All of this introductory material on light and its behaviour brings us to the most critical aspect of optical mineralogy - that of Polarization of Light.
Light emanating from some source, sun, or a light bulb, vibrates in all directions at right angles to the direction of propagation and is unpolarized.
In optical mineralogy we need to produce light which vibrates in a single direction and we need to know the vibration direction of the light ray. These two requirements can be easily met but polarizing the light coming from the light source, by means of a polarizing filter.Three types of polarization are possible.
Three Types of Polarized Light
In the petrographic microscope plane polarized light is used. For plane polarized light the electric vector of the light ray is allowed to vibrate in a single plane, producing a simple sine wave with a vibration direction lying in the plane of polarization - this is termed plane light or plane polarized light.
Plane ploarized light may be produced by reflection, selective absorption, double refraction and scattering.
Unpolarized light strikes a smooth surface, such as a pane of glass, tabletop, and the reflected light is polarized such that its vibration direction is parallel to the reflecting surface.
The reflected light is completely polarized only when the angle between the reflected and the refracted ray = 90°.
This method is used to produce plane polarized light in microscopes, using polarized filters.
Some anisotropic materials have the ability to strongly absorb light vibrating in one direction and transmitting light vibrating at right angles more easily. The ability to selectively transmit and absorb light is termed pleochroism, seen in minerals such as tourmaline, biotite, hornblende, (most amphiboles), some pyroxenes.
Upon entering an anisotropic material, unpolarized light is split into two plane polarized rays whose vibratioin directions are perpendicular to each other, with each ray having about half the total light energy.
If anisotropic material is thick enough and strongly pleochroic, one ray is completely absorbed, the other ray passes through the material to emerge and retain its polarization.