Polarized Sunglasses
J. Wright
Polarized sunglasses work by filtering out certain frequencies and orientations of light, such as ultra-violet, which is harmful to human eyes. In order to polarize a material for light, etches of scratches must be microscopically be put into the material, so that only the light waves that are lined up with the scratches can pass through. This is the basis behind polarized sunglasses.
See How Sunglasses Work for more information.