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.
"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.
A great story, though it seems a little odd to me since Rayleigh scattering was covered in my undergrad exoplanets course. I'd expect an astrophysics PhD to have a better first answer than "scattered sunlight".
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.
> “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
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.
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.
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
> 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?
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.
One technical point about resonance: the fourth-power law only applies when the driving frequency (i.e., the frequency of visible light) is far below the resonant frequency (which is in the UV region of the spectrum). The approximation breaks down the closer you get to the resonant frequency and also above.
I was confused about how a monotonic function (f^4) could possibly describe a resonance phenomenon (which ought to have a strong local extremum), and this is the answer.
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 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?
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.
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.
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.
In space, there is only direct sunlight. On Earth, you have to consider illumination from the whole sky.
On Earth, the sun's disc at noon appears yellow-ish due to being white minus some blue light scattered away by the atmosphere. But there is also the rest of the sky illuminating the ground, as you can still easily see in shadowed areas. That is blue light from the sky hemisphere.
It very approximately adds back up to white overall for something exposed to direct sun and the whole sky, depending on the sun's position in the sky.
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 interesting part is not that much Rayleigh scattering - it is obviously very important, but the better question was on WHAT THE HELL light is scattered. Rayleigh had some ideas, like some "dust" that is in the atmosphere, etc.
One day, long time ago, a now forgotten Polish physicist Marian Smoluchowski [1], was on the walk in the Alps Mountains (except being physicist he was a professional mountain climber/hiker), he looked at the sky and said - no freaking way, there is no dust over there.
So he started thinking and thinking (and Albert Einstein said, after premature death of Smoluchowski, that he was the best thinker he knew) and this moved him into consideration of fairly fresh and new branch of physics at that time - quantum mechanics.
As it turned out, that was it. He figured out that light is scattered on quantum fluctuation of the air density, what explained fully blue color of the sky as well as gave a birth to critical opalescence theory.
[1] https://en.wikipedia.org/wiki/Marian_Smoluchowski - if Smoluchowski hadn't died early, in 1917 from dysentery, he would have been better known, as he would have been awarded three Nobel Prizes (in physics and chemistry), as researches he did were awarded (other guys get the prize).
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?
Although dogs can see violet better than humans, the sky appears blue to them like it does to us, not violet (despite violet being closer to the resonance frequency of N and O atoms).
That's because dogs only have two types of cones (color receptors) in their eyes, blue and yellow, not three as we do, so they see violet as blue.
Therefore, the sky is blue to us and to dogs, but for different reasons!
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.
The mental model that helps visualize why the sky is blue—blue photons flying in every direction—is also the mental model that helps understand the greenhouse effect.
Visible light passes through the atmosphere, gets absorbed by the ground, and then re-radiated at infrared (IR) wavelengths. Atmospheric gases like water vapor, methane, and CO2 cause a certain amount of that IR light to shoot in every direction. So a portion of inbound solar energy gets trapped bouncing around inside the atmosphere for a while before it finally makes it out.
Basically the greenhouse effect is the sky glowing in infrared in addition to blue. We can’t see infrared, but we can feel it with our skin. It feels like warmth.
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!
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