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…
"""
It also needs a bit of biology. Our eyes don't have a flat response over frequency, they're more sensitive to blue than violet. Violet gets scattered even more than blue, and the violet light does shift our perception of the color. But it does so less than it would if we had photoreceptors more sensitive to violet, so the resulting perceptual color depends not just on the intensity of the light at different frequencies but also on our particular biology. People with tritanopia (blue-yellow color blindness) don't have blue-sensitive cones (S cones) and thus to them there is no perceived blue. Not to mention the linguistic history of the word "blue" and why English uses "blue" instead of "青" or some other word, the questions around qualia & what it means to perceive color, etc.
There are differences in receptor behavior across species, but they are understandably clustered around the parts of the spectrum in which sol is most luminous. An earth-like planet orbiting a different star would likely have evolved photoreceptor arrangements which match that star instead. So after scratching the biology itch we'll probably need to talk about fusion byproducts in sol-like stars.
Some animals have more cone types than humans, especially various birds, so would probably see a violet sky.
We don't have this because common ancestor for all mammals lost all cones but one, perhaps due to being nocturnal, and a second was re-evolved as mammals became more dominant (after dinosaur extension). A third cone was evolved in primates due to a gene duplication that gave us our green cone
We already have mutations, generally in women, for tetrachromaticism, who usually have male relatives with severe or moderate color blindness, in which the X chromosome encodes a different green cone. So they end up seeing red, strange-green, green, and blue, where strange-green is somewhere closer to red than green.
Only a few on record but they tend to have absolutely insane color matching and color perception. One of note worked in the fashion industry and could match fabrics perfectly even in varying lighting (e.g. working under fluorescent but able to match colors that would stay matched in halogen/stage lighting)
I puttered on a color interactive where, to emphasize this distinction between world-spectra vs brain-color, you could swap in color deficiencies, a non-primate mammal ( dichromats), and a monochromat.
this is fascinating because I'm red/green color deficient yet I have no problem seeing most reds or greens. I feel there's a "spectrum" of color that we all see and each of us is slightly different. My shade of green may not be your shade of green. Yet, when I point out my shade of green - it matches your shade of green because of our eyes. Even though we may be perceiving entirely different colors.
Most colorblind people aren't dichromats, they're so-called anomalous trichromats. Basically, the genes coding opsins in your eyes have a number of functional sites that tune the spectral sensitivity. Those sites are tuned as far apart as they can be in color-normal humans. Anomalous trichromats usually have a genetic error that causes their opsins' sensitivity curves to overlap more, which manifests as reduced color sensitivity.
"Could you be more specific" is a great question to find out more what the person knows and how they thing. You give an answer that, just due to the nature of knowledge and the limitation of language, has some black boxes. And "could you be more specific" is basically asking to go through the black boxes.
Its like asking how does Java work or something like that? You can go from "The JVM interprets java byte code" to quite a lot of depth on how various parts work if you have enough knowledge.
Hah - that is exactly what I did. Someone asked me this question and after 5 minutes in the weeds of the debounce on the mouse click they said "look all we wanted was to find out if you'd ever heard of DNS, let's move on, that was great".
A great response is "What exactly do you want to know?", so we don't end up like Cliff giving answer after answer. In his case it was a great test question, but such a vague question is a horrible communication tactic if abused.
I am positively excited about the upcoming first generation of humans who will have all their questions answered, correctly and in the way they can best understand, and as often and many of them as they want – and what that is going to enable.
I childishly looked for a historical quote on how we should all be doing science at home now. Google referred me to a gorgeous article written by Isaac Asimov:
While computers and robots are doing the scut-work of society so that the world, in 2019, will seem more and more to be “running itself,” more and more human beings will find themselves living a life rich in leisure.
This does not mean leisure to do nothing, but leisure to do something one wants to do; to be free to engage in scientific research, in literature and the arts, to pursue out-of-the-way interests and fascinating hobbies of all kinds.
Fortunately our good friends at the Public Gaming Research Institute have republished the article originally published in the Toronto Star where Asimov imagined the world 35+ years in his future.
Unfortunately the link seems to contain some advertisements so perhaps google yourself to find a better source. I looked for a filetype:pdf but that didn't help me (although Gemini AI did helpfully summarise the same article).
We are definitely fortunate to live in a world with free access to information.
Unfortunately my skills at search are getting rusty.
The same anticipation of great things happening preceded the arrival of widely available internet, but all we really got was cat videos initially, and doomscrolling more recently. I don’t have much hope for great things anymore.
I saw a Microsoft talk decades back, that was a dispirited "the people of India could be buying educational materials and... but no, all the money is in ringtones". For some kinds of business perspective, ok I guess. But for others, and for civilizational change, what's going on in the tail can matter a lot. Does China become a US engineering/science peer in early 21st C absent an internet/WWW?
Does anyone have experience of early-childhood "Why?"-phase meets speech-enabled LLMs?
Startup wise, there's old work on conversational agents for toddlers, language acquisition, etc. But pre‑literate developmental pedagogy, patient, adaptive, endlessly repetitive, responsive, fun... seems a potential fit for LLMs, and not much explored? Explain It Like I'm 2-4. Hmm, there's a 3-12 "Curio" Grok plushie.
Me too but I don't think these sorts of Solved Society endgames are likely to show up. Basically presents the same issue with a utopia.
Progression and regression are always going to be at war with each other. There will always be humans that want to hurt instead of help, there will always be humans who TRY to help but ultimately hurt. There will always be misinformation, there will always be lies, and there will always be liars.
The good news is there will also always be people trying to pull humanity forwards, to help other people, to save lives, to eradicate disease, educate, and expose the truth.
I don't think society will ever be solved in the way you're saying because there will always be hurtful people, but there will also always be good people to keep up the fight.
... and due to that, people will not appreciate all the knowledge, we will take it as air - invisible but cut the access in a myriad ways and its a catastrophe.
