Why are shadows never sharp?
In: Physics
They can be!
I use shadow puppets in my work and different kinds of lights make different kinds of shadows.
Think of photons like little water balloons hurling through the air. When they hit a surface SPLASH, it makes that surface look bright (not getting into how they bounce off and get to your eye, that’s a different ELI5) The places the balloons don’t splash are dark and that leaves a shadow
A standard lightbulb is made to send those water balloons in as many directions as it can. It wants to splash the whole room to make it nice and bright. They used to be a hot piece of tungsten, now they’re a bundle of little LEDs. Some lights use reflectors or diffusers which make the light bounce around at even more angles. Because they’re sending light in as many directions as they can, it means they’re coming in at a lot of different angles. When they hit the middle of an object, the angle doesn’t really matter, the light is blocked either way. But closer to the edges of an object, the angle starts to make a difference. Some of the light coming in is blocked because of the way the angle hits the edge of the object, some scoots right past it. This is why the edges of shadows are often blurry.
If you wanted a crisp, sharp shadow, you would want light all coming from one very small source so it’s mostly going in the same direction when it hits something.
A single, blight LED without a reflector or a small halogen bulb without a reflector, or an overhead projector can create a very crisp shadow.
In fact, you’ve probably seen this in action. If you’ve ever seen a digital projector show black text, you were looking at a sharp shadow! It works because the projector is throwing all the light in the same direction.
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