The absolute refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in that medium. So, absolute refractive index of a medium (µ), µ =speed of light in vacuum/speed in light in that medium This means that higher the speed of light in a medium, lower will be the absolute refractive index of that medium. Light travels faster in an optically less dense medium than in an optically more dense medium.