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.

Isotropic Indicatrix

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.

  1. a wave normal, is constructed through the centre of the indicatrix
  2. a slice through the indicatrix perpendicular to the wave normal is taken.
  3. the wave normal for isotropic minerals is parallel to the direction of propagation of light ray.
  4. index of refraction of this light ray is the radius of this slice that is parallel to the vibration direction of the light.

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.