Why does a shadow become more fuzzy when the object that casts the shadow is further removed from the surface it casts the shadow on?

Question

Why isn’t a shadow as sharp as one can see the object?

In: Physics

Answer 1

Imagine you have two point source lights that are near each other. A surface that’s exposed to both lights will be illuminated twice as brightly as a surface that’s only exposed to one.

Now, put an object between the light and the surface. Because there are two lights, the object actually casts two shadows. But, if the object is very close to the surface, the two shadows will be close to each other, even overlapping. As you move the object away from the surface, the two shadows get farther apart.

Where the two shadows overlap, you get a dark shadow. Where there’s only one shadow, you get a light shadow.

Now, the thing to realize is that no real light source is a single point. It takes up space, which looks the same as multiple light sources very close to each other.

So, in summary, the fuzzy border of the shadow is the region where only part of the light source is visible.