Spies have been using “laser listening devices” that can record sound waves off of windows since the idea was invented several decades ago. When someone is talking in the room, the sound waves cause the window to vibrate, and the laser is able pick up those vibrations. This video describes the process:

Now you can experiment with the process using an extremely simple technique shown in this video. You only need 3 parts (a cheap laser pointer, a microphone jack and a photo cell) to try it out:

The laser reflects off the glass and hits the photo cell. It is acting like a mini solar cell and produces a voltage change that the microphone circuit in the laptop interprets as microphone signals.

As long as we are talking about listening devices, we should probably mention this one as well. The following video shows you how to build a simple parabolic microphone:

[Side note - you could also use a window in the opposite direction. If you force a window to vibrate it turns into a speaker as shown here:

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