The index of refraction of any material is the speed of light in a vacuum divided by the speed divided by the speed of light in the medium. It is denoted with the letter "n."
The index of refraction is different for every material in the known world. Since it depends on how quickly light passes through the material, materials that are thicker have larger indices of refraction. For example, the index of refraction for diamond is 2.417, while the index of refraction in air is 1.00. Index of refraction values can be used to determine the speed of light through the material and can be used theoretically to determine if light would bend away form the normal or towards the normal as it moves from one media to another.