Not at all how a 6 year old would think, but I'm actually a little intrigued by this - mind if I have a crack? (Bit silly to ask, really, since I'll have posted by the time you're reading this anyway...)
TOC said that light is a photon - a discrete packet of energy, which is absolutely true. Thing about our eyes is, they can't recognise a single photon of light. Light, thankfully, is also a wave - sort of like a photon beam - and when we perceive it, our retinas are bombarded with lots of these photons in succession. Which is great - yay! - cos this means we can see.
So, we need light to be a wave to see it - thing about light waves, though, is they abide by the 'Inverse Square Law' which means, as they move further and further away, they spread further and further apart - this reduces the intensity of light (lots and lots of the photons in a small space = very intense, same number of photons spread over a much larger space = not nearly as intense). This isn't good in terms of the succession of photons thing that we need so we can see.
More so, every time they bounce against a wall, not only are we moving over more distance and becoming less intense, the 'bounce' off the wall isn't perfect. (Think of a stream of water, shooting out of a hose and hitting the floor - it doesn't bounce back perfectly, it scatters and goes everywhere. Same thing happens with light.) Which, again, is not good for the succession of photons we need so we can see.
Between the scattering of the photon beam, the rapid drop in intensity and the speed at which this happens, light appears to vanish when we cut off the source.
Dave (who's apologetic for how long that was)
