If your eyes respond to light, then how can you see black ?

Actually the answer Ken is quite clever, because your eye sees light by switching off its activity when light goes into the eye, and switching on its activity when it doesn't receive any light. So there's a thing called the dark current, nothing to do with Star Wars, so when you close your eyes your retina becomes more active than when they're open. So in other words what you're seeing is actually inactivity rather than activity !