Whereas differing wavelengths in light are manifested as differing colors, a change in sound wavelength indicates a change in pitch. Wavelengths for visible light range from 400 (violet) to 700 nm (red): hence, it would be possible to fit about 5,000 of even the longest visible-light wavelengths on the head of a pin! Light waves, on the other hand, have a wavelength, typically measured in nanometers (nm), which are equal to one-millionth of a millimeter. The waves by which sound is transmitted are larger, or comparable in size to, the column or the door-which is an example of an aperture-and, hence, they pass easily through apertures and around obstacles. Longitudinal waves radiate outward in concentric circles, rather like the rings of a bull's-eye. Sound travels by longitudinal waves, or waves in which the movement of vibration is in the same direction as the wave itself.
The reason for the difference-that is, why sound diffraction is more pronounced than light diffraction-is that sound waves are much, much larger than light waves. But, if you moved away from the door and stood with your back to the building, you would see little light, whereas the sound would still be easily audible. And if you stood right in front of the doorway, you would be able to see light from inside the concert hall. The sound quality would be far from perfect, of course, but you would still be able to hear the music well enough. Suppose, now, that you had failed to obtain a ticket, but a friend who worked at the concert venue arranged to let you stand outside an open door and hear the band. Light waves diffract slightly in such a situation, but not enough to make a difference with regard to your enjoyment of the concert: if you looked closely while sitting behind the post, you would be able to observe the diffraction of the light waves glowing slightly, as they widened around the post. But you have little trouble hearing the music, since sound waves simply diffract around the pillar.
You cannot see the band, of course, because the light waves from the stage are blocked. Imagine going to a concert hall to hear a band, and to your chagrin, you discover that your seat is directly behind a wide post. C OMPARING S OUND AND L IGHT D IFFRACTION
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