Sellmeier equation
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 1872 by Wolfgang Sellmeier and was a development of the work of Augustin Cauchy on Cauchy's equation for modelling dispersion. The equation. In its original and the most general form, the Sellmeier equation is given as where "n" is the refractive index, "λ" is the wavelength, and "B" and "C" are experimentally determined "Sellmeier coefficients". These coefficients are usually quoted for λ  in micrometres. Note that this λ is the vacuum wavelength, not that in the material itself, which is λ/n. A different form of the equation is sometimes used for certain types of materials, e.g. crystals. Each term of the sum representing an absorption resonance of strength "B" at a wavelength . For example, the coefficients for BK7 below correspond to two absorption resonances in the ultraviolet, and one in the mid-infrared region. Close to each absorption peak, the equation gives non-physical values of "n" = ±∞, and in these wavelength regions a more precise model of dispersion such  as Helmholtz's must be used. If all terms are specified for a material, at long wavelengths far from the absorption peaks the value of "n" tends to where ε is the relative permittivity of the medium. For characterization of glasses the equation consisting of three terms is commonly used: As an example, the coefficients for a common borosilicate crown glass known as "BK7" are shown below: For common optical glasses, the refractive index calculated with the three-term Sellmeier equation deviates from the actual refractive index by less than 5×10 over the wavelengths' range of 365 nm to 2.3  μm, which is of the order of the homogeneity of a glass sample. Additional terms are sometimes added to make the calculation even more precise. Sometimes the Sellmeier equation is used in two-term form: Here the coefficient "A" is an approximation of the short-wavelength (e.g., ultraviolet) absorption contributions to the refractive index at longer wavelengths. Other variants of the Sellmeier equation exist that can account for a material's refractive index change due to temperature, pressure, and other parameters.