The Zapper really only consisted of two working parts: The trigger, and a photodiode. When the trigger is pulled, the photodiode kicks into gear, recording light levels from your television screen (usually, at least.) Once the NES realized you were firing, the entire screen would go black for a single frame. On the next frame, the area your target duck was in flashed white. If the photodiode registered the change from black to white, that meant you were pointing the gun in the right direction. For multiple targets, multiple target frames flash in sequence, telling the game which target was hit.
http://kotaku.com/5719971/ever-wonder-how-the-nintendo-zapper-light-gun-worked
http://kotaku.com/5719971/ever-wonder-how-the-nintendo-zapper-light-gun-worked
