☺Double Refraction And Dispersion☺
Amber beautiful Rings.
By Aamir Mannan.
When discussing the use of the refractometer in the preceding chapter, reference was made to the curious phenomenon of double refraction, which is characteristic of all minerals which crystallise in systems other than the cubic system. this means that the spilitting of a single ray into two polarised rays which have different velocities and therefore different refrangibility is observable in all the common gem minerals except the 'strenght' of the effect in the various stones differs considerably, and this fact is highly important as an aid to identification.
Double refraction is measured numarically as the difference between the least and greatest refractive indices for the stone; some figures will be found in the table of refractive indies given here. in diminishing order of birefringence (birefringence is another term for double refraction) we have.☺
in some minerals the figures vary somewhat; thus in zircorn, while the blue, white, and golden 'fired' types most popular in modern jewellery are almost constant in their birefringence of 0.059, many of the ceylonese zircons, particularly the green varieties, have a much lower double refraction, sinking in some cases practically to zero. ☺
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