Relief can also vary within the same grain
When a ray of polarised light crosses a crystal, the refractive index which acts corresponds to the direction of vibration.
Let us consider a crystal under the microscope. In case (1), the light waves arrive from the polariser vibrating in accordance with one of the two directions of vibration of the mineral. For these waves, the vibration index of the crystal would be n1. If we turn the stage round 90º (case 2), the waves now would be propagated according to the other index, n2. If this crystal is very anisotropic and n1 is very different from n2, the relief will change according to whether one or the other is acting.
The refractive index of all anisotropic minerals varies continuously (between a maximun value and a minimun value) depending on the vibration direction of the light within the crystal. The maximun possible difference in refractive indices of a mineral is called the birefringence.
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