(published on 10/22/2007)
(published on 03/25/2011)
(published on 07/27/2012)
Well, this was an easy experiment to reproduce. It's not really that the lower hand shadow reaches out toward the upper one. Whichever shadow has fingers pointing toward the broad part of the other one seems to reach out. What that means is just that the small gap between the shadows fills in, and our brains do the "reaching out" interpretation.
So now to the physics. Diffraction isn't important here because your hands are very big compared to the wavelength of light and the distance to the ground isn't big enough for a tiny angular spread of the light to matter. What does matter is that the sun isn't a point, it has some spread. There's a region of the shadow that's dark because your hand blocks off light from all parts of the sun. There's a "penumbra" around that which is not so dark because your hand blocks off light from only part of the sun. When the penumbras overlap, you get a region where one hand blocks off light from part of the sun and the other blocks off light from the rest, forming a dark shadow.
(published on 06/12/2013)
As you move into the region of the shadow of the Moon in a total eclipse, to pick an example, there is an exact point at which the Moon just barely starts to block of some light from the Sun. That's the edge of the penumbra. Farther in, there's an exact point at which the Moon blocks off the last bit of light from the Sun. That's the edge of the umbra. Although these edges are sharp, the change in light intensity is gradual. Just outside the umbra, light only reaches the shadow from a little bit of the Sun, so it's still very dark. Just inside the penumbra, light still reaches the shadow from most of the Sun, so it's not very dark.
(published on 09/20/2016)