What does matter is that the sun isn't a point, it has some spread. 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 that means is just that the small gap between the shadows fills in, and our brains do the "reaching out" interpretation. Whichever shadow has fingers pointing toward the broad part of the other one seems to reach out. It's not really that the lower hand shadow reaches out toward the upper one. Well, this was an easy experiment to reproduce. If you are in the penumbra region, you will see part of the sun ("a 'bite' has been taken out of the sun!" where the moon is in the way). In a solar eclipse, if you are standing in the umbra region, the light of the sun will be blocked out and it will be very dark. Here is a diagram of the umbra and penumbra regions of the sun and the moon. The sun is a very large source of light and it does not cast sharp shadows because of its size. Photographers often used point light sources when they want shadows in their pictures with sharp edges on them (that is, without penumbras), and multiple or extended sources to make the shadow edges "fuzzier" or "to fill in the shadows with light". A point light source will either be all visible or all blocked, but an extended source can be partially viewable beyond the edge of the shadowing object. If you are in the penumbra looking towards the light source, you will see part of it visible, and part of it blocked. These form because while some of the light from the source gets blocked by the shadowing object, not all of it does. You get these when the light source is larger than a single point. A "penumbra" is that region around the umbra where the shadow is only partial, or imperfect. An "umbra" is the part of the shadow where all of the light from the source is blocked by the shadowing object. Umbras and penumbras are the names for two kinds of regions in a shadow that have different amounts of light in them.
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