According to researchers from the Ben-Gurion University of the Negev and the Weizmann Institute of Science in Israel, they have found that by using an electro-optical sensor-equipped telescope, they can aim it at a light bulb in the room that the conversation is being held and in theory, listen in on private conversations.
As our voices are essentially sound waves, these waves cause all kinds of vibrations that might be invisible to the naked eye, but with the correct equipment, hackers could theoretically pickup on those vibrations caused by our voices and convert it into audio through an analog-to-digital converter.
Now it is obviously a lot more complex than what we are making it out to be, but that’s essentially the theory it is based on. That being said, it might not be quite as easy to pull off. This is because depending on the thickness of the glass of the bulb and its brightness, and assuming that nothing is in the way like a draw curtain, it might not necessarily work, but if anything it is rather ingenious.