The index of refraction of a material is actually the ratio of the speed of light in vacuum over the speed of light inside the material. Since the speed of light is the reciprocal of the square root of the material's permittivity times its permeability, then the index of refraction is also given by the square root of the materials's relative permittivity times its relative permeability. Refraction index can be complex when considering the material's absorption. Birefringent materials have two refraction indices, one for each incoming polarization. These two indices are named ordinary index and extraordinary index. Following the iconal equation, one can see the index of refraction as the modulus of the optical path gradient at any given point of a medium.