what is the refractive index for a vacuum?
By convention, the refractive index of a vacuum is defined as having a value of 1.0. The index of refraction, n, of other transparent materials is defined through the equation: n = c/v. where c is the speed of light, and v is the velocity of light in that material. Since the refractive index of a vacuum is defined as 1.0 and a vacuum is devoid of any material, the refractive indices of all transparent materials are therefore greater than 1.0.