Polarization by Absorption

A number of crystalline materials absorb more light in one incident plane than another, so that light progressing through the material become more and more polarized as they proceed. This anisotropy in absorption is called dichroism. There are several naturally occurring dichroic materials, and the commercial material polaroid also polarizes by selective absorption.

Dichroic Materials

Materials which have different absorption for perpendicular incident planes for light are said to be dichroic. The mineral tourmaline is the best known of natural materials. Tourmaline refers to a class of boron silicates. A tourmaline crystal has a unique optic axis, and any electric field vector which is perpendicular to that axis is strongly absorbed.

Polaroid is strongly dichroic and therefore an effective polarizer. If the transmission axes of ideal polarizers are perpendicular, no light is transmitted. The light tranmitted at other angles follows the Law of Malus.

Polaroid Material

Polaroid is the trade name for the most commonly used dichroic material. It selectively absorbs light from one plane, typically transmitting less than 1% through a sheet of polaroid. It may transmit more than 80% of light in the perpendicular plane. The word "polaroid" usually refers to polaroid H-sheet, which is a sheet of iodine-impregnated polyvinyl alcohol. A sheet of polyvinyl alcohol is heated and stretched in one direction while softened, which has the effect of aligning the long polymeric molecules in the direction of stretch. When dipped in iodine, the iodine atoms attach themselves to the aligned chains. The iodine atoms provide electrons which can move easily along the aligned chains, but not perpendicular to them. Light waves with electric fields parallel to these chains are strongly absorbed because of the dissipative effects of the electron motion in the chains. The direction perpendicular to the polyvinyl alcohol chains is the "pass" direction since the electrons cannot move freely to absorb energy.

Polaroid Sunglasses

The polaroid material used in sunglasses makes use of dichroism, or selective absorption, to achieve polarization.