We value what we achieve with effort, I would say proportionally to energy put in (certainly true for me, thus I like harder efforts in activities and ie sport climbing).
> “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. This is indeed a funny gap in the language.
"Show" work for any sort of visual thing you might want to present to someone. It's a bitransitive verb: it takes both a direct and indirect object in addition to the subject:
"Bill showed Marsha her new car."
^^^^ ^^^^^^ ^^^^^^^^^^^
Subject D.Obj Indirect Obj.
For an auditory thing, our common words seem to subdivide it based on the sound source: "tell" for presenting speech to someone, "play" for presenting something musical:
"Amy told Fred a story."
"Bill played Fred a song."
"Play" has grown to encompass recorded audio, so is probably the closest thing to an auditory equivalent to "show".
There is also "audition" which can be used transitively, but I don't think it works bitransitively. You can say "I auditioned a bunch of saxophone recordings.", but you can't audition something to someone.
Ah! That's not bad but it's not the same thing. Good nevertheless.
I can 'show' (or point someone to a) a sight that I am not myself creating in anyway. The word I am looking for would mean to 'make you hear' in the same may to show is to make you see.
"the bell was rung" illustrates a cause (and introduces a question: who rang the bell?)
"the bell rang" illustrates an effect (the vibration and sound of the bell as it rings).
i think this is more an illustration of the ambiguity of the root word "ring", which can be an action by a subject upon an object, or to describe the behavior of the object itself.
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
Not just butterflies, birds too! But what selection pressure drove the evolution of these structural colors? Presumably signaling, the opposite of muted, camouflaging colors.
Also, as many might know, blue eyes are the result of a lack of pigment (eumelanin). The iris is translucent, but Rayleigh scattering preferentially backscatters blue photons. Green eyes have some pigment, making them a mix of brown and blue.
I wonder if the interference-based-blue of the morpho butterfly evolved because it's difficult to make blue pigment for some reason having to do the chemistry of our biosphere, or if it's an evolutionary response to humans who may have captured the blue ones and ground them up for pigment (much like we did with https://en.wikipedia.org/wiki/Tyrian_purple snails).
I'm not aware of any record of us having done so, but it's absolutely the kind of thing we would do, and there's much more pre-history than history when it might've happened.
Inventions can be useful recombinations or applications of other inventions. They don't need to be wholly unique unto themselves. Indeed, the vast majority of them are not wholly unique.
As far as the patent system is concerned, 20 years and a million years are the same thing. If you combine some million year old things in a new way, you can get a patent.
I went down this rabbit hole a while back — there's a fascinating history of various scientists' investigations into the blue sky, across many decades, with some back-and-forth between Russia and Europe. Einstein eventually made a connection between it and the seemingly-unrelated issue of "critical opalescence," by showing that the fluctuations of densities is responsible for the scattering, not just a simple "individual molecules floating in space" analysis that Rayleigh originally performed. But funnily, for an ideal gas (such as our atmosphere), the formula works out to be the same.
So, "Rayleigh scattering" is the common term still used today, but there is a deeper reason for the formula being correct — it remains correct even when molecules are relatively close together, such as in the lower layers of our atmosphere.
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.
As confusion elsewhere on this page illustrates, one also needs to clarify absorption. "It's just blue" sky and "it's just blue" stained-glass have quite different behavior. Both side scatter some blue, but while one mostly transmits the rest, the other mostly absorbs the rest, for very different experiences peering through it.
Yes, I came here to say this. The whole topic drives me crazy. Air is just blue. Everything is a color because of some physics reason. Some birds have blue wings due to microscopic structures and how light interacts with them, rather than pigment.
If you took a large column of air into space and shined white light through it, it would be blue.
No, it would look red. The weird thing about air is that it's not reflection or absorption that gives the color, but scattering, and that means the color is strongly dependent on what direction you are looking at it from in a way that most transparent mediums aren't.
Ok, so the air would be red from one angle, blue from another. In each case, that is what color the air “really” is, in the same sense that a butterfly’s wings are blue (but not from every angle)
Here's one: why does sunlight appear "sharper" or "harsher" during cold winters?
The reason, I believe, is that cold air tends to be drier, and drier air scatters less, leading to more of the overall flux you receive coming straight from the sun rather than from the diffuse source of the sky around you. But I'm not certain of this.
"To whom have I given blue from my sunbeam?!?" might be another fun question. I explored it as a potential interactive, to see, geographically, where your direct sunlight is donating sky blue-ification. Especially around sunset - IIRC, think a 100 km neon tube, at 7ish km altitude, near-end 150 km up range, with a 15-ish km wide ground footprint with 3/4-ish of the ground-impinging light, and the rest of a 100 km wide path with the 1/4-ish.
> Of course, a reasonable question is why are blue and violet absorbed so strongly by these dust particles?
> Well, those are the only photons with enough energy to bump the dust molecules’s electrons up to a new energy state.
This is also why posters in a window turn blue. The warm-colored organic pigments that produce the yellows and reds do that because they absorb blue light and UV. And that light has the energy to knock apart their bonds and break them down. The dyes that absorb the lower energy waves, passing through blue, last longer resulting in bluing.
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.
The article has a section on "why are clouds white?", but it doesn't really address the reason I thought that question should be covered.
It just says that clouds act like a collection of randomly-oriented prisms, such that whenever light of any wavelength comes into the cloud, it is dispersed from the cloud evenly in all directions.
This would explain why a cloud was white if even white light was coming into the cloud. But the rest of the article establishes that the light coming into the cloud is predominantly blue and purple. Why isn't that also true of the light leaving the cloud?
Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The same like a heap of powdered salt in the palm of your hand, held up to the sun.
Clouds illuminated by the setting sun aren't white.
