If you rotate a pair of polarizing sunglasses, you will find that they cut road glare much better in some positions than in others.
When light reflects from water, asphalt, or other non-metallic surfaces, it becomes polarized. That is, the reflected light is usually vibrating more in one direction than in others. Polarizing sunglasses reduce this reflection, known as glare, but only when the polarizing lenses are oriented properly.
(15 minutes or more)
Place the lit bulb with its filament parallel to the surface of the plastic. Orient the bulb so that you can see the reflection of the bulb in the plastic.
Look at the reflection through a piece of polarizer. Rotate the polarizer and vary the angle at which you look at the plastic until you get the dimmest reflection. You'll probably get the best results when there's about a 35-degree angle between your eyes and the piece of plastic (see drawing). Rotate the polarizer 90 degrees as you watch the reflection. The reflection should become notably brighter.
Observe reflections elsewhere around you. Rotate the polarizer and vary the angle of viewing to vary the brightness. Try looking at a reflection from a metallic surface, such as an ordinary mirror. There should be no difference in the brightness of an image reflected in the mirror as you rotate the polarizer or vary the angle of viewing.
Look at the sky through the polarizing lens. Notice that the brightness of the sky changes as you rotate the polarizer. That's because the light in the sky is polarized. (For more information about this effect, see the Blue Sky Snack.)
Look through a polarizer at the surface of a pond on a bright, sunny day. Rotate the polarizer and notice that at one orientation of the polarizer, the surface reflections are greatly reduced and you can see beneath the surface of the water. Rotate the polarizer 90 degrees from this orientation, and the surface reflections block your view of the underwater world. This is why fishermen wear polarizing sunglasses.