Why does a tree in the path of sunlight create a shadow instead of of light spreading around light the tree?
1 Answer
A second factor involves diffraction.
Explanation:
If a wave is blocked by an obstacle, let us say it's a tree, then the part of the wave that goes around the tree will act as a source of its own waves which then tend to fill the space behind the tree. See https://en.wikipedia.org/wiki/Diffraction, most notably the "Mechanism" section.
That works well enough for sound whose wavelength is about as large or larger than the size of the tree. But with visible light the ability to fill the space behind the tree is limited by the wavelength being less than a micron. Light diffracion is effective for only very small objects, whereas sound with its longer wavelength can diffract around things as big as a tree. So you hear sounds from the other side of the tree even though the light source leaves you in shadow.
