As the refractive index varies with the direction of vibration of the light waves for most crystals, it is of great assistance to visualise the values of "n" for all the directions of vibration and relate them to the directions of propagation, which will be perpendicular. The geometric figure which results is called the optic indicatrix.
The indicatrices are the representation of the values for "n" for all the directions of vibration of a crystal.
By thus representing the minerals, three types of geometric shapes appear:
-For some minerals, the optic indicatrix is a sphere. This is the case of those that are isotropic.
- For others, it is an ellipsoid of revolution (with two principle axes), which are known as uniaxial anisotropic minerals.
- Finally, another group has an indicatrix which is an ellipsoid with three principle axes, and these are called biaxial anisotropic minerals.
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