How do polarized sunglasses block out the glare without affecting your vision?

Polarized glasses cut out the glare from your field of view by filtering out all the horizontal light waves, which happen to be what “glare” is made of.

The lens that performs this feat was invented by Edwin H. Land at the age of eighteen, working in a laboratory he set up in his rented apartment on New York’s West Side. Land later founded Polaroid, invented the Polaroid camera, and became quite wealthy.

Light, like all energy, travels in waves, just as the energy of motion travels in waves through the ocean. Ocean waves oscillate (vibrate) vertically, describing an up and down motion as they move toward the shore; anyone who has been clobbered by a six foot breaker can attest to this. Light waves from the sun and from ordinary light bulbs vibrate at all different angles: horizontally, vertically, and at every angle in between.

When you look down a brightly lit roadway, with the sun’s glare reflecting off your car’s hood and blinding you, the light reaching your eye from the glare spots contains a large percentage of horizontal waves; this happens because light that strikes a shiny surface at the special angle of 57 degrees (called Brewster’s angle) is filtered or polarized by the surface so that only waves that vibrate parallel to the surface bounce off and reach your eye.

In the case of a car hood, horizontal waves reach you, since the hood is horizontal. This polarized light has not penetrated the paint on the hood or been changed by it, it simply bounces, so it appears the same color as the sun that generated it: blinding white.

Polaroid sunglasses contain vertical ribbing as fine as the height of a single light wave, about a millionth of an inch, which polarizes light vertically, stopping all but the vertically vibrating waves. The ribbing is made by stretching a sheet of heat softened plastic, which makes parallel “stretch marks” or stress lines in the sheet. A light absorbing dye is applied to the plastic, which settles along the minute lines to form the ribbing.

This material wipes out glare from horizontal surfaces because the horizontal light waves coming off at Brewster’s angle can’t fit through the Polaroid ribbing, which runs crosswise to them. Most things that are important for us to see move on horizontal surfaces, such as roads, floors, and bodies of water, so a vertically polarizing lens takes out the worst glare. We see clearly through the lens because there are enough vertical waves that do get through to light the scene for us.

You can test polarization for yourself at the drugstore racks where the glasses are sold. Put on one pair, and hold another pair in front of you as if you were about to put it on. The second set of lenses looks as clear as the set you have on, through it you’re seeing vertical waves that have made it through both polarizers.

Now, swing the second pair sideways so that its lenses line up one above the other instead of side by side. The second set of lenses now looks black. Turned on its side, set 2 transmits only horizontal waves, whereas set 1 cuts out all of them, transmitting only vertical ones.

Thus nothing can reach your eyes through both pairs, between them they have polarized out all the light.