If you haven't done this, it's kind of cool, and it's a good object lesson on how a camera works.
1. Sit in a completely dark room for 20-30 minutes. (back in my younger days this was a great trick to show the ladies... :whistling: )
2. Open your eyes, turn on the light for 1 second or less (while looking in one direction only) then turn off the light/close eyes again.
If you've done it right you should be able to see a very clear, red-purple-ish 'photograph' of what you were looking at during the second the light was on. The picture lasts anywhere from 10-20 seconds to several minutes, depending on your eye chemistry/light levels/etc.
What happens is that in the dark your retina becomes coated with the very light sensitive rhodospin- much like an emulsion coated film. Your eyes are the camera lens, eyelids the shutter. Your pupils control the aperture, in this case... wide open. The light coming in essentially bleaches the rhodospin, leaving a captured image on the retina which your brain can 'see'.