Curious properties of the edges of shadows?

Question

Let's do a thought experiment. Suppose you have two objects a few feet up from the ground, and they're casting shadows. When you move to merge these two objects, their shadows will suddenly "suck" together, just like water droplets will - their edges will meet, and suddenly the edges will expand. In effect, it looks like the shadows are fluid, merging together more than they visually should.

Can anyone explain this? I've been puzzling over this problem for a while, and it's really starting to bother me.

Thanks!

Answer 1

First, do some calculations on your thought experiment. Calculate the light intensity as a function of position relative to the two objects. But don't assume the sun is a point source.

Then, do the physical experiment. First, you'll see that they don't suddenly suck together. The merge is gradual. You'll also notice other effects, like edge diffraction, and the incident light from the whole sky, not just the sun.

Answer 2

This is a penumbra effect. Given an extended light source like the sun, the umbra of a shadow is the region where the sun is completely block. It is darkest there. The penumbra is where the sun is only partially blocked, so it is just partially lighted there. If you overlap the penumbra of two separated objects, the combined penumbra will be darker than before the overlapping occurs. That would give the appearance of the dark umbra (what we notice most) flowing out toward the other object.

Answer 3

diffraction can be a possible explaination