Your intuition isn’t far off; there is an angle where the weight of green relative to the sum over wavelengths sees a local maximum. But it doesn’t dominate. In that transition zone, there is still an overlapping, transitioning abundance of redder and bluer wavelengths, adding with the green. Consequently, you see red, going into a red+green transition (== oranges, yellows), go into into a green+blue transition (== cyan), which already has few photons relative to the red and yellow zones, so it’s a dark/weak cyan, before it blends into the darker blue of the night sky.
Because the color of the sky is determined by a shifting mixture of wavelengths, not a single shifting wavelength.
Basically, the scattering process that "remove" blue from the spectrum also removes green, albeit to a lesser extent. There are some greenish and yellowish wavelengths in the sunset sky, but they're dominated by red, so the overall color appears red or orange.
In order for the sky to look noticeably green, there would have to be something that scattered reds and blues, without significantly absorbing green.
If you try to interpolate between sky-blue and orange using graphics software, the result depends on what "color space" you're using. If your software interpolates based on hue, you might see green (or purple) in the middle. But that's not physically realistic.
A realistic model is to interpolate each wavelength of the continuous spectrum separately. Interpolating in RGB color space is a crude approximation to this. And if you try the experiment, you'll see that the midpoint between sky-blue and orange is a kind of muddy brown, not green.
You won't get a green sky, but at least there is a meteorological optical phenomenon called the green flash around sunset. To see it, I think, you have to know what you're looking for - and you need good conditions.
That is the natural transition from overhead sun to sunset as each higher energy wavelength gets cut off more and more. When blue is mostly gone and green starts to fade we call it the Golden Hour.
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.
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.
The explanation that made it click for me a while ago was by someone who implemented a shader https://www.alanzucconi.com/2017/10/10/atmospheric-scatterin... — the explanations that don't end up producing an image all seemed to skip over one detail or another.
> the explanations that don't end up producing an image all seemed to skip over one detail or another.
Implementation can be wonderfully useful as both a test of, and a forcing function for, really understanding something. At least when ground-truth (ie tests) is available.
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?
I think at this point you need to consider how the human eye see color. It's not like each wavelength gets picked up and then communicated perfectly.
(I'm going to skip over some basic stuff, and use some generalities)
Each Cone in the eye responds to a range of frequencies. This means that things that unless it's on the extreme low, or high, end of the frequencies that the human eye can discern you are going to have two, or all three, Cone types responding. The strength of those responses is what your brain uses to interpret the color that you see.
The real problem is that out in space there is no attenuation of sunlight, it's bright. Super crazy bright. It basically overloads all of your Cones, and Rods, all at once, there is no way for your brain to find a signal of "oh there's more higher wavelengths here so interpret bluer than normal" because all of the signals got maxed out. If you max out all of the signals, you get white.
It doesn't matter that in absolute terms there's more blue, the lower and mid frequencies are also maxed out.
IIUC, saturation is a (not uncommon) distractor here. As you get the same observation when desaturated by a neutral filter. Even on the "ground" with low air mass (Sun vertical, at altitude, etc).
Mid-day sun in a clear sky is very white, in the 5k-6k color temperature range. It's hard to get a sense of how white it is because of how bright it is. In fact, the color temperature on the surface can be even higher than in outer-space!
Compare this to a "warm" light bulb, which is around 2.5K. Sunrise/sunset is also around that range.
Perhaps the "warm color" sun mindset comes from the only times that people can look directly at it. That is to say, around sunrise or sunset.
Perhaps because one's world is often blue-lit? While whole-hemisphere illumination generalizes as warmer, local conditions vary. Absent direct (yellow-ish) sunlight, outdoor daylight illumination can be quite blue-ish. I've had fun recently with photos in a park under clear blue skies, shadowed by tall buildings... but with a gap, resulting in a narrow strip of bright sunlit ground. My phone will take a bit of sunlit snow as its whitepoint, and provide a blue-tinted world. Similarly for sunlit buildings in background.
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!)
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.
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.
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 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 perfectly 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.
I'm surprised that there were downvotes. This is an excellent answer, and better interfaces between linguistic definitions of color and physicists' than saying "Rayleigh scattering impacts blue more than red"!
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."
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?_
If you say that air isn’t “really” blue because the reason it is blue is different than the reason that other things are blue (e.g. most blue things in daily life absorb frequencies other than blue), this is equivalent to saying that birds with blue feathers due to structural coloration—where instead of pigment, microscopic structures interfere with light—don’t “really” have blue feathers.
This is just silly. Some birds have blue feathers. There are various ways to be blue. Similarly, air is blue.
> To get something purple, you’d need to find a material whose electrons were excited by low-energy red photons, but had no use for higher-energy violet photons.
Nope! Purple is not violet! It a color that the eye perceives when stimulated by both blue and red wavelengths at the same time; there is no wavelength that produces purple by itself.
I think this and other facts about how we see color demonstrates that colors are not out there in the world, but rather a conscious perception of how we see the world, with animals having varying color schemes, some colors which we don't see, depending on their eyes and brains.
Why should only visible EM radiation have colors, but not radio, X-rays, etc?
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.
Air molecules are much smaller than the wavelength of visible light, by several orders of magnitude. This is why you can't resolve individual molecules in an optical microscope, and why photolithography with visible light doesn't go down to molecular feature sizes.
Fs is the frequency at which whatever your measuring is most efficient at vibrating
So it’s a combination of the composition of the thing and the environmental coupling with other vibrating things
Size and material composition are the primary factors
So for this case, the photon spectrum interact with nitrogen-oxygen mixture most efficiently at the frequency that reflects blue
I mostly studied sound frequency mixing with static objects (matching or cancelling the fs of room/space with the fs of a driver) but the principles of resonance hold across media
that's interesting. I thought it was because our sun's spectrum has the most energy in visible light band - therefore we evolved to see the light which can give us the highest SNR.
Both the "because that's what the sun emits" and "because we are mostly water" explanations are incomplete. There are plenty of other animals [1] that can "see" infrared.
