Anisotropic minerals are covered in Chapter 5 of Nesse.
Anisotropic minerals differ from isotropic minerals because:
When light enters an anisotropic mineral it is split into two rays of different velocity which vibrate at right angles to each other.
In anisotropic minerals there are one or two directions, through the mineral, along which light behaves as though the mineral were isotropic. This direction or these directions are referred to as the optic axis.
Hexagonal and tetragonal minerals have one optic axis and are optically UNIAXIAL.
Orthorhombic, monoclinic and triclinic minerals have two optic axes and are optically BIAXIAL.
In Lab # 3, you will examine double refraction in anisotropic minerals, using calcite rhombs.
Calcite Rhomb Displaying Double RefractionLight travelling through the calcite rhomb is split into two rays which vibrate at right angles to each other. The two rays and the corresponding images produced by the two rays are apparent in the above image. The two rays are:
The vibration directions for the ordinary and extraordinary rays, the two rays which exit the calcite rhomb, can be determined using a piece of polarized film. The polarized film has a single vibration direction and as such only allows light, which has the same vibration direction as the filter, to pass through the filter to be detected by your eye.
With the polaroid filter in this orientation only one row of dots is visible within the area of the calcite rhomb covered by the filter. This row of dots corresponds to the light ray which has a vibration direction parallel to the filter's preferred or permitted vibration direction and as such it passes through the filter. The other light ray represented by the other row of dots, clearly visible on the left, in the calcite rhomb is completely absorbed by the filter.
With the polaroid filter in this orientation again only one row of dots is visible, within the area of the calcite coverd by the filter. This is the other row of dots thatn that observed in the previous image. The light corresponding to this row has a vibration direction parallel to the filter's preferred vibration direction.
It is possible to measure the index of refraction for the two rays using the immersion oils, and one index will be higher than the other.
Remember the difference between:
Electromagnetic theory can be used to explain why light velocity varies with the direction it travels through an anisotropic mineral.
Velocity of light travelling though an anisotropic mineral is dependant on the interaction between the vibration direction of the electric vector of the light and the resonant frequency of the electron clouds. Resulting in the variation in velocity with direction.
Can also use electromagnetic theory to explain why light entering an anisotropic mineral is split into two rays (fast and slow rays) which vibrate at right angles to each other.