In The Cuckoo's Egg Cliff Stoll recounts an episode from the oral defense of his astrophysics PhD thesis. A bunch of people ask questions but one prof holds back until...

""" “I’ve got just one question, Cliff,” he says, carving his way through the Eberhard-Faber. “Why is the sky blue?”

My mind is absolutely, profoundly blank. I have no idea. I look out the window at the sky with the primitive, uncomprehending wonder of a Neanderthal contemplating fire. I force myself to say something—anything. “Scattered light,” I reply. “Uh, yeah, scattered sunlight.”

“Could you be more specific?”

Well, words came from somewhere, out of some deep instinct of self-preservation. I babbled about the spectrum of sunlight, the upper atmosphere, and how light interacts with molecules of air.

“Could you be more specific?”

I’m describing how air molecules have dipole moments, the wave-particle duality of light, scribbling equations on the blackboard, and . . .

“Could you be more specific?”

An hour later, I’m sweating hard. His simple question—a five-year-old’s question—has drawn together oscillator theory, electricity and magnetism, thermodynamics, even quantum mechanics. Even in my miserable writhing, I admired the guy… """

Really cool article! Tangential:

> “Scattering” is the scientific term of art for molecules deflecting photons. Linguistically, it’s used somewhat inconsistently. You’ll hear both “blue light scatters more” (the subject is the light) and “atmospheric molecules scatter blue light more” (the subject is the molecule). In any case, they means the same thing

There's nothing ambiguous or inconsistent about this. In English a verb is transitive if it takes one or more objects in addition to the subject. In "Anna carries a book", "carries" is transitive. A verb is intransivite if it takes no object as with "jumps" in "The frog jumps.".

Many verbs in English are "ambitransitive" where they can either take an object or not, and the meaning often shifts depending on how it's used. There is a whole category of verbs called "labile verbs" where the subject of the intransitive form becomes the object of the transitive form:

* Intransitive: The bell rang.

* Transitive: John rang the bell.

"Scatter" is simply a labile verb:

* Intransitive: Blue light scatters.

* Transitive: Atmospheric molecules scatter blue light more.

Interesting here is: Actually, for most blue butterflies, it’s not even a pigment-it’s just a trick of the light. Since blue is so rare in the biological world (hardly any plants or animals can produce real blue chemicals), they evolved structural colors. Their wings have these microscopic ridges that reflect blue light while canceling out other colors.

It’s basically the same reason the sky looks blue, just built into a wing. If you were to look at the wings from a different angle or get them wet, the blue often disappears because you're messing with that physical structure

I think we can simplify the answer to this question for most audience and say "the air is blue".

If they say, the air appears to be clear when I stare at something other than sky, the answer is you need more of air to be able to see its blue-ness, in much the same way that a small amount of murky water in your palm appears clear, but a lot of it does not.

If they ask, why don't I see that blue-ness at dawn or dusk, the answer is that the light source is at a different angle. The color of most objects changes when the light source is at a flat angle. And sun lights hits at a flat angle at dawn and dusk.

If they ask, what exactly is the inside phenomenon to see the sky color to be blue, then explanations like this blog are relevant.

If they ask, what exactly is a color, the answer is that it is a fiction made up by our brain.

This was both very informative, easy to understand, and fun to read! That's a winning combo. I now know a bit more about why the sky is the color it is.

Thank you for making it. :)

(The blog post, that is, not the sky. If you made the sky - please let me know!)

It's also not just why the setting or rising sun is red, but why it's yellow when high in the sky. The sun doesn't look yellow when viewed from outside the atmospheric veil.

Good explanation of Rayleigh scattering, but I find many summaries miss that the scattering cross-section goes as wavelength, which is why blue light is so much more affected than red.

This level if geekiness is amazing. I hope more, a lot more, Americans can get into STEMS with this level of passion. It's sad that in the past few decades more and more people seemed to forget that STEM is a pillar of the modern civilization that we enjoy.

For the sunset example then, a natural question (for me) is then why isn't the sky green in the transition from blue sky to red sunset sky?

Why is the sky black?

- at night (of course)

- there are ~1 septillion stars that are all shiny

Implementing an atmospheric shader in three.js is a fun way get an understanding of the interaction of the different scattering components, light, and observer’s position. Plus you get a pretty cool effect to play around with after you’re done.

I have a related but deeper question about sun and colors:

Sunlight in space is considered white. When it reaches earth surface, it's considered a warmer color. Why human eyes that never (during evolution) saw sunlight without the atmosphere, consider it true white, and not colder color?

the pupil asked, why is the sky blue? the master answered, because the sun is yellow, and the pupil was enlightened

Where I live, the sky is grey much of the time... Most of last week anyway!

This was great as it went farther than Rayleigh scattering. On this topic you have to watch this fantastic undergrad physics lecture demonstration: https://www.youtube.com/watch?v=sJG-rXBbmCc&t=1674s

In terms of "qualia", its the other way round probably? Like the way we see colours would have evolved (within the available environment of wavelengths and scatterings and the possibilities with rods and cones) so that the things we want to see are more likely to stand out. So we see the sky as blue because leaves are green and berries are red.

So, does that mean, and bear with me here, that… air is blue?

This post is so good! You are a hero.

I dislike with passion the answer "because Rayleigh scattering". When someone asks why, especially if a child asks, the default answer should be the simplest correct answer:

Because it's the color of the atmosphere, specifically nitrogen and oxygen! It's technically correct to state this.

