Index of Refraction

Index of Refraction

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.
   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: n* = n (1 - ik) When the wave passes from one medium n₁ to another n₂, the angle of incidence φ and the angle of refraction φ', both measured with respect to the normal to the interface, are related by:
   sin φ / sin φ' = n₁* / n₂* = constant
   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.
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.

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use