Polarized Light in Crystals

If you observe calcite through a polarizing filter, the doubled images blink in and out as you rotate the polarizer, confirming that the two images are made of light polarized in perpendicular directions. These directions are called the privileged directions.

Before cheap plastic polarizers became available, the best source of polarized light was a prism of calcite cut so that one beam passed through but the other was reflected away. Since the beams only diverge by a few degrees, the prism had to be very precisely manufactured. Also, given calcite's good cleavage, it was quite fragile. Such a prism was called a Nicol Prism and mineralogists still often refer to crossed polarizers as "crossed Nicols."

Unpolarized light (A) passes through polarizer B and becomes polarized. When it enters a crystalline material, it splits into a slow ray (C: higher refractive index) and a fast ray (D: lower refractive index). When these rays exit the crystal (E) they recombine into a single ray. Their electric field vectors add together and the result usually does not have the same orientation as the initial beam.

If there is a second polarizer above the material at right angles to the first, some of the exiting light will pass through it. The material will not be black as would be the case if we simply looked through two crossed polarizers.

When one privileged direction of the mineral is parallel to the bottom polarizer, the light splits into two components with perpendicular polarizations, but the component perpendicular to the incoming polarized light is zero. The light exits with the same polarization as when it entered, and is blocked by the upper polarizer.

Under the microscope, as the specimen is rotated 360 degrees, it has a privileged direction parallel to the bottom polarizer every 90 degrees. No light gets through the upper polarizer, the specimen appears black, and is said to be at extinction. At all other orientations, some light is transmitted by the upper polarizer, with maximum transmission at 45 degrees to the extinction directions.