Gasp! But aren't nitrogen and oxygen usually described as "colorless"? Well, yes but... If they were truly colorless, the sky would be black. It's technically more correct to describe them as nearly colorless and very slightly blue. Very slightly because you need to see through kilometers of atmosphere to perceive the blue. It doesn't matter if the color is caused by absorption, or reflection, or (Rayleigh) scattering of certain wavelengths. The "color" of an object is simply the color you perceive with your eyes. If you perceive blue, it's technically correct to say its color is blue.

It's like saying plants are green because green is the color of chlorophyll. And in the case of chlorophyll, the color is caused by absorption not by scattering. But the physics is irrelevant. Green is its color.

Q: But sunsets/sunrises are red & orange not blue! A: the simplest answer is: color of an object can change under different light conditions. Specifically in this example, when seeing the sun through not kilometers but hundred of kilometers of atmosphere, all the blue-ish wavelengths have been scattered in random directions so only the red-ish wavelengths remain, thus the atmosphere is illuminated by progressively redder and redder light as the photons travel longer and longer distances through the atmosphere.

Great article! I have to admit I had also heard of "Rayleigh scattering", but didn't really know more than that, until today.

Actually, I liked it so much that I went to the homepage of the blog, only to find out that this is the only article. Oh well... I hope there will be more to come!

I always loved this question when I played the 'Why' game with my kids: They ask why, and I'd ELI5. Then they'd ask why, and the process continued until I could excitedly say "We don't know for sure!! We think it might be XYZ, but we're still exploring that frontier."

So are there animals that see a violet sky?

Anyone else immediately think of this commercial?

https://www.youtube.com/watch?v=PbKsC4GCT5k

*Since blue is the shortest wave length...*

Back in my youth, after the Internet became common but before Wikipedia, I tried to discover the answer to this and came away disappointed again and again. Every article I could find simply stated "because light scattering", and barely much more.

How does scattering work? Why does light scatter? _What does scattering even mean in the context of light?_

Okay, why does visible light have that range of frequencies?

Funniest memory re: Rayleigh scattering: in anime show Aldnoah Zero, the uber-genius protagonist mansplains about it to a high profile girl, basically completely out of blue. An impostor of the girl later appears on an in-universe pirate broadcast, making an agitating environmentalism talking point using a technically incorrect explanation of the phenomenon that isn't consistent with the fact. The ever-right protagonist immediately notices it, having enlightened the girl previously on that exact topic, and it leads to actions.

Like, dude, as if anyone would care about such a highly technical point, like eg some React framework quirk or race condition mitigation for specific generation of Intel procesdor or a semi-well known edge cases with btrfs inode behavior, even if I had been on that exact camp.

Brilliant explanation and beautifully presented. I wish I had a technical writer who could write up our business case this well!

Here is a wonderful lecture with real-world demonstrations of the effect:

https://www.youtube.com/watch?v=4a0FbQdH3dY

https://en.wikipedia.org/wiki/Rayleigh_scattering

I do have a question though.

The article says:

> blue and violet have the closest frequencies to a “resonant frequency” of nitrogen and oxygen molecules’s electron clouds

I thought it was more to do with the photon frequency matching the physical size of the air molecules? Or is that the same as its resonant frequency?

This is a really great piece, the bit at the end showing why IR works in smokey environments and guessing the planet's composition based on color was really good.

The same reason it's polarized.

didn’t cv raman prove just that via his raman-effect for which he got the noble prize ?

Not discussed but should be:

Prior to the great oxygenation event, Earth's sky was not blue; it was likely red-orange, carbon dioxide and methane being primary components.

Brilliant, thank you

Going to be that guy, even though I think this is a really nice work overall...

But the winking and "cool guy" emojis are so grating. In general, technical explanations that apologize for themselves with constant reassurances like "don't worry" and "it's actually simple" undermine their own aim.

Your job -- if you're making content for people with double digit ages -- is to make the explanation as clear as you can, not to patronize and emotionally hand-hold the reader.

If you think about it "because air is blue when you look at it from the side" is about all the explanation we'd require if the sky was some normal object like an apple. Nobody asks "why is wood brown?" as if it's some deep question, but "why is the sky blue?" is somehow given greater gravitas, as if the reason is more mystical. I guess because the sky is so big and uniform?

The sky isn’t blue. It’s transparent. That’s why you can see stars that aren’t blue at night. When struck by sunlight at the right angles it appears blue, but saying it is blue is like saying the ocean is green when a bucket of it clearly isn’t.

Some of the demonstrations are not working correctly, at least on my machine (Windows + MS Edge). Any demo with a "reference image" is not correctly updating the reference.

Nitrogen.

Very well explained. I love the in-depthness of the article.

Air is mostly nitrogen. Nitrogen gas is blue.

There.

Obligatory xkcd: "Sky Color" https://m.xkcd.com/1145/

Obligatory xkcd[2]: "Rayleigh Scattering" https://m.xkcd.com/1818/

Others?

to piss me off, goddammit

It’s not. It’s raining here.

Let's be real. The sky is blue because God thought it was a pretty color, simple as. All this stuff about wavelengths and resonant frequencies and human color perception got retconned into the physics engine at some point in the past millennium, that's why all these epicycles are needed.