Absolute refractive index
The absolute refractive index is the refractive index of a medium in relation to the refractive index of a vacuum. It is calculated in relation to the vacuum in which light travels at the speed of light, i.e. the highest possible speed of 299,792,458 m/s. In practice, however, the air we breathe is usually considered the reference medium, in which light travels slightly slower than in a vacuum (1.0003 times slower). The absolute refractive index measures the factor by which the speed of light is higher in a vacuum or in the air than in any other medium.
