Index of Refraction (n)

The ratio of the speed of light in a vacuum (c) to the speed of light in a refractive material (v). The index of refraction of a given medium varies with wavelength because the speed of light varies with wavelength. Common indices of refraction include n_d (the yellow helium line at 587.6nm), n_f  (the blue hydrogen line at 486.1nm) and n_c (the red hydrogen line at 656.3nm).