![]() The higher the pitch, the greater the frequency, and, hence, the shorter the wavelength. 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. HOW IT WORKS Comparing Sound and Light Diffraction (Because sound waves are much larger than light waves, however, diffraction of sound is a part of daily life that most people take for granted.) Diffraction of light waves, on the other hand, is much more complicated, and has a number of applications in science and technology, including the use of diffraction gratings in the production of holograms. Any type of energy that travels in a wave is capable of diffraction, and the diffraction of sound and light waves produces a number of effects. The aperture or the diffracting object effectively then becomes the second source of the wave.Diffraction is the bending of waves around obstacles, or the spreading of waves by passing them through an aperture, or opening. The wave then bends around the corners of an obstacle, through apertures into the regions of the shadow of the obstacle. Note: Diffraction refers to the phenomenon of a wave encountering an opening or obstacle. Therefore to encounter diffraction on electromagnetic waves in our normal lives, we would require microwaves and not visible light since microwaves have a much higher wavelength and the longer wavelengths of about $3\ cm$ can be seen in low light conditions. This does not happen in electromagnetic waves.įor observing the phenomenon of diffraction, the order of the magnitude of the wavelength of the waves should be comparable to that of the slit width. The motion of vibration in longitudinal waves is in the same direction as the wave propagation. Sound travels by longitudinal waves which radiate outward in concentric circles. The general wavelength of visible light ranges from $7000 \times m$. The wavelength of sound generally ranges from $17\ m$ to $15\ mm$. The frequency of human audible sound waves lies from $20\ Hz$ to $20\ kHz$. The wavelength of sound waves is much higher than that of visible light. This condition is satisfied only for sound waves in everyday life. For diffraction to occur, the slit width should be comparable to the wavelength of the light or sound waves. Hint: The reason for the diffraction of sound waves being more evident in daily experience than light waves is that sound waves have much higher wavelength compared to the visible light waves.
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