Polarized Sunglasses
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.
One or two pieces of polarizing material (such as old lenses from polarizing
sunglasses).
One clear light bulb with socket and cord.
One piece of shiny opaque plastic (the shiny black side of a cassette tape case will
work well).
No assembly necessary.
(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.
The light bulb 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 to 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 that is vibrating in one direction and absorbs light that is
vibrating in all other directions. When the black surface is horizontal, the reflection looks
dimmest when you hold the filter so that it lets through just vertically vibrating light. The
reflection looks brightest when you hold the filter so that 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.
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 angles. Brewster's angle is traditionally measured from a line that is perpendicular to
the surface. To find the angle measured directly from the surface you must subtract
Brewster's angle from 90 degrees.