As a kid (in the early 80's!) I accidentally figured this out while "hacking" a sound based "walkie talkie" with a photo cell and a LED pointed at each other th

As a kid (in the early 80's!) I accidentally figured this out while "hacking" a sound based "walkie talkie" with a photo cell and a LED pointed at each other through a little telescope, from my house to my neighbors across the bock. The device worked, but had a bug. a hummmm on the receiver. For the longest time I could not figure out what was the "hummm", till I noticed it was there even with the transmitter off.. and then, I heard strange voices even with the transmitter being off! Then I realized the humm was the reflection of incandescent light on the window, and the voices where the people in the room making the window vibrate too.. Ahh.. those where great fun days early on "hacker" life as a hacker. :)