Why does the edge of a shadow sometimes appear to shift when another shadow gets close to it?
Details: I was in front of a window. The edge of a chair cast a shadow on the floor from the window light. When I moved such that the shadow of my arm got very close to the shadow of the chair, part of the edge of the chair’s shadow was “pulled towards” the shadow being cast by my arm. The shadow of my arms didn’t appear to move. My arm was closer to the sun than the chair.
Interesting! I can see this happening when I wave my hand in front of my window. When the shadow of my hand gets too close to the shadow of my window frame, the shadow of my hand seems to elongate. The window frame is closer to the light than my arm is. It doesn’t work if my hand is too close to the wall. It also seems to bend a little, depending on the angles; and if I have my fingers in a /\ shape and bring them together, I can make a shadow grow between them towards the tips, kind of like an A shape.
One thing I notice is that shadows don’t have hard edges, they fade out. When two penumbras overlap, you might start to perceive shadow where you weren’t expecting it. Whatever is closer to the sun will have a wider penumbra, and this might cause the shadow to seem to grow on the other object. My two fingers were the same distance from the sun, so it grew equally from them.
Unfortunately the sun went behind a building while I was writing this post, so I can’t experiment further. If I’m right, I’d expect this to happen less with lightbulbs and other spot lights, where the penumbra will be smaller.
Yes! I think this is it. The wikipedia article links to these ray diagrams, which I found helpful (particularly the fourth picture).
I suspected it had to do with an overlap in the penumbra, or the “fuzzy edges”, of the shadow, but I kept getting confused because the observation isn’t what you would expect, if you think of the penumbra as two separate pictures that you’re simply “adding together” as they overlap.
Why does the edge of a shadow sometimes appear to shift when another shadow gets close to it?
Details: I was in front of a window. The edge of a chair cast a shadow on the floor from the window light. When I moved such that the shadow of my arm got very close to the shadow of the chair, part of the edge of the chair’s shadow was “pulled towards” the shadow being cast by my arm. The shadow of my arms didn’t appear to move. My arm was closer to the sun than the chair.
Interesting! I can see this happening when I wave my hand in front of my window. When the shadow of my hand gets too close to the shadow of my window frame, the shadow of my hand seems to elongate. The window frame is closer to the light than my arm is. It doesn’t work if my hand is too close to the wall. It also seems to bend a little, depending on the angles; and if I have my fingers in a /\ shape and bring them together, I can make a shadow grow between them towards the tips, kind of like an A shape.
One thing I notice is that shadows don’t have hard edges, they fade out. When two penumbras overlap, you might start to perceive shadow where you weren’t expecting it. Whatever is closer to the sun will have a wider penumbra, and this might cause the shadow to seem to grow on the other object. My two fingers were the same distance from the sun, so it grew equally from them.
Unfortunately the sun went behind a building while I was writing this post, so I can’t experiment further. If I’m right, I’d expect this to happen less with lightbulbs and other spot lights, where the penumbra will be smaller.
Quick research… http://en.wikipedia.org/wiki/Shadow_blister_effect isn’t very detailed, but it looks like I’m correct?
Yes! I think this is it. The wikipedia article links to these ray diagrams, which I found helpful (particularly the fourth picture).
I suspected it had to do with an overlap in the penumbra, or the “fuzzy edges”, of the shadow, but I kept getting confused because the observation isn’t what you would expect, if you think of the penumbra as two separate pictures that you’re simply “adding together” as they overlap.
See also this highly-upvoted question on the Physics Stack Exchange, which deals with your question.