# Refractive index

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The refractive index or index of refraction of a substance is a measure of the speed of light in that substance. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium.[note 1] The velocity at which light travels in vacuum is a physical constant, and the fastest speed at which energy or information can be transferred. However, light travels slower through any given material, or medium, that is not vacuum. (See: light in a medium).[1][2][3][4]

A simple, mathematical description of refractive index is as follows:

Hence, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in a vacuum as it does in water.

As light exits a medium, such as air, water or glass, it may also change its propagation direction in proportion to the refractive index (see Snell's law). By measuring the angle of incidence and angle of refraction of the light beam, the refractive index n can be determined. Refractive index of materials varies with the frequency of radiated light. This results in a slightly different refractive index for each color. The cited values of refractive indexes, such as 1.33 for water, are taken for yellow light of a sodium source which has the wavelength of 589.3 nanometers.[1][2][5]