The real reason is simply because that's how we evolved. That's how the "because those are the frequencies that pass through water" explanation comes into play: vision first evolved in aquatic animals, so frequencies that don't penetrate water wouldn't have been all that helpful to their survival and reproductive success, and so wouldn't be selected for. But that's incomplete too: salmon are one of the top IR-sensing animals and they live in water, so when there's an evolutionary need to select for IR vision, it happens. The reason we "see" in the visible light range is simply that that's how we've defined "visible".
There are some physics reasons as well, notably that most mammalian body structures emit heat, which would blind an animal that relies on infrared to see (notice how most of the animals that can see infrared are cold-blooded reptiles, fish, and insects), and that most of the high-resolution biochemical mechanisms that can convert electromagnetic waves to electrochemical nerve impulses operate in the visible light range. Structures that convert infrared radiation to nerve impulses are more complex and more costly to support, so unless there's a clear survival benefit for the species, they tend to get selected away.
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.
If something appears blue, it is blue. That’s all color is.
Also, if you took a sufficiently large quantity of air and put it into empty space and shined very bright white through it, it would experience rayleigh scattering—-meaning that air, when you have enough of it and shine a bright enough light through it, is blue.
Rayleigh scattering is elastic (only the direction changes), whereas Raman scattering is inelastic (energy, that is color changes in addition to direction) scattering.
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.
No, your job is to help your reader get to the end of the text. That means writing in a way that most of your audience finds compelling, readable, and not intimidating.
Not all readers are the same, so you will fail at your job for some readers.
But few readers are emotionless automatons that need nothing but dry technical content, unless it’s a topic they are very motivated to understand.
> That means writing in a way that most of your audience finds compelling, readable, and not intimidating.
I would agree with that. And I think emojis and unnecessary reassurances subvert that goal. It's fluff, it's more to read, and if the writing isn't already clear, they don't fix the problem.
> But few readers are emotionless automatons that need nothing but dry technical content
Nothing in my post argues for dry technical content.
Bartosz Ciechanowski's superb work, which may have inspired the author, gets the balance just right without any hand-holding asides:
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.
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.
The "Rayleigh Scattering" comic is really spot on.
Air is blue. The reason air is blue is blah blah blah physics, see the article we're all commenting on, but at the end of the day air is blue. We don't demand the same elaborate physics questions for why a ripe banana peel is yellow.
Though is some cases it is a very interesting question, like why gold and copper the color they are instead of being boring and silvery like all the other metals?
Not really. If the explanation was "air is blue" then the naive expectation would be that sun would appear blue against blackish background, basically the image of sun is being filtered through the atmosphere; if sun is white and air is blue then white filtered through blue should be blue? But sun appears yellowish against blue background. So clearly something different is going on.
But a banana is yellow for a very different reason (and a much easier to explain reason) than why the sky is blue. And air isn’t blue, because it’s red at the end of the day?
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.
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… """
It also needs a bit of biology. Our eyes don't have a flat response over frequency, they're more sensitive to blue than violet. Violet gets scattered even more than blue, and the violet light does shift our perception of the color. But it does so less than it would if we had photoreceptors more sensitive to violet, so the resulting perceptual color depends not just on the intensity of the light at different frequencies but also on our particular biology. People with tritanopia (blue-yellow color blindness) don't have blue-sensitive cones (S cones) and thus to them there is no perceived blue. Not to mention the linguistic history of the word "blue" and why English uses "blue" instead of "青" or some other word, the questions around qualia & what it means to perceive color, etc.
There are differences in receptor behavior across species, but they are understandably clustered around the parts of the spectrum in which sol is most luminous. An earth-like planet orbiting a different star would likely have evolved photoreceptor arrangements which match that star instead. So after scratching the biology itch we'll probably need to talk about fusion byproducts in sol-like stars.
The real question is, is the sky blue for everyone? Some creatures can see ultraviolet. Some lack color at all…
Some animals have more cone types than humans, especially various birds, so would probably see a violet sky.
We don't have this because common ancestor for all mammals lost all cones but one, perhaps due to being nocturnal, and a second was re-evolved as mammals became more dominant (after dinosaur extension). A third cone was evolved in primates due to a gene duplication that gave us our green cone
https://www.sciencedirect.com/science/article/pii/S004269890...
maybe in 100M years we'll get a 4th cone or rod. Probably from nuclear mutation...
We already have mutations, generally in women, for tetrachromaticism, who usually have male relatives with severe or moderate color blindness, in which the X chromosome encodes a different green cone. So they end up seeing red, strange-green, green, and blue, where strange-green is somewhere closer to red than green.
Only a few on record but they tend to have absolutely insane color matching and color perception. One of note worked in the fashion industry and could match fabrics perfectly even in varying lighting (e.g. working under fluorescent but able to match colors that would stay matched in halogen/stage lighting)
There's some evidence that tetrachromacy already exists in a few humans. If so we have the gene already. But why would it spread?
I puttered on a color interactive where, to emphasize this distinction between world-spectra vs brain-color, you could swap in color deficiencies, a non-primate mammal ( dichromats), and a monochromat.
this is fascinating because I'm red/green color deficient yet I have no problem seeing most reds or greens. I feel there's a "spectrum" of color that we all see and each of us is slightly different. My shade of green may not be your shade of green. Yet, when I point out my shade of green - it matches your shade of green because of our eyes. Even though we may be perceiving entirely different colors.
Most colorblind people aren't dichromats, they're so-called anomalous trichromats. Basically, the genes coding opsins in your eyes have a number of functional sites that tune the spectral sensitivity. Those sites are tuned as far apart as they can be in color-normal humans. Anomalous trichromats usually have a genetic error that causes their opsins' sensitivity curves to overlap more, which manifests as reduced color sensitivity.
How would Lieutenant Geordi La Forge from ST Next Generations see the sky with his visor?
