[Photo] Polarizer

Polarizers remove distorting reflections of light, increase color saturation, and in addition increase the contrast.

Scattered light, glare and other strong reflections can degrade photographs in many ways—e.g., by diluting colors, by obscuring or distracting the viewer from important image details, or by forcing suboptimal exposure compromises. Luckily, nature tends to tag such "bad light" with varying degrees of polarization, and that marker provides an easy way to suppress the "bad light" while capturing the good.

Most of the primary light sources encountered in photography—the sun, moon, indoor lighting, flash lamps—are unpolarized, meaning that the electric field fluctuations accompanying the light are oriented equally and randomly in all directions perpendicular to the light's direction of travel. If all the light's electric fields were oriented in the same direction, we'd say it's linearly or plane polarized. Other types of polarization—e.g., circular and elliptical—are seldom encountered by photographers outside their cameras.

Unpolarized light typically acquires polarization through absorption, scattering or reflection.

see also:
camerapedia on polarizer
B+W filters FAQ
using a polarizer effectively