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Polarization (Brit. polarisation) is a property of waves that describes the orientation of their oscillations. Longitudinal waves such as sound waves in liquids and gases do not exhibit polarization, , because for these waves the direction of oscillation is by definition along the direction of travel. Electromagnetic waves (radio, light, microwave, ...) are polarized.

Most light is unpolarized. Light can be made horizontally or vertically polarized by reflecting off different surfaces at various angles. Light reflecting off water is horizontally polarized, as is the glare off the back window of most cars. Light reflecting off snow is unpolarized.
Polarized sunglass lenses are vertically polarized to cancel out horizontally polarized light. For activities such as fishing, where most light glare will be horizontally polarized off the water, a pair of polarized sunglasses is a good bet. For snow-sports or hiking, polarization is helpful but not absolutely necessary.
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Links:
The polarization of the sky

last updated 14 July 2004