This is some kind of effective index of refraction for a randomly oriented polycrystalline material. In practice, this only works if the diameter of the crystallites is smaller than about one tenth of the wavelength, meaning that at a wavelength of 500 nm as in your example the crystallites would have to be smaller than 50 nm. If the material consists of larger crystallites, the material is still isotropic, but depolarizing, since in every crystallite double refraction occurs. Then a description with an average index of refraction is not possible (but this is quite often simply ignored). See e.g.
Article Optical isotropy in polycrystalline Ba_ {2} TiSi_ {2} O_ {8}...
Mr Thomas I really Thank you for your response and your article.
I have one question again, how can I find this effective refractive index at any wavelength?
There are a couple of different theories out there. You can find a comparison of these in the following paper.
Article New Method of Modeling Infrared Spectra of Non-Cubic Single-...
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