Index of refraction

The index of refraction is a quantity that expresses the degree to which a medium refracts light. The index is given by $n=c/v_{phase}$, where c is the speed of light in a vacuum and $v_{phase}$ is the phase velocity of light in the medium. For media that absorb as well as refract light, the index of refraction is expressed as a complex number whose imaginary component expresses the absorption.