The sensors in digital cameras pick up more than just visible light. They pick up a lot of infrared too, that's light below red that we're not able to detect with our eyes.
This can make pictures look funny, so manufacturers add a filter to block the infrared and give you a more natural image. Actually the filters don't do a perfect job and if you fire a TV remote control at your camera phone you should see it light up on the screen.
All this means you can have a bit of fun with your old web cam. You can remove the infrared filter then add a visible light filter so your webcam only sees infrared!
