What does the interference colour depend on?
The interference colour is the result of a retardation between the waves which are vibrating inside the mineral. The retardation represents a distance, and thus will depend on the path followed by the waves and their difference in velocity. The interference colour can therefore be represented by:
R= e (n1 - n2)
where "R" represents the retardation, "e" is the thickness of the mineral, and "n1" and "n2" are the refractive indices of the waves.
The interference colour thus depends on the thickness and the birefringence of the mineral.
The thicker the mineral, the longer the fast component will have to get ahead of the slow one. Similarly, the faster the fast component and the slower the slow one, the greater the final phasal difference.
Grains are normally wedge-shaped at their margins, and it is possible to observe colour rings.
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