Why do shadows seem blurry around the edges?
There are several reasons for shadows being blurry around the edges
rather than suddenly going from totally dark to full brightness.
The first reason is that all light sources have a finite size. For
example, on a sunny day the shadows cast by the sun go from full
darkness at a point where the sun is fully obstructed to full
brightness at a point where the sun is fully visible. In between,
where the sun is, for example half obscured, the shadow will be half
Another reason which always applies, even when the light source is
negligibly small, is that light is diffracted around edges. Though
light goes mostly in straight lines, it does change direction a
little when it passes an edge. This is because light is a wave
motion and light waves can go around corners just like water waves
or sound waves. They do not do that as much because the wavelength
of light is very much shorter than the wavelength of water waves or
Diffraction of water waves is easy to see when you are at the
shore. Light diffraction is much harder to see -- unless you have a
laser and a thin slit.
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012