I happened to notice the shadow of my legs in the sunlight on the floor and I noticed that I could see the shadow of the hairs on my legs only when I moved my leg slowly (slowly moving onto my toes etc.) As soon as the movement stopped the shadow of the hair seemed to diffuse. Why? Many Thanks, Terry
I think that its related to how quickly your eyes can perceive small and fast moving things. Take an annoying mosquito for instance. While a mosquito is slightly larger than your leg hairs, its awfully difficult to keep track of them flying around you with your eyes. I'm willing to bet its related to this concept of human visual processing speeds.
According to the Wikipedia article on Frame rate: http://en.wikipedia.org/wiki/Frame_rate
It states that at 48 frames per second is what's being used in the film industry to make things look more real. Taking this information and applying it here, it would mean that information that changes faster than 1/48 of a second, would probably be unnoticed by the human eye. The information would be the change of image for film, and for our case, the information would be what you see, which is the change of position of your leg.
So if you move your leg slowly, it makes sense that you'll be able to keep track of your leg hair shadows, where as moving it fast, you wouldn't because they are very thin/small and moves really fast like a mosquito.
If you have lots of leg hair like a forest, than I think the concept of looking through a fence while driving by quickly makes sense. Have you ever drove by a fenced off playing ground and noticed that you can kinda "filter out" the fence because you are seeing through the fence at different positions really fast? When your going slow, you're almost always obstructed, until a certain crucial speed.
Moreover, when you focus on the background, the image of the playground, you can see it, but if you focus on the foreground, the fence, then you'll see the fence and not the background moving really fast. It's difficult to see the foreground and the background at the same time.
I think this would be the same with your leg hair shadows too if there are a lot. It would make a cris-cross pattern like a fence and you can "see through" it if you move your leg fast, but you can't see through it if you move your leg slow.
The retina in the eyes detect changed impressions of moving objects. It cannot detect small variation in illumination compared to position change due to speed. If you are near a tiger in forest an advice to escape danger is to be steady, and not move at all. (It is risky,but if you move, the tiger is faster :) )