Therefore, signifi cant diffraction of sound waves occurs around hand-sized obstacles. However, sound waves have wavelengths that are comparable to the dimensions of the hand or even larger. Q38.2 The wavelength of visible light is extremely small in comparison to the dimensions of your hand, so the diffraction of light around an obstacle the size of your hand is totally negligible. This sound is clear on the stage but does not diffract around the prompter's box enough for the audience to hear it. Try it-you will sound like a birdwatcher trying to lure out a curious bird. Then the prompter may make rhythmic kissing noises with her lips and teeth. Suppose an opera singer loses the tempo and cannot immediately get it from the orchestra conductor. The high-frequency, short-wavelength, information-carrying components of the sound do not diffract around his head enough for you to understand his words. Out-of-doors, away from refl ecting surfaces, have someone a few meters distant face away from you and whisper. The side fringes in Figure 38.1 and the Arago spot in the center of Figure 38.3 show this diffraction. Another way of phrasing the answer: We can see by a small angle around a small obstacle or around the edge of a small opening. But λ a is a tiny fraction for visible light passing ordinary-size objects or apertures, so light changes its direction by only very small angles when it diffracts. In this world of breadbox-sized objects, λ a is large for sound, and sound diffracts around behind walls with doorways. ANSWERS TO QUESTIONS Q38.1 Audible sound has wavelengths on the order of meters or centimeters, while visible light has a wavelength on the order of half a micrometer.
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