In addition to other replies, you can easily "visualize" IR light from say a remote control. Just point your phone's camera to the bulb on the end of a remote t

In addition to other replies, you can easily "visualize" IR light from say a remote control. Just point your phone's camera to the bulb on the end of a remote that you point at the device it controls. You'll see a flash on the display because the IR flash from the remote saturates the CCD in your phone.