Polarization is a peculiar modification which, under certain
conditions, a ray of light undergoes. This property is easier to
observe than double refraction.
If from a transparent prism of tourmaline two thin plates are cut,
parallel to its axis, they will transmit light when they are placed
above each other with the chief axis of each in the same direction.
When one of the plates is turned at right angles to the other, no
light, or but very little, is transmitted, so that the plates appear
black.
In passing through the first slip, the rays of light have acquired a
peculiar property, which renders them incapable of being transmitted
through the second, except when the two are held in a parallel
position, and the rays are then said to be polarized.
In some doubly refracting crystals the two oppositely polarized beams
are of different colors, so upon double refraction and polarization
depends the property of many gems which is called pleiochroism.