You only have to go to
the beach and be hit by a wave to appreciate that waves carry energy. But they do
so in surprisingly diverse ways. Some waves, such as sea waves and sound waves,
physically move the particles of water, or air, or whatever medium they are traveling
through. These waves come in two types. A sound wave is
"longitudinal" -it creates vibrations that move air parallel to the
direction in which the wave is moving. "Transverse" waves, such as
electromagnetic waves, oscillate in a direction perpendicular to their
direction of travel. Polaroid sunglasses work because they block out transverse
oscillations moving in a certain orientation-for example, up and down.
Any light waves oscillating in another direction from side to side, for
example-pass through unaffected. If light was a longitudinal wave, polaroid
lenses would have no effect at all.
Most of wave theory was worked out in the 19th century when
pioneers such as Thomas Young showed how waves can be manipulated. Waves are reflected by certain materials, refracted
as they cross the boundary between two media, or diffracted, which means they spread
out as they pass through a narrow opening. They can also interfere with each other,
canceling each other out completely in some regions of the medium while
combining into larger, more powerful waves in other areas.
by Michael
Brooks, 30 Second Theory
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