To examine how light travels through a mineral, either isotropic or anisotropic, an indicatrix is used.
|INDICATRIX - a 3 dimensional geometric figure on which the index of refraction for the mineral and the vibration direction for light travelling through the mineral are related.|
Indicatrix is constructed such that the indices of refraction are plotted on lines from the origin that are parallel to the vibration directions.
It is possible to determine the index of a refraction for a light wave of random orientation travelling in any direction through the indicatrix.
For isotropic minerals the indicatrix is not needed to tell that the index of refraction is the same in all directions.
Indicatrix introduced to prepare for its application with anisotropic materials.