"Could you be more specific" is a great question to find out more what the person knows and how they thing. You give an answer that, just due to the nature of knowledge and the limitation of language, has some black boxes. And "could you be more specific" is basically asking to go through the black boxes.
Its like asking how does Java work or something like that? You can go from "The JVM interprets java byte code" to quite a lot of depth on how various parts work if you have enough knowledge.
i used something like this in unstructured technical interviews all the time.
"you type a phrase into google search, you press enter, get some results. tell me, in technical detail, what happened in that chain of actions"
the diversity of replies is fascinating, you learn a lot about a "full stack" candidate this way.
Feynman's classic "Why?" chain: https://www.youtube.com/watch?v=36GT2zI8lVA
I'd probably spend at least 20 minutes just to get through how the keyboard works, much more if it's a USB-HID device.
Hah - that is exactly what I did. Someone asked me this question and after 5 minutes in the weeds of the debounce on the mouse click they said "look all we wanted was to find out if you'd ever heard of DNS, let's move on, that was great".
the good ones would usually follow up with, "how much detail do you _really_ want ;D"
A great response is "What exactly do you want to know?", so we don't end up like Cliff giving answer after answer. In his case it was a great test question, but such a vague question is a horrible communication tactic if abused.
It's reminds me of that scene from Fargo: "He was kinda funny lookin'" ... "Could ya be any more specific?"
I am positively excited about the upcoming first generation of humans who will have all their questions answered, correctly and in the way they can best understand, and as often and many of them as they want – and what that is going to enable.
I childishly looked for a historical quote on how we should all be doing science at home now. Google referred me to a gorgeous article written by Isaac Asimov:
Fortunately our good friends at the Public Gaming Research Institute have republished the article originally published in the Toronto Star where Asimov imagined the world 35+ years in his future.Unfortunately the link seems to contain some advertisements so perhaps google yourself to find a better source. I looked for a filetype:pdf but that didn't help me (although Gemini AI did helpfully summarise the same article).
We are definitely fortunate to live in a world with free access to information.
Unfortunately my skills at search are getting rusty.
The same anticipation of great things happening preceded the arrival of widely available internet, but all we really got was cat videos initially, and doomscrolling more recently. I don’t have much hope for great things anymore.
I saw a Microsoft talk decades back, that was a dispirited "the people of India could be buying educational materials and... but no, all the money is in ringtones". For some kinds of business perspective, ok I guess. But for others, and for civilizational change, what's going on in the tail can matter a lot. Does China become a US engineering/science peer in early 21st C absent an internet/WWW?
We got more than that. We got 24/7 surveillance.
Does anyone have experience of early-childhood "Why?"-phase meets speech-enabled LLMs?
Startup wise, there's old work on conversational agents for toddlers, language acquisition, etc. But pre‑literate developmental pedagogy, patient, adaptive, endlessly repetitive, responsive, fun... seems a potential fit for LLMs, and not much explored? Explain It Like I'm 2-4. Hmm, there's a 3-12 "Curio" Grok plushie.
I presume you're referring to LLMs here, but if so, your presumption that their questions will be answered "correctly" seems a bit optimistic.
Me too but I don't think these sorts of Solved Society endgames are likely to show up. Basically presents the same issue with a utopia.
Progression and regression are always going to be at war with each other. There will always be humans that want to hurt instead of help, there will always be humans who TRY to help but ultimately hurt. There will always be misinformation, there will always be lies, and there will always be liars.
The good news is there will also always be people trying to pull humanity forwards, to help other people, to save lives, to eradicate disease, educate, and expose the truth.
I don't think society will ever be solved in the way you're saying because there will always be hurtful people, but there will also always be good people to keep up the fight.
When is that going to be?
... and due to that, people will not appreciate all the knowledge, we will take it as air - invisible but cut the access in a myriad ways and its a catastrophe.
We value what we achieve with effort, I would say proportionally to energy put in (certainly true for me, thus I like harder efforts in activities and ie sport climbing).
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.
There are many verbs like this, and English is somewhat open toward using verbs that way, or becoming so.
Did English speakers say "this novel reads well" two, three hundred years ago?
TIL!
Debates whether to update the sidenote with an explainer on ambitransitive and labile verbs
I have always wondered about this. The verb for the first person is to 'see'. To a third person you 'show'
For the first person there is 'listen' (or 'hear'). Does English not have a corresponding word for the third person ?
What about Germanaic or Nordic languages ? Do they have a third person analogue of 'listen' ?
Interesting. This is indeed a funny gap in the language.
"Show" work for any sort of visual thing you might want to present to someone. It's a bitransitive verb: it takes both a direct and indirect object in addition to the subject:
For an auditory thing, our common words seem to subdivide it based on the sound source: "tell" for presenting speech to someone, "play" for presenting something musical: "Play" has grown to encompass recorded audio, so is probably the closest thing to an auditory equivalent to "show".There is also "audition" which can be used transitively, but I don't think it works bitransitively. You can say "I auditioned a bunch of saxophone recordings.", but you can't audition something to someone.
AFAIK listen used to be used therefor[sic] but it has fallen out of use nowadays. From wiktionary:
> Listen the watchman’s cry upon the wall.
Edit: formatting
'Hear the watchman’s cry upon the wall' works the same way, no ?
I have clarified what I am looking for in a cousin comment.
"tell"?
Ah! That's not bad but it's not the same thing. Good nevertheless.
I can 'show' (or point someone to a) a sight that I am not myself creating in anyway. The word I am looking for would mean to 'make you hear' in the same may to show is to make you see.
I showed him the distant tower.
I ??? him the faint sound.
You appear to be looking for the word show, which is not specific to visual phenomena.
play?
I played him the faint sound.
Labile verbs is a source of ambiguity of natural languages (only western ones?) that we are all accustomed to.
The bell rang should become The bell was rung, either way it means The bell rang another bell.
"the bell was rung" illustrates a cause (and introduces a question: who rang the bell?)
