UNIAXIAL OPTICS

Uniaxial minerals have only one optic axis, and belong to the hexagonal and tetragonal systems.

Minerals in this group include:

In the last lab you examined the calcite rhomb and the two rays formed by the light travelling through the rhomb, with each ray corresponding to a different RI of the calcite.

On rotating the calcite rhomb one dot remained stationary but the other dot rotated with the calcite about the stationary dot.

The vibration direction of the ordinary ray lies in the {0001} plane of the calcite and is at right angles to the c-axis.

The extraordinary ray vibrates perpendicular to the ordinary ray vibration direction in the plane which contains the c-axis of the calcite.

If instead of using a calcite rhomb we had used a slab of calcite which had been cut in a random orientation and placed that on the dots, two images would still appear.

If the random cuts were such that they were perpendicular to the c-axis, then light travelling through the calcite, along the c-axis would produce only one image andwould not become polarized.

The c-axis coincides with the optic axis, which is the direction through the mineral along which light propogates without being split into two rays.

For calcite,

  1. The index of refraction for the ordinary ray is uniform nomega = 1.658, regardless of the direction through the grain that the light follows.
  2. The index of refraction for the extraordinary ray, nepsilon, is variable ranging from 1.486 to 1.658. The index is dependant on the direction that the light travels through the mineral.

Calcite is used as an example of the formation of the two rays because of the large difference between the refractive indices (birefringence (delta)).

For minerals with a lower birefringence, e.g. quartz, delta = 0.009, the two images are still produced but show very little separation. The quartz would have to be 20-25X as thick as the calcite to see the same separation of the dots.