High Quality Content by WIKIPEDIA articles! The Sellmeier equation is an empirical relationship between refractive index and wavelength for a particular transparent medium. The equation is used to determine the dispersion of light in the medium. It was first proposed in 1871 by W. Sellmeier, and was a development of the work of Augustin Cauchy on Cauchy's equation for modelling dispersion.where n is the refractive index, is the wavelength, and B1,2,3 and C1,2,3 are experimentally determined Sellmeier coefficients. These coefficients are usually quoted for in micrometres. Note that this is the vacuum wavelength; not that in the material itself, which is /n( ). A different form of the equation is sometimes used for certain types of materials, e.g. crystals.