You’ve probably seen drugstore sunglasses labeled “polarized.” Maybe you looked at those next to sunglasses that weren’t polarized, and wondered what the difference was. Or maybe your eye dr. recommended getting polarized lenses, but you weren’t sure how they worked and what benefit they offered.
Well, we’re going to tell you! Very simply, here’s what polarized lenses do: they reduce glare by blocking horizontal light rays.
Why horizontal? Here’s a little background on how polarization works: light comes from the sun in all directions, and it is reflected in every direction, too. When it reflects from light-colored horizontal surfaces – such as a white sidewalk, a white-sand beach, snow, or sunlight reflecting on water – it is said to be polarized horizontally. Light that is polarized horizontally is responsible for most of the glare that interferes with our vision.
How do polarized lenses work? To understand this, it’s useful to think of window blinds. Let’s start with Venetian blinds, which are horizontal. When Venetian blinds are open, light comes into the room in horizontal stripes, depending on the angle of the sun and the angle the blinds are open.
Polarized lenses are more like vertical blinds. When vertical blinds are open at a certain angle, light comes into the room in vertical stripes. Since horizontally reflected light is responsible for most of the glare we see, the polarized tint is placed on the sunglass lens in vertically angled strips. These vertical strips of tint allow vertically reflected light into the eye but block horizontally reflected light – greatly reducing horizontally polarized glare.
Most polarized lenses are sunglasses. Polarized sunglasses are especially popular with boaters, since water is a very reflective surface on a sunny day. Polarized sunglasses are also great for other outdoor activities, such as golfing and tennis, since they can help to sharpen the focus on the ball. They can also be good for driving, since polarized lenses reduce the glare that reflects from the shiny hood of a car, or the glare from the surface of the road on a hot, sunny day.
However, in some situations there can be drawbacks to polarized lenses. They can be problematic for skiing, since blocking the reflected glare off an icy patch might prevent the skier from noticing and steering away from a potential hazard.
In addition, it can sometimes be difficult to read liquid crystal display (LCD) or light-emitting diode (LED) screens on a boat or plane’s instrument panel while wearing polarized sunglasses. This could interfere with the pilot’s ability to clearly read and quickly respond to the information on the instrument panel. This could also apply to the global positioning system (GPS) and other displays of a car’s dashboard, a smart-phone, an ATM, or a self-service gas pump.
An oddity you may notice while wearing polarized sunglasses is that when you look at your car’s rear or side window from the outside, or perhaps the windows on an office building, you may see splotchy, iridescent spots. When you remove your polarized sunglasses, these spots are invisible.
This effect is created because you are looking at heat-tempered glass. The heat-tempering creates several stress points on the glass, enabling the glass, when broken, to crumble into small, granular chunks, which are safer than splitting the glass into sharp, jagged shards. The stress points also reflect the light in different directions from the parts of the glass that don’t have these stress points. The stress points prevent the polarized lens from filtering out the light evenly across the surface of the glass, creating the splotchy, iridescent effect.
If you are unsure if your sunglasses are polarized, here’s a fun, simple test to see if they are. Hold your sunglasses up to a computer screen, which has an anti-glare coating similar to the anti-glare coating on a polarized lens. Angle your sunglasses about 60 degrees, with one side of the frame at 10 o’clock and the other at 4 o’clock. If the lenses are polarized, they will turn black.
You could also take the test with two pairs of polarized sunglasses – hold one pair at a horizontal (180-degree) angle. Now hold the other pair in front of the first pair, but rotate this second pair of sunglasses a half-turn, till it’s straight up and down, at a vertical (90-degree) angle. You will see that the lenses of both pairs of sunglasses turn considerably darker where the two lenses overlap when they are perpendicular. This is because when you angle one polarized lens to another perpendicularly, they block glare both horizontally.