# Why are shadows never sharp?

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In: Physics

Because a light source like the sun is too broad/big. And indoors there are too many light sources

If you used a dark room with only focused light source (think spot light) you will get a crisp shadow

The only way a shadow can be 100% sharp is if the source of light is a point that has zero dimensions, but such a source doesn’t exist.

Have you seen shadows on the moon? Super sharp and crisp because there’s less in the way to diffuse the light. It all depends on the source and what’s in the way

* Size of the light source. If the light source is large enough, any object will block some light before it blocks all light. In the area where that transition lands a shadow, it will look like the shadow is fading in, which is not sharp.
* Reflections. As long as there is enough light to reflect off of objects that are near the shadow (in other words, enough light to see the surroundings), some of it will decrease the effect of the shadow. And these reflections are almost always wide and imprecise (especially if there’s any curved surfaces), so they will mess with the sharpness of shadows.
* Translucent materials. While some things are very effective at blocking light, most organic examples are at least a little translucent (especially to a strong light, such as the sun). And when that effect becomes most noticeable at the edge of an object, a little light will come through. This small effect will make shadows become less sharp.

So there are examples of sharp shadows, but they tend to counter as much of the above as possible. Small light source, dark/light absorbing surroundings, and material that is more opaque than the strength of the light source.

ETA, I want to point out that the 1st bullet point is the biggest cause (and it *is* what most of the other comments are pointing out). But the other 2 still have a minor effect.

No light is emitted from a single pin-point source. The sun is a disc, and at the edges of shadows only a part of the sun will be obscured (meaning that it’s less illuminated). This is a matter of angles and the sun itself takes up about half a degree (which is scattered a bit by the atmosphere) so this will contribute to half a degree of blurry shadow. This becomes more visible the further away a shadow is from the object that created it.

So if you’re out in the sun, and you cast a shadow on an object that’s 5 meters away, then about 4.5cm of the edge of the shadow will be blurry due to the sun being a big ball of superhot gas and not a pin-point source of light.

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.

[Measuring sunlight: what instrument to use? (hukseflux.com)](https://www.hukseflux.com/library/measuring-sunlight-what-instrument-to-use)

The thing to be aware of is that sunlight scatters as it passes through the atmosphere, on everything it touches. clouds. the trees around you. Even clear air scatters sunlight; that’s your blue sky.

So you can think of sunlight that falls on your body as being in two buckets: (1) direct sunlight and (2) diffuse sunlight. There’s more direct sunlight than diffuse, BUT, light is coming from all directions.

All that light, even the diffuse light, casts shadows. That’s why your shadow is blurry, it’s because of diffuse sunlight. That light from many directions also casts shadow, which gives all shadows a fuzzy edge.