Refractive Index

The refractive index is a constant for a given pair of materials. It can be defined as

speed of light in material 1

speed of light in material 2

This is usually written 1n2 and is the refractive index of material 2 relative to material 1. The incident light is in material 1 and the refracted light is in material 2.

If the incident light is in a vacuum this value is called the absolute refractive index of material 2. This is the value given in data books.

By definition the refractive index of a vacuum is 1. In practice, air makes little difference to the refraction of light with an absolute refractive index of 1.0008, so the value of the absolute refractive index can be used assuming the incident light is in air.

Material

Absolute Refractive Index

Air 1.0008
Water 1.330
Glass, soda-lime 1.510
Diamond 2.417
Ruby 1.760