What are Polarized Sunglasses?

Polarized sunglasses are specialized eyewear designed to reduce glare and increase contrast sensitivity. Glare distorts the true color of objects and makes them harder to distinguish.

Unlike traditional sunglasses, polarized sunglasses selectively block out glare instead of dimming the entire field of vision. Traditional sunglasses may reduce glare, but they may also block out subtle details about the surroundings because of the dye used in tinting the lenses.

Polarized sunglasses have been enjoyed for years by boaters and fishermen to reduce unwanted glare from the water. This glare prevents fishermen from seeing fish swimming below the surface. Wearing polarized sunglasses reduces this glare, allowing fish to be more easily seen.

Today, polarized sunglasses are becoming more and more popular with other sports enthusiasts, including skiers, golfers, bikers and joggers, as well as the general public. Although they are more expensive than traditional sunglasses, polarized sunglasses effectively reduce glare from surfaces other than water, such as snow and glass. Drivers can also benefit from polarization, as the special lenses help reduce glare and reflections from the surface of the road.

Polarization: Light Waves, Rainbows, and Cheap Sunglasses
In this lecture taped before a live audience of elementary and middle school students and their families, MIT Physics Professor Walter Lewin explains polarization, and demonstrates properties of light in rainbows, smoke and the sky. He answers the perennial question, "why is the sky blue?" and creates a red sunset in the laboratory.

Polarization Effects in Everyday Life

Light reflected by shiny transparent materials is partly or fully polarized, except when the light is normal (perpendicular) to the surface. It was through this effect that polarization was first discovered in 1808 by the mathematician Etienne Louis Malus. A polarizing filter, such as a pair of polarizing sunglasses, can be used to observe this effect by rotating the filter while looking through it at the reflection off of a distant horizontal surface. At certain rotation angles, the reflected light will be reduced or eliminated. Polarizing filters remove light polarized at 90° to the filter's polarization axis. If two polarizers are placed atop one another at 90° angles to one another, there is minimal light transmission.

Polarization by scattering is observed as light passes through the atmosphere. The scattered light produces the brightness and color in clear skies. This partial polarization of scattered light can be used to darken the sky in photographs, increasing the contrast. This effect is easiest to observe at sunset, on the horizon at a 90° angle from the setting sun. Another easily observed effect is the drastic reduction in brightness of images of the sky and clouds reflected from horizontal surfaces (see Brewster's angle), which is the main reason polarizing filters are often used in sunglasses. Also frequently visible through polarizing sunglasses are rainbow-like patterns caused by color-dependent birefringent effects, for example in toughened glass (e.g., car windows) or items made from transparent plastics. The role played by polarization in the operation of liquid crystal displays (LCDs) is also frequently apparent to the wearer of polarizing sunglasses, which may reduce the contrast or even make the display unreadable.

Light from the sky is reflected by car windshields at an angle, making it mostly horizontally polarized. The rear window is made of tempered glass. Stress in the glass, left from its heat treatment, causes it to alter the polarization of light passing through it, like a wave plate. Without this effect, the sunglasses would block the horizontally polarized light reflected from the other car's window. The stress in the rear window, however, changes some of the horizontally polarized light into vertically polarized light that can pass through the glasses. As a result, the regular pattern of the heat treatment becomes visible.