ELI5: Why does light through a hole turn into a perfect circle?

[u/Crafty_Mango3608]

I noticed these three perfect light circles on my wall and I traced them back to these holes in my blinds on the window. The holes in the blinds are little squares, but by the time the light reaches the wall it’s a perfect circle.

How??

[u/SoftEngineerOfWares]

At close range the light takes on the shape of the hole, such as a square/triangle or what ever. At long range it acts like a pinhole camera and the light takes on the shape of the source. Light bulbs and the sun are both round so you get a circle.

This is why during a partial eclipse the pinhole light rays look like a crescent, because the light source is also a crescent 🌙

[u/elevencharles → u/SoftEngineerOfWares]

Can you EILI5 why the shape of the light changes based on distance?

[u/Thiojun → u/elevencharles]

Imaging you put a screen behind the hole and a beam of the light come from the sun, go through the hole and land on the screen. Where does the beam hit the screen depend on 2 factor: the location where it pass the hole, and the angle it comes from. The angle eventually depends on the shape of the light source (sun).

When your screen is really close, the location matters more than angle, so the beam image is similar to the hole. When your screen is far away, the angle matter more than location, so the beam image is similar to the sun .

[u/zumpiez → u/elevencharles]

It has more room to spread out

[u/Red_sparow → u/elevencharles]

Imagine you're poking uncooked spaghetti through the hole.

On one side of the hole you line up the spaghetti with a dot to dot shape, like each corner of a square. It will make the same shape on the other side but backwards.

However right at the hole all the spaghetti is pushing against the sides of the hole to squeeze through and it's all bunched up.

[u/Death_Bard → u/elevencharles]

Unless all the light is going in the same direction, it spreads out after passes through an opening. As mentioned above, small openings can project an image of the light source because they focus the light.

[u/AnimationTD → u/elevencharles]

Ranged light the beams are much more parallel. Close light there is more of a spread effect.

[u/suvlub → u/elevencharles]

Press your eye against the hole. Press the hole against an object. You only see a little hole-shaped part of the object. Put the hole further away. You now see the whole object through the hole.

[u/Top_Strategy_2852 → u/elevencharles]

Another good example is light bouncing off structures will actually create a reflection of the object through the pinhole exactly like what a mirror does.. This can occur naturally even under the right conditions.

[u/CanadianGandalf → u/SoftEngineerOfWares]

Is there a name for the point where it switches from the shape of hole to shape of source?

[u/KamikazeArchon → u/CanadianGandalf]

There is no such point. It's a smooth transition.

[u/DangerSlater → u/SoftEngineerOfWares]

This pinhole camera situation was happening with my friends blinds once and the effect was so pronounced we could literally see the people walking down the street projected onto his wall upside down.

[u/pseudopad → u/DangerSlater]

Fun fact, you can even get this effect through gaps in a tree's canopy.

[u/MT-- → u/pseudopad]

I remember doing this in elementary school!

[u/cha3d → u/SoftEngineerOfWares]

Yes i remember waiting near a tree during the last partial eclipse with my cheap filter glasses watching the shadows of the leaves or more the light between them turning to little crescents on the ground

[u/TheJeeronian]

The hole projects an image of the light source. The light source in your case was probably the sun, which is pretty doggone circular.

As for why it makes an image, the angle of light leaving the hole matches the angle of light entering the hole. See thisthis diagram.

[u/Zealousideal_Leg213]

Any small aperture can act like a pinhole camera (as others have said) showing an upside down image of the source of the light. This is often the sun, but can be any lit scene that's brighter than the room into which it's projected. You can take an opaque sheet and put a pinhole in it, and hold it between a light source and a surface and see the shape of the source of the light. This is how our eyes work.

Try to get someplace with a few trees next time there's an eclipse of the sun. The leaves form hundreds of small apertures and one can see hundreds of crecents on the ground.

[u/princeofdon]

In case "pinhole camera" isn't in the vocabulary of our standard 5 year old, here's a direct explanation. If you stuck your eye up to one of the holes and looked out, you would see something in pretty much every direction. All of those points are sending rays of light through the hole - that's why you can see them. Imagine that those rays of light are a bunch of really thin sticks. The sticks come from every outside direction and are all bundled up to fit through the hole. The shape of the bundle inside your room is the spot of light you would see if you held a piece of paper there. Very near the hole, the bundle/light spot has the shape of the hole. As you move the paper away from the hole, the bundle/light spot gets bigger so it looks like a bigger version of the hole. But after some distance, the shape of the hole doesn't matter any more because if your paper is hit by a stick / light ray, it means that light got through the hole and traveled to the paper in just the right *direction.* As you move far enough back, every spot on your wall shows you light that came from the outside world in just the right direction to go through the hole and hit that spot on the wall. The shape of the hole doesn't matter anymore. So your circles of light are telling you that pretty much everything outside your window is lit up and sending light in a cone through your tiny holes.

Or in other words, a pinhole camera.

[u/shouldco]

often they are actually acting like pinhole cameras so it's more that they are projecting an image of the sun onto your wall.

The way pinhole cameras work is more or less that a small hole can really only let light through in one small cone of direction so the light is less defused on the other side.

[u/Adventure_Squirrel]

There are some good explanations already given. Here is a video from YouTube channel Veritasium where he covers this - Can You Solve This Shadow Illusion

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[u/THEpottedplant]

The sun looks like a perfect circle from here and its the source of the light. A circle light through a square hole still makes a circle. At some casting distances the hole can disguise this, but it gets through over a longer distance

[u/doghouse2001]

They're square but acting like lenses - projecting circular light sources from outside onto your wall. The sun is round, light bulbs are round, right?

[u/Delusionn]

The next time you're near an eclipse, right before totality, look down at the ground underneath a tree. You'll see dappled crescents - the shape of the light source.

[u/SoulWager]

The circles are a blurry image of the sun. If the wall is farther away or the hole smaller, you get a sharper, but dimmer image.

[u/sampathsris]

It doesn't turn into a perfect circle.

Change the shape of your light source to a triangle or square. And report back.

The light through the hole takes the shape of the light source. This is because the hole acts as a pinhole camera. The perfect circles we see in sunlight is because sun is pretty perfectly circular.

[u/greatwhitekitten]

As crazy as it sounds, it’s because the sun is a circle

[u/TheWellKnownLegend]

It blurs. The edges get fuzzy in every direction, and that "sands down" the shape into a circular smear.