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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 illuminated.

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 sound waves.

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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