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