https://arstechnica.com/information-technology/2020/06/spies-can-eavesdrop-by-watching-a-light-bulbs-variations/

The list of sophisticated eavesdropping techniques has grown steadily over years: wiretaps, hacked phones, bugs in the wall—even bouncing lasers off of a building's glass to pick up conversations inside. Now add another tool for audio spies: Any light bulb in a room that might be visible from a window.

Researchers from Israeli's Ben-Gurion University of the Negev and the Weizmann Institute of Science today revealed a new technique for long-distance eavesdropping they call "lamphone." They say it allows anyone with a laptop and less than a thousand dollars of equipment—just a telescope and a $400 electro-optical sensor—to listen in on any sounds in a room that's hundreds of feet away in real-time, simply by observing the minuscule vibrations those sounds create on the glass surface of a light bulb inside. By measuring the tiny changes in light output from the bulb that those vibrations cause, the researchers show that a spy can pick up sound clearly enough to discern the contents of conversations or even recognize a piece of music.