Waves bend when they go from one medium to another when the speed changes. It’s a really important topic in light (not so much with sound), because it’s how lenses, eyes, cameras, and telescopes work. The bending of sound waves happens naturally in the air above the earth when it’s warmer than the surface of the earth. The sound waves that travel through the warmer air are faster and the ones that travel through cooler air are slower. When the sound waves go from warmer to cooler air (less dense to more dense air), they become bent back down toward the surface.
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So refraction can bend sound downward which in turn amplifies the sound by adding to the direct (original source) sound. If you’ve ever been near a cool lake, you’ve heard one of nature’s amplifiers!


But why does sound bend? You can imagine a toy car going from a wood floor to carpeting. One wheel hits the carpet first and slows down before the other, causing the toy to turn. The direction of the sound changes in addition to the speed. The slower speed must also shorten its wavelength since the frequency of the wave doesn’t change. Here’s the wave equation again from the previous section:



Note the first v is velocity, and the last ν is frequency in the equation.


Click here to go to next lesson on Diffraction.

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