Note 1 to entry: The value of the refractive index can depend on the frequency, polarization, and direction.

Note 2 to entry: The refractive index is expressed by n = c₀/c, where c₀ is the speed of light in vacuum and c is the speed of light in the medium.

Note 3 to entry: See also "complex refractive index.

Note 4 to entry: The refractivity is expressed by n − 1, where n is refractive index.

Note 5 to entry: The refractive index has unit one.

Note 6 to entry: This entry was numbered 845-04-101 in IEC 60050-845:1987.