Index of refraction , ∨ Refractive index Meaning, Definition & Usage

Definitions
  1. (Opt.), the number which expresses the ratio of the sine of the angle of incidence to the sine of the angle of refraction. Thus the index of refraction for sulphur is 2, because, when light passes out of air into sulphur, the sine of the angle of incidence is double the sine of the angle of refraction.

Webster 1913