Refractive index

R.I. is the comparative ability of a gem to bend, or refract light. It is expressed as the ratio of the velocity of light in air to its velocity in the substance e.g. diamond has an R.I. of 2.42. This means that light travels in air at a velocity 2.42 times its velocity in the diamond. Since R.I. is a measure of the optical density, the higher the R.I., the greater the degree of bending for the same angle of incidence.

The optical density and therefore the strength of refraction vary from species to species.
For a given species, the velocity at which light passes through is constant.
The angle of refraction for any given angle of incidence is constant for every gem species, since the reduction in velocity determines the degree of refraction or bending of light, as it enters the gem.

Refractive index can be measured by an instrument known as the refractometer, as well as by other simple methods to be discussed later.
Total Internal Reflection and Critical Angle





accurate determination of Refractive Index
cutting of gemstones, especially in the cutting of colourless stones like diamond so as to obtain maximum amount of light reflection back to the observer, as a result of total internal reflection at the back facets.