The idea is to make a proof-of-concept device that shows that you can obtain an audio signal from a video feed. This would be useful for eavesdropping (assuming you didn't have a mic for some reason), or for getting audio using a telephoto lens at long distance - farther than the sound would travel. A low-mass, thin object like a piece of paper, metal foil, or plastic could change an audio signal into an observable signal that could be captured in video and converted to audio. A known technique for eavesdropping is to bounce a laser off a glass window: the audio signal causes the window to vibrate, modulating the reflected optical signal.

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