Index of Refraction
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Index of Refraction
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1. A measure of the amount of refraction
(a property of a dielectric
substance). It is the ratio of the wavelength or phase velocity of an electromagnetic
wave in a vacuum to that in the substance. Also called refractive
index, absolute index of refraction, absolute refractive index, refractivity.
See modified
index of refraction, N-unit, potential
index of refraction.
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It can be a function of wavelength, temperature, and pressure. If the
substance is nonabsorbing and nonmagnetic at any wavelength, then n2 is equal to the dielectric
constant at that wavelength. The complex index of refraction is obtained
when the attenuation of the wave power radian, called the absorptive index k,
is paired with the index of refraction. It is written
the angle of incidence φ and the angle of refraction φ', both measured with respect to the normal to the
interface, are related by
which becomes, for a nonabsorbing medium, the
ratios of the (noncomplex) indices of refraction. In the particular case that
medium 2 is a vacuum, this ratio is the index of refraction of medium 1. This
is known as Snell law, named after Willebrord Snell who discovered it about
1621.
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2. A measure of the amount of refraction
experienced by a ray as it passes through a refractive interface, i.e., a
surface separating two media of different densities. It is the ratio of the
absolute indices of refraction of the two media (see sense 1 above). Also
called refractive index, relative index of refraction.
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References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use