"the bell rang" illustrates an effect (the vibration and sound of the bell as it rings).
i think this is more an illustration of the ambiguity of the root word "ring", which can be an action by a subject upon an object, or to describe the behavior of the object itself.
Now do clam steamers and shrimp fried rice.
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
Another great example of "structural" blue that can be created artificially by heating steel:
https://www.youtube.com/watch?v=NhjiIPohUyw
Not just butterflies, birds too! But what selection pressure drove the evolution of these structural colors? Presumably signaling, the opposite of muted, camouflaging colors.
Also, as many might know, blue eyes are the result of a lack of pigment (eumelanin). The iris is translucent, but Rayleigh scattering preferentially backscatters blue photons. Green eyes have some pigment, making them a mix of brown and blue.
Also the blood veins that you see as bluish through the skin are blue for the same reason, due to light scattered in their walls.
I thought they are green.
Definitely more blue/purple.
I wonder if the interference-based-blue of the morpho butterfly evolved because it's difficult to make blue pigment for some reason having to do the chemistry of our biosphere, or if it's an evolutionary response to humans who may have captured the blue ones and ground them up for pigment (much like we did with https://en.wikipedia.org/wiki/Tyrian_purple snails).
I'm not aware of any record of us having done so, but it's absolutely the kind of thing we would do, and there's much more pre-history than history when it might've happened.
It's also the trick employed by Iridigm, which Qualcomm acquired in late 2004 (i was there then).
https://en.wikipedia.org/wiki/Interferometric_modulator_disp...
I'm curious how they were able to patent a technique invented by nature millions of years ago.
the displays have an array of switchable mirrors individually addressable, unlike nature in this case.
(but sort of like chromophores in an octopus or cuttlefish, perhaps).
I see, but those MEMs mirrors were already invented.
Inventions can be useful recombinations or applications of other inventions. They don't need to be wholly unique unto themselves. Indeed, the vast majority of them are not wholly unique.
We're talking about a millions year old invention here.
As far as the patent system is concerned, 20 years and a million years are the same thing. If you combine some million year old things in a new way, you can get a patent.
I went down this rabbit hole a while back — there's a fascinating history of various scientists' investigations into the blue sky, across many decades, with some back-and-forth between Russia and Europe. Einstein eventually made a connection between it and the seemingly-unrelated issue of "critical opalescence," by showing that the fluctuations of densities is responsible for the scattering, not just a simple "individual molecules floating in space" analysis that Rayleigh originally performed. But funnily, for an ideal gas (such as our atmosphere), the formula works out to be the same.
So, "Rayleigh scattering" is the common term still used today, but there is a deeper reason for the formula being correct — it remains correct even when molecules are relatively close together, such as in the lower layers of our atmosphere.
I found this nice paper[1] giving an overview of the timeline, various discoveries, etc: http://users.df.uba.ar/bragas/Web%20roberto/Papers/sobelman%...
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.
As confusion elsewhere on this page illustrates, one also needs to clarify absorption. "It's just blue" sky and "it's just blue" stained-glass have quite different behavior. Both side scatter some blue, but while one mostly transmits the rest, the other mostly absorbs the rest, for very different experiences peering through it.
So perhaps "clear with a blue tint"?
Yes, I came here to say this. The whole topic drives me crazy. Air is just blue. Everything is a color because of some physics reason. Some birds have blue wings due to microscopic structures and how light interacts with them, rather than pigment.
If you took a large column of air into space and shined white light through it, it would be blue.
No, it would look red. The weird thing about air is that it's not reflection or absorption that gives the color, but scattering, and that means the color is strongly dependent on what direction you are looking at it from in a way that most transparent mediums aren't.
Ok, so the air would be red from one angle, blue from another. In each case, that is what color the air “really” is, in the same sense that a butterfly’s wings are blue (but not from every angle)
Here's one: why does sunlight appear "sharper" or "harsher" during cold winters?
The reason, I believe, is that cold air tends to be drier, and drier air scatters less, leading to more of the overall flux you receive coming straight from the sun rather than from the diffuse source of the sky around you. But I'm not certain of this.
"To whom have I given blue from my sunbeam?!?" might be another fun question. I explored it as a potential interactive, to see, geographically, where your direct sunlight is donating sky blue-ification. Especially around sunset - IIRC, think a 100 km neon tube, at 7ish km altitude, near-end 150 km up range, with a 15-ish km wide ground footprint with 3/4-ish of the ground-impinging light, and the rest of a 100 km wide path with the 1/4-ish.
> Of course, a reasonable question is why are blue and violet absorbed so strongly by these dust particles?
> Well, those are the only photons with enough energy to bump the dust molecules’s electrons up to a new energy state.
This is also why posters in a window turn blue. The warm-colored organic pigments that produce the yellows and reds do that because they absorb blue light and UV. And that light has the energy to knock apart their bonds and break them down. The dyes that absorb the lower energy waves, passing through blue, last longer resulting in bluing.
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.
The article has a section on "why are clouds white?", but it doesn't really address the reason I thought that question should be covered.
It just says that clouds act like a collection of randomly-oriented prisms, such that whenever light of any wavelength comes into the cloud, it is dispersed from the cloud evenly in all directions.
This would explain why a cloud was white if even white light was coming into the cloud. But the rest of the article establishes that the light coming into the cloud is predominantly blue and purple. Why isn't that also true of the light leaving the cloud?
Well, no; what's coming into the cloud is sunlight, filtered through the atmosphere. It scatters that light and so it has that color. The same like a heap of powdered salt in the palm of your hand, held up to the sun.
Clouds illuminated by the setting sun aren't white.
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?
