Polarized lenses are used in sunglasses to reduce glare from reflective surfaces such as the surface of a lake or the hood of a car. They accomplish this feat through a process called polarization, much like a Venetian blind controls the amount of light entering a room through a window.

"Polarization" refers to the fact that reflected light rays (glare) are oriented in a plane that is parallel to the surface off which they are reflected. Sunlight itself is not polarized but glare created when sunlight bounces off a highly reflective surface is polarized.

Since there are many more horizontal glare-producing surfaces in the world around us, most glare we encounter will be horizontally polarized. A polarized lens has a laminated surface containing invisible vertical "stripes". These invisible stripes act like a vertical grating and block horizontally-polarized light from passing through the lens. The illustration below explains how lens orientation affects the transmission of polarized light.
 

The bulb produces light that is not polarized. The polarizer lens here transmits only vertically-polarized light. The analyzer lens lets the light pass when the "stripes" are vertical but blocks the light when the stripes are horizontal.  If a wearer of polarized sunglasses tilts their head left or right while looking at a source of polarized light (glare off a lake or chrome car bumper), the same effect is seen.

For outdoor use, polarized lenses are an excellent choice. Fishermen and boaters in particular benefit from polarized lenses because they deal almost exclusively with horizontal-surface glare. Fishermen or boaters wearing polarized sunglasses can more easily see below the surface of the water to spot fish or submerged obstacles. Some experts feel that polarized sunglasses are also a good choice for snow skiers but other experts disagree. Polarized sunglasses do cut the polarized glare coming off the surface of snow but they also tend to reduce contrast and make icy patches or bumps (moguls) harder to see. Yellow or amber lenses actually increase contrast and are generally regarded as a better option for snow skiers.

There are some limitations to the use of polarized lenses. In laptop computers, the Liquid Crystal Display (LCD) uses polarized light to control both brightness and color. Since they block transmission of some polarized light, polarized lenses can cause distortions when viewing the laptop screen. LCDs are also used in automobiles so clocks and other instrument displays may become temporarily unreadable.