The band spacing of the fringes of equal chromatic order of a thin Fabry–Perot interferometer is compared when this interferometer contains air, a solid, or a liquid. This comparison enables the dispersion of the group velocity of light in these media to be known accurately to 2.4 parts in one thousand. The Sellmeier dispersion function is used to deduce the refractive indices with the same degree of accuracy. The order-transformation method is used to find the exact order values from the roughly known optical thickness. The exact order values for air and the sample are used to find the refractive index accurately to approximately 3 × 10-5. A least-squares fitting of the accurate experimental data to the Sellmeier dispersion function enables the coefficients of the latter to be more precisely defined for solids such as glass and mica and for liquids such as glycerin and distilled water. The atomic parameters such as the density of states and the absorption wavelengths in the ultraviolet region of the spectrum for the given samples are deduced from the more precisely found Sellmeier coefficients.
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