Transmission of quasi-monochromatic light wave through dispersive medium and measurement of refractive index

Relations used in the measurements of the refractive index in a non-absorbing dispersive medium are discussed, taking into account the quasi-monochromaticity of the light wave. It is shown that when interference methods using compensation are employed, it is always necessary to allow for corrections for dispersion. Relations describing the diffraction of a ray at the boundary of vacuum and a non-absorbing dispersive medium can be obtained only from the continuity of the Poynting vector at the interface of these media. The stationary phase method may lead to erroneous conclusions. The group velocity vector, which for diffraction describes correctly not only the absolute magnitude but also the direction of this velocity in a dispersive non-absorbing medium, is of the form $$v_g = \frac{{k(\omega _0 )}}{{|k|}}|\upsilon _g |$$ whereω ₀ is the frequency belonging to the maximum amplitude of a given quasi-monochromatic wave.