What is the interference colour?

The interference colour is the false colour which anisotropic crystals present when they are observed under the petrographic microscope using the polariser and the analyser with perpendicular directions of vibration, known as "crossed polars" (or crossed nicols).

In the figure, we have only used two polarising plates whose planes of vibration are perpendicular. The upper one (which acts as an analyser) only covers the left hand-side, meaning that on the right, the grains appear with their natural colour, whilst on the left we can see the interference colours. The latter are totally different from the natural colours (right). The interference colours vary from one part of the grain to another, due to the fact that it does not have a uniform thickness.

But why does this happen?

 

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