So, when an unpolarized light is incident on a birefringent material it is split into two types of polarized rays one of these rays has polarization in a direction perpendicular to the optical axis (ordinary rays) and the other in the direction of the optical axis of the medium (extraordinary rays).
So, what is polarization by double refraction?
Light passing through a calcite crystal is split into two rays. This process, first reported by Erasmus Bartholinus in 1669, is called double refraction. The two rays of light are each plane polarized by the calcite such that the planes of polarization are mutually perpendicular.
What is double refraction in gems?
Some gemstones have more than one refractive index (RI) because these stones belong to crystal systems (anisotropic) that have atomic structures that cause an incident ray of light to be resolved into two rays traveling at different velocities. Refraction is when light changes direction as it passes through a surface.