What are Laser microphones?

Laser microphones are often portrayed in movies as spy gadgets. A laser beam is aimed at the surface of a window or other plane surface that is affected by sound. The slight vibrations of this surface displace the returned beam, causing it to trace the sound wave. The vibrating laser spot is then converted back to sound. In a more robust and expensive implementation, the returned light is split and fed to an interferometer, which detects frequency changes due to the Doppler effect. The former implementation is a fun tabletop experiment; the latter requires an extremely stable laser and precise optics.

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