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When I look for some materials like https://en.wikipedia.org/wiki/Barium_sulfate and want to extract the refractive index then there is written: (nD)=1.636 (alpha). And sometimes also for beta and gamma.

What does this mean? The refractive index is mostly dependent on the wavelength so why is alpha, beta and gamma the only value given?

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The Greek letters are not related to wavelength but to the directions in crystals. In anisotropic crystals the speed of light (and so the index of refraction) depends on the polarization of light and the direction of propagation relative to the crystalline axes. As your link shows, Barium sulfate has an orthorombic structure so probably is biaxial. You can learn about those by searching "Biaxial crystal optics". For example, this: http://edafologia.ugr.es/optmine/intro/indibiaw.htm

Here you can find all three indices for this type of crystal - orthorombic (see section 4.3): https://pubchem.ncbi.nlm.nih.gov/compound/barium_sulfate#section=pH You will see that they are labeled with alpha, beta and gamma and they are all for this orthorombic crystal and not for three different crystal forms of the same compound.

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  • $\begingroup$ Greek letters are also used to distinguish between different crystal structures for the same compound (like e.g. beta barium borate), so you should support this interpretation with suitable references. $\endgroup$ – Emilio Pisanty Aug 18 '17 at 13:59
  • $\begingroup$ @nasu Thanks a lot! How do I know at which wavelength the refractive indices are given? Or more or less the same question: Why is there no varying refractive index given for the range of the wavelengths? Does it not depend on it? $\endgroup$ – Ben Aug 20 '17 at 8:39
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Take RI in two of the three optic axis directions in Biaxial gemstones. ie. Alexandrites and tanzanite positive signs.

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  • $\begingroup$ In other words, in order to have a positive sign in gemstones, the alpha or lowest reading is closest to the intermediate beta and vice versa for negative sign as the gamma or highest reading is closer to the intermediate beta. $\endgroup$ – user244331 Oct 9 at 15:18

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