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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

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As with most of this "illusion" questions this is probably not a physics question but a human-visual-apparatus question. I'm not closing, however, as I don't have a good explanation. – dmckee Jan 1 '13 at 16:48
It could be a diffraction question, perhaps there is some kind of aliasing effect with the effective "frame rate" of the eye and the movement of the hairs, if it is dependent on the physics of the eye it is a good question, I never noticed this before myself... – daaxix Jan 1 '13 at 17:21
Well, in direct sunlight, at about 25cms away from an A4page there is no difference in seeing the shadows of the fuzz on my arm. Individual hairs show up whether moving or not. At a distance of +50cm I can see nothing, whether moving or not, except a difractive pattern on the shape of the arm. Glasses? – anna v Jan 2 '13 at 7:13

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:

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.

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Hi, Many thanks for the answer. It was interesting that today I was trying to take a photo of this effect - but my camera is not good enough to be able to see the effect. I did notice that while I was standing there looking at the shadow (of my leg) there is a tree outside the window and the wind was blowing and as the shadow moved up and down the shadow of my leg the shadow of the hairs appeared and my leg was not moving at all. It seems counter-intuitive - you would think that the shadow of the hairs would show when not moving instead of when moving. Again, Many thanks, Terry. – Terry Flannery Jan 2 '13 at 18:33
What your referring to probably has to do with reference frames. Although you are not moving, the tree is moving due to the wind being blown. Therefore, there are moments when sunlight directly hits you, and moments when it's indirect, because the tree is blocking the sunlight. When you don't have a bright enough light, you won't be able to make out small thin shadows. Kinds like when your sitting inside a train adjacent to another train. When your train moves vs. the other train makes it look as though the scenario is the same. Something is moving relative to another, in your case, the tree – QEntanglement Jan 3 '13 at 5:27
Many thanks for the explaination. Cheers, Terry – Terry Flannery Jan 3 '13 at 13:02

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 :) )

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