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 <math>n=c/v_{phase}</math>, where c is the speed of light in a vacuum and <math>v_{phase}</math> 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.