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Polarized sunglasses are made by applying an electric charge to the lens during the manufacturing process. This charge creates "lines" in the tint. Sunlight bounces around off of everything and comes at your eyes from all different angles. Only the light waves that are in line with the polarization will pass through the lens. That is how they reduce glare.

Take 2 polarized lenses and hold them together. Then turn them and you will see them change from transparent to opaque as you allign and mis-allign the polarization.

Anyway, that's why they work so well. Because they only allow a small percentage of the light waves through.