In optics, the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as the ratio of the speed of light in vacuum relative to that in the considered medium. This can be written mathematically as: n = speed of light in a vacuum/speed of light in a medium Since refractive index is the ratio of same quantities, so it has no unit and is a dimensionless quantity.