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Q & A: light scattering off sound

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Most recent answer: 05/15/2015
Q:
Hello, I am doing a physics project involving light waves and sound waves. I'm wondering if sound waves (placed perpendicularly to the travel path of light waves, or at any other orientation) can alter the light waves (by constructive or destructive interference) to alter the perceived color? I have even more questions on this as I can find very little research, but first I want to see if it is possible. So if you could lead me to research on this topic or provide insightful thoughts that would be very much appreciated. Thank you!
- Meta (age 18)
Nacogdoches, Tx
A:

Yes, the process of light scattering off sound waves is well known. It includes Brillouin scattering, described here: https://en.wikipedia.org/wiki/Brillouin_scattering. Inelastic Brillouin scattering does transfer energy to or from the light wave, changing its frequency, and thus changing the perceived color.

Here's a way you can start to think about it. A sinusoidal sound wave gives a periodic wiggle in the index of refraction of a material. Light scatters off that, just as it scatters off a diffraction grating. Now if the diffraction grating is moving (with respect to the lab frame), the scattered light will be Doppler shifted in frequency, just like light or radar bouncing off a moving car. 

Mike W.


(published on 05/15/2015)

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