for total internal reflection, and the reflectivity of a surface also depend on the refractive index, as described by the Fresnel equations. The refractive index can be seen as the factor by which the velocity and the wavelength of the radiation are reduced with respect to their vacuum values: The speed of light in a medium is and similarly the wavelength in that medium is, where is the wavelength of that light in vacuum. This implies that vacuum has a refractive index of 1. Historically other reference media have been common.