What are Polarized sunglasses? How do they work?

Edwin H. Land
Before we answer these questions it will be necessary to explain what Polarized sunglasses do. As you know, ordinary light travels in waves, or vibrations. When the waves hit a smooth surface, such as a mirror, a pool of water, a wall or even the leaves of trees, they behave like a handful of flat skipping-stones thrown across a pond. Those that happen to strike the surface edge-on will plunge in, those that strike the surface with their flat sides do not plunge in, but I skip off. The light waves that skip off are the ones that make up most of the dazzling glare you see on a bright day. Since most glare surfaces are horizontal, these glare waves strike at your eyes with their vibrations running side to side rather than up and down. Polarized sunglasses have lenses of a special plastic that can block these side to side vibrations. Think of two boys, one on either side of a picket fence, each holding the end of a jumping rope passed between two pickets. The boys can make the rope go up and down in a wave-like motion; but they cannot give it a circular motion as for jumping rope. Polarized material acts somewhat like picket fence. In the sunglasses, the Polarized lenses are set with their ‘pickets’ vertical. They permit light vibrations that are moving crosswise.
There are certain natural substances that will polarize light. A mineral called Iceland spar is one ot them; tourmaline is another. Researchers have tried for a long time to find or invent a polarizing material that would be available on a large scale, for polarized light has many uses in industry and in optics. Shortly after 1925 an American scientist, Edwin H. Land, (photo, above) brought out a plastic polarizing sheet that he called Polarized film. In the plastic were tiny polarizing crystals. Later polarizers invented by Land have no crystals. Instead, their molecules are lined up in such a way that they polarize light.
Sometimes even light that has passed through a Polarized lens is much too bright for comfort. Polarized sunglasses with double lenses, or discs, reduce the light as much as you wish. One disc rotates. You may set it so that the two discs polarize the light in the same direction, or you may turn it so that they partly cancel each other. You can even block off all the light. (To go back to your picket fence, you could set a second fence crosswise, behind the upright fence. Now your rope could not move freely in any direction). Perhaps you have ridden in a train that has Polarized windows. They have double panes that can be adjusted, like the need shades, for you can block out a little, or all, of the light by turning one of the discs. Perhaps someday all our automobiles will have their headlights and windshields made of Polarized glass. This would block off glare and make driving far safer and more comfortable.

Additional reading:
Sunglasses (Wikipedia)

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