Your intuition isn’t far off; there is an angle where the weight of green relative to the sum over wavelengths sees a local maximum. But it doesn’t dominate. In that transition zone, there is still an overlapping, transitioning abundance of redder and bluer wavelengths, adding with the green. Consequently, you see red, going into a red+green transition (== oranges, yellows), go into into a green+blue transition (== cyan), which already has few photons relative to the red and yellow zones, so it’s a dark/weak cyan, before it blends into the darker blue of the night sky.
Because the color of the sky is determined by a shifting mixture of wavelengths, not a single shifting wavelength.
Basically, the scattering process that "remove" blue from the spectrum also removes green, albeit to a lesser extent. There are some greenish and yellowish wavelengths in the sunset sky, but they're dominated by red, so the overall color appears red or orange.
In order for the sky to look noticeably green, there would have to be something that scattered reds and blues, without significantly absorbing green.
If you try to interpolate between sky-blue and orange using graphics software, the result depends on what "color space" you're using. If your software interpolates based on hue, you might see green (or purple) in the middle. But that's not physically realistic.
A realistic model is to interpolate each wavelength of the continuous spectrum separately. Interpolating in RGB color space is a crude approximation to this. And if you try the experiment, you'll see that the midpoint between sky-blue and orange is a kind of muddy brown, not green.
You won't get a green sky, but at least there is a meteorological optical phenomenon called the green flash around sunset. To see it, I think, you have to know what you're looking for - and you need good conditions.
It can be - https://en.wikipedia.org/wiki/Green_flash
That's refraction not scattering though.
Blue + Green + Red = White
Green + Red = Yellow
Red = Red
That is the natural transition from overhead sun to sunset as each higher energy wavelength gets cut off more and more. When blue is mostly gone and green starts to fade we call it the Golden Hour.
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.
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.
The explanation that made it click for me a while ago was by someone who implemented a shader https://www.alanzucconi.com/2017/10/10/atmospheric-scatterin... — the explanations that don't end up producing an image all seemed to skip over one detail or another.
> the explanations that don't end up producing an image all seemed to skip over one detail or another.
Implementation can be wonderfully useful as both a test of, and a forcing function for, really understanding something. At least when ground-truth (ie tests) is available.
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?
I think at this point you need to consider how the human eye see color. It's not like each wavelength gets picked up and then communicated perfectly.
(I'm going to skip over some basic stuff, and use some generalities)
Each Cone in the eye responds to a range of frequencies. This means that things that unless it's on the extreme low, or high, end of the frequencies that the human eye can discern you are going to have two, or all three, Cone types responding. The strength of those responses is what your brain uses to interpret the color that you see.
The real problem is that out in space there is no attenuation of sunlight, it's bright. Super crazy bright. It basically overloads all of your Cones, and Rods, all at once, there is no way for your brain to find a signal of "oh there's more higher wavelengths here so interpret bluer than normal" because all of the signals got maxed out. If you max out all of the signals, you get white. It doesn't matter that in absolute terms there's more blue, the lower and mid frequencies are also maxed out.
IIUC, saturation is a (not uncommon) distractor here. As you get the same observation when desaturated by a neutral filter. Even on the "ground" with low air mass (Sun vertical, at altitude, etc).
Is it considered a warmer color on the surface?
Mid-day sun in a clear sky is very white, in the 5k-6k color temperature range. It's hard to get a sense of how white it is because of how bright it is. In fact, the color temperature on the surface can be even higher than in outer-space!
Compare this to a "warm" light bulb, which is around 2.5K. Sunrise/sunset is also around that range.
Perhaps the "warm color" sun mindset comes from the only times that people can look directly at it. That is to say, around sunrise or sunset.
Perhaps because one's world is often blue-lit? While whole-hemisphere illumination generalizes as warmer, local conditions vary. Absent direct (yellow-ish) sunlight, outdoor daylight illumination can be quite blue-ish. I've had fun recently with photos in a park under clear blue skies, shadowed by tall buildings... but with a gap, resulting in a narrow strip of bright sunlit ground. My phone will take a bit of sunlit snow as its whitepoint, and provide a blue-tinted world. Similarly for sunlit buildings in background.
So, does that mean, and bear with me here, that… air is blue?
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!)
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 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.
https://www.youtube.com/watch?v=yV-KiTAAcrY
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!
There will be! Requests welcome!
(I will almost certainly do one on quantum mechanics, but that's such a big explanation that I want to do some simpler ones first)
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 perfectly 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.
I'm surprised that there were downvotes. This is an excellent answer, and better interfaces between linguistic definitions of color and physicists' than saying "Rayleigh scattering impacts blue more than red"!
Anyone else immediately think of this commercial?
https://www.youtube.com/watch?v=PbKsC4GCT5k
*Since blue is the shortest wave length...*
Why is the sky black?
- at night (of course)
- there are ~1 septillion stars that are all shiny
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."
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?_
Yes! This is exactly why I wrote this article :)
Any other questions give you the same disappointment?
How can light "bounce off" something if it doesn't have mass?
If you say that air isn’t “really” blue because the reason it is blue is different than the reason that other things are blue (e.g. most blue things in daily life absorb frequencies other than blue), this is equivalent to saying that birds with blue feathers due to structural coloration—where instead of pigment, microscopic structures interfere with light—don’t “really” have blue feathers.
This is just silly. Some birds have blue feathers. There are various ways to be blue. Similarly, air is blue.
> To get something purple, you’d need to find a material whose electrons were excited by low-energy red photons, but had no use for higher-energy violet photons.
Nope! Purple is not violet! It a color that the eye perceives when stimulated by both blue and red wavelengths at the same time; there is no wavelength that produces purple by itself.
I think this and other facts about how we see color demonstrates that colors are not out there in the world, but rather a conscious perception of how we see the world, with animals having varying color schemes, some colors which we don't see, depending on their eyes and brains.
Why should only visible EM radiation have colors, but not radio, X-rays, etc?
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.
Because there are too many DemocRats in the US, impacting climate, weather, sky.
Brilliant explanation and beautifully presented. I wish I had a technical writer who could write up our business case this well!
I'm the writer of the article, and happy to chat. Email is my username at gmail.
Your blog layout, particularly on desktop, is brilliant.
My day job is UI design, so I especially appreciate this
(Is there something in particular you're referring to? I feel like sticky nav and sidenotes aren't particularly unusual?)
the pupil asked, why is the sky blue? the master answered, because the sun is yellow, and the pupil was enlightened
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?
Air molecules are much smaller than the wavelength of visible light, by several orders of magnitude. This is why you can't resolve individual molecules in an optical microscope, and why photolithography with visible light doesn't go down to molecular feature sizes.
https://www.youtube.com/watch?v=4a0FbQdH3dY&t=2038
Direct link to timestamp 33:56
Fs is the frequency at which whatever your measuring is most efficient at vibrating
So it’s a combination of the composition of the thing and the environmental coupling with other vibrating things
Size and material composition are the primary factors
So for this case, the photon spectrum interact with nitrogen-oxygen mixture most efficiently at the frequency that reflects blue
I mostly studied sound frequency mixing with static objects (matching or cancelling the fs of room/space with the fs of a driver) but the principles of resonance hold across media
Okay, why does visible light have that range of frequencies?
because they are the frequencies that pass though water most readily, and we are made of mostly water
that's interesting. I thought it was because our sun's spectrum has the most energy in visible light band - therefore we evolved to see the light which can give us the highest SNR.
Can you be more specific?
The sun's spectrum doesn't have the most energy in the visible light band, though it's close. Most of the energy is in the infrared band:
https://sunwindsolar.com/blog/solar-radiation-spectrum/?v=0b...
Both the "because that's what the sun emits" and "because we are mostly water" explanations are incomplete. There are plenty of other animals [1] that can "see" infrared.
The real reason is simply because that's how we evolved. That's how the "because those are the frequencies that pass through water" explanation comes into play: vision first evolved in aquatic animals, so frequencies that don't penetrate water wouldn't have been all that helpful to their survival and reproductive success, and so wouldn't be selected for. But that's incomplete too: salmon are one of the top IR-sensing animals and they live in water, so when there's an evolutionary need to select for IR vision, it happens. The reason we "see" in the visible light range is simply that that's how we've defined "visible".
There are some physics reasons as well, notably that most mammalian body structures emit heat, which would blind an animal that relies on infrared to see (notice how most of the animals that can see infrared are cold-blooded reptiles, fish, and insects), and that most of the high-resolution biochemical mechanisms that can convert electromagnetic waves to electrochemical nerve impulses operate in the visible light range. Structures that convert infrared radiation to nerve impulses are more complex and more costly to support, so unless there's a clear survival benefit for the species, they tend to get selected away.
[1] https://a-z-animals.com/animals/lists/animals-that-can-see-i...
So are there animals that see a violet sky?
If the sun is emitting pure white light, why is it yellow?
How does a 3D raytracing engine accomplish this?
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?
It could also be this sorta thought:
"There's air in my room, it appears transparent. The sky is made of air, it appears blue. Why the difference"
The same reason it's polarized.
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.
If something appears blue, it is blue. That’s all color is.
Also, if you took a sufficiently large quantity of air and put it into empty space and shined very bright white through it, it would experience rayleigh scattering—-meaning that air, when you have enough of it and shine a bright enough light through it, is blue.
Perhaps "transparent with a blue tint"?
didn’t cv raman prove just that via his raman-effect for which he got the noble prize ?
Kind of, but not really.
Rayleigh scattering is elastic (only the direction changes), whereas Raman scattering is inelastic (energy, that is color changes in addition to direction) scattering.
Where I live, the sky is grey much of the time... Most of last week anyway!
This post is so good! You are a hero.
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.
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.
No, your job is to help your reader get to the end of the text. That means writing in a way that most of your audience finds compelling, readable, and not intimidating.
Not all readers are the same, so you will fail at your job for some readers.
But few readers are emotionless automatons that need nothing but dry technical content, unless it’s a topic they are very motivated to understand.
> That means writing in a way that most of your audience finds compelling, readable, and not intimidating.
I would agree with that. And I think emojis and unnecessary reassurances subvert that goal. It's fluff, it's more to read, and if the writing isn't already clear, they don't fix the problem.
> But few readers are emotionless automatons that need nothing but dry technical content
Nothing in my post argues for dry technical content.
Bartosz Ciechanowski's superb work, which may have inspired the author, gets the balance just right without any hand-holding asides:
https://ciechanow.ski/mechanical-watch/
Brilliant, thank you
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.
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.
Go watch a video of nitrogen gas evaporating from liquid nitrogen ... tell me what colour you see?
Oxygen is blue actually. That only contributes to the sky blueness a little tho.
Obligatory xkcd: "Sky Color" https://m.xkcd.com/1145/
Obligatory xkcd[2]: "Rayleigh Scattering" https://m.xkcd.com/1818/
Others?
The "Rayleigh Scattering" comic is really spot on.
Air is blue. The reason air is blue is blah blah blah physics, see the article we're all commenting on, but at the end of the day air is blue. We don't demand the same elaborate physics questions for why a ripe banana peel is yellow.
Though is some cases it is a very interesting question, like why gold and copper the color they are instead of being boring and silvery like all the other metals?
Not really. If the explanation was "air is blue" then the naive expectation would be that sun would appear blue against blackish background, basically the image of sun is being filtered through the atmosphere; if sun is white and air is blue then white filtered through blue should be blue? But sun appears yellowish against blue background. So clearly something different is going on.
But a banana is yellow for a very different reason (and a much easier to explain reason) than why the sky is blue. And air isn’t blue, because it’s red at the end of the day?
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.
Our lord Zeus always thinks of everything
His noodly appendage touches all.
> thought it was a pretty color
So was blue intrinsically pretty and thus made into the sky, or considered pretty and thus imprinted in the minds of humans that way?