Polarization by Scattering
Also known as Rayleigh scattering. Unpolarized light scattering from air molecules shakes their electrons perpendicular to the direction of the original ray. The scattered light therefore has a polarization perpendicular to the original direction and none parallel to the original direction.
Source
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"OpenStax College, Polarization. January 13, 2013."
http://cnx.org/content/m42522/latest/
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