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Polarized Sunglasses

Science Snack
Polarized Sunglasses
Polarizing sunglasses cut road glare better in some positions than in others.
Polarized Sunglasses
Polarizing sunglasses cut road glare better in some positions than in others.

When light reflects from water, asphalt, or other nonmetallic surfaces, it becomes polarized—that is, the reflected light is usually vibrating more in one direction than others. Polarizing sunglasses reduce this reflection, known as glare, but only when the polarizing lenses are oriented properly.

Tools and Materials
  • One clear lightbulb with socket and cord
  • One piece of shiny opaque plastic, such as a black plastic party plate
  • One or two pieces of polarizing material (such as old lenses from polarizing sunglasses)

None needed.

To Do and Notice

Place the lit bulb upright and the plastic plate on a flat surface nearby (see diagram). Orient the bulb and your eye so 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 (click to enlarge drawing below). 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 change 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.

On a sunny day, look at the sky through the polarizing lens. Notice that in some places the brightness of the blue sky changes as you rotate the polarizer. That’s because the light in the sky is polarized.

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 people wear polarizing sunglasses when they go fishing.

What's Going On?

The lightbulb produces unpolarized light—each photon is vibrating in its own different direction.

Nonmetallic surfaces, such as black plastic, tend to reflect light that is vibrating parallel to the surface and transmit or absorb light vibrating in all other directions. If the black plastic is horizontal, then it reflects light that is vibrating horizontally, creating horizontally polarized light. The horizontal black plastic reflects less light that is vibrating vertically.

The polarizer lets through light vibrating in one direction and absorbs light vibrating in all other directions. When the black surface is horizontal, the reflection looks dimmest when you hold the filter so it lets through just vertically vibrating light. The reflection looks brightest when you hold the filter so it lets through just horizontally vibrating light.

Horizontal surfaces in the environment, such as the asphalt of a street or the surface of a lake, reflect light that is vibrating horizontally. Polarizing sunglasses absorb this horizontally oriented glare. If you tilt your head sideways, this horizontally oriented glare passes through the glasses, making the surface look brighter. (Click to enlarge the diagram below.)

Going Further

Light becomes completely polarized parallel to the surface at one particular angle of reflection, called Brewster’s angle. Brewster’s angle for water is 53 degrees; for glass, it is 56 degrees; for plastic, the angle varies but, in general, will be somewhere between these two numbers. Brewster’s angle is traditionally measured from a line that is perpendicular to a surface. To find the angle measured directly from the surface you must subtract Brewster’s angle from 90 degrees.