The index of refraction is defined by

(1)

where c is the speed of light and is the phase velocity. It gives the amount of refraction which takes place for light passing from one medium to another. A complex index of refraction can be defined for substances which absorb as well as refract as

(2)

where k is the wavenumber, is the angular frequency, is the electric permittivity, and is the electrical conductivity.

Cauchy's Formula, Dielectric, Dielectric Constant, Dispersion, Electric Permittivity, Ewald-Oseen Extinction Theorem, Kronig-Kramers Relations, Magnetic Permeability, Maxwell Relation--Index of Refraction, Refraction, Refractivity, Relative Permeability, Snell's Law, Stress Refringence