Spies can eavesdrop by watching a light bulb's vibrations
In college for a project I wanted to encode messages over small brightness variations in light bulbs. It's a pain to deal with AC, so I ended up doing it with a laser pointer. It turned out to be stupidly easy with a laser pointer and photodiode attached almost directly to a pair of PC speakers. All these devices have a +5v DC signal. I could attach my iPod headphones to the laser pointer directly, then the photodiode to the speakers, and boom, music at about 100 yards, with almost no quality decrease, though I didn't do anything quantitative :)I've always wondered what places communication started getting hidden since we went online and no longer rely on things like numbers stations or drop boxes (though I assume dead drops still exist)