You and Your Students!
Vicki Cobb, Education World Science Editor
Discover how polarized sunglasses reduce glare.
Physical Science, Optics
Setting the Scene (Background)
What advantage do polarized sunglasses have over ordinary sunglasses? The answer to that question is glaringly obvious. (That’s a little “light” humor.) Polarized sunglasses reduce glare. Non-polarized sunglasses are designed just to block light. Polarized sunglasses are also designed to block light, but they block glare as well.
Collect polarized sunglasses to use for this activity. The ideal is to have two pairs of polarized sunglasses for every two or three students. You might send home a note a few days in advance to ask parents to send in polarized sunglasses.
It's easy to find glare on a sunny day. Sunlight that reflects off water, a highway, a snowy field, a car hood, or glass is glare, or polarized light.
Have students look at a glaring car windshield without glasses. If the glare is strong enough, they will not be able to see through the windshield. Have them focus on any "glaring situation" as they put on the polarized sunglasses. The polarizers should let them see right through the glare. Have them tilt their heads sideways while looking at the windshield. See how the glare disappears and reappears as they change their viewing angle.
Ordinary light waves vibrate in every direction. Polarized light vibrates in a single plane.
Much of the glare people are exposed to comes from horizontal surfaces (for example, a lake or a highway). When light strikes the a reflective horizontal surface, it will produce a lot of horizontally polarized light (instead of normal light, which vibrates in every direction).
The polarized lenses in sunglasses are fixed at an angle that only allows vertically polarized light to enter. Therefore, the lenses act like a "gate" for light waves. So long as the gate is turned at a right angle to the polarized horizontal light, the glare is blocked. However, if you tilt your head with the polarized sunglasses on, that's like turning the gate sideways. Doing that allows horizontally polarized light through.
Have your students put on one pair of polarized sunglasses and hold another pair up to the light. They should look at the light through both pairs of glasses. They should rotate the glasses they are holding and notice the lenses change from light to dark and back again.
Next, have the students stick a piece of transparent cellophane tape across one of the polarized lenses. While wearing the other pair, have them hold the taped sunglasses up to the light and rotate the glasses again. They should notice that when the lens gets dark, the tape gets light. Keep rotating, and this situation reverses.
Behind the Scenes
Light that passes through a polarizer emerges as polarized light. Let's say that the light that passes through the held pair of sunglasses gets polarized horizontally. That means that the light that is perpendicular to the ground is blocked and the emerging light coming at the sunglass wearer is parallel to the ground. If the worn sunglasses block light that is also parallel to the ground, the held lenses will appear dark. When the held lenses are rotated, the orientation slowly changes until it passes vertical light that can pass through the worn sunglasses. Now the held lenses appear light.
The cellophane has the amazing ability to "flop" polarized light 90 degrees. So the appearance of the lens will be the opposite of the appearance of the tape.
How can you tell if a pair of sunglasses is polarized or not? You know that they are polarized when you buy them, because there is a sticker on them labeling them as such. (Be careful; some sunglasses that are advertised as "polarized" might not be.) The only other way to know is to put one lens of a pair of sunglasses over another and rotate them. If they darken and then lighten, the lenses are both polarized. If there is no change in the lenses, one pair is definitely not polarized; the other pair may or may not be.
The word polarize, means to break into opposing factions. For example,
More experiments with light can be found in my book Light Action: Amazing Experiments with Optics, published by the International Society for Optical Engineering (2005). My son Josh, an optical engineer, was my co-author on this book. My son Theo was the illustrator.
Article By Vicki Cobb
Copyright © 2005 Education World