Merging shadows

[AndiFlatland]

I've often noticed that when 2 shadows approach each other, just before they meet there seems to be a sort of 'reaching out' to each other. It's as if they are being gravitationally drawn to each other, like the graphics you often see of 2 stars passing close to each other, where each draws out a tongue of material from the other's surface. This is more noticeable when the shadows are not sharp-edged. Is this just an optical illusion, or is it a real phenomenon with a name?

[vascop]

I guess what you're referring to is the partial shadow or penumbra, which surrounds the full shadow or umbra.

[Laasya]

I think there is no such real phenomenon.

[john91]

There's a similar effect if you hold your hand up in front of a light and slowly move your finger and thumb together, just before they physically touch the outlines merge together.
John