refractive index (n) When light travels from air into a substance its velocity is reduced. The light path is also refracted into the substance, and the relationship between the angle of incidence (i) and the angle of refraction (r) is a constant (Snell's law). This constant (n) is the refractive index of the mineral and is determined by: sin i/sin r = n. The refractive index is also the ratio of the velocity of light in air (V) to the velocity of light in the mineral (v): n = V/v.

refractive index Measure of the bending or refraction of a beam of light on entering a denser medium (the ratio between the sine of the angle of incidence of the ray of light and the sine of the angle of refraction). It is constant for pure substances under standard conditions. Used as a measure of sugar or total solids in solution, purity of oils, etc.