If you were a video game aficionado in the 1980s, you probably remember the game Duck Hunt. If you played this game, you used what was technically called the "Nintendo Entertainment System (NES) Zapper" but was actually a small plastic gun that you aimed at the television.
Today, we have Nintendo's Wii and Microsoft Kinect, which are far more sophisticated than the Zapper, but still, just how did this little gun work? The Zapper had a photodiode in the barrel that detected light.
When you shot at the television (or rather at the ducks on the screen), the NES blacked out the screen causing the photodiode to start detecting.
Then the computer flashed a solid white block around the targets you were shooting at. The Zapper detects the change in light and tells the computer that it is pointed at a lit target. Basically, you just hit a target. Go you!