In optics the refractive index (or index of refraction) n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium. Its most elementary occurrence (and historically the first one) is 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.
More fundamentally, n is defined as the factor by which the wavelength and the velocity of the radiation are reduced with respect to their vacuum values: The speed of light in a medium is v = c/n, where c is the speed in vacuum. Similarly, for a given vacuum wavelength λ0, the wavelength in the medium is λ=λ0/n. This implies that vacuum has a refractive index of 1. Historically other reference media (e.g., air at a standardized pressure and temperature) have been common.
Refractive index of materials varies with the wavelength. This is called dispersion; it causes the splitting of white light in prisms and rainbows, and chromatic aberration in lenses. In opaque media, the refractive index is a complex number: while the real part describes refraction, the imaginary part accounts for absorption.
The concept of refractive index is widely used within the full electromagnetic spectrum, from x-rays to radio waves. It can also be used with wave phenomena other than light (e.g., sound). In this case the speed of sound is used instead of that of light and a reference medium other than vacuum must be chosen.
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