Lots of home video games and arcade games use some sort of gun as an input device. You point the gun at the screen and pull the trigger, and if you hit the target on the screen, the target explodes.

To create this effect, the gun contains a photodiode (or a phototransistor) in the barrel. The photodiode is able to sense light coming from the screen. The gun also contains a trigger switch. The output of the photodiode and the switch are fed to the computer controlling the game.
At the same …