Polarization of light and Malus's law
Light is a transverse wave, and in ordinary (unpolarized) light the waves vibrate in many directions perpendicular to the direction of travel. A polarizing filter lets through only the part of the light vibrating along one direction, so unpolarized light passing through it loses about half its intensity. Only transverse waves can be polarized; sound, a longitudinal wave, cannot. Malus's law describes light that is already polarized passing through a second filter: the transmitted intensity is I = I0 x cos^2(theta), where theta is the angle between the light's polarization and the filter's axis. At 0 degrees nearly all passes, at 60 degrees one quarter passes, and at 90 degrees (crossed filters) none does. Polarization has everyday uses: polarized sunglasses block glare reflected off water and roads, 3-D movie glasses send differently polarized images to each eye, and LCD screens use polarizing layers to control which pixels let light through.
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