For electromagnetic waves, and especially for light, it is customary to express the above results in terms of refractive indices. The refractive index of a medium with normal velocity v 1 is defined as ‍ n 1 = c / v 1 , where c is the speed of light in vacuum. Hence ‍ v 1 = c / n 1 . Similarly, ‍ v 2 = c / n 2 . Making these substitutions in Eqs. (1) and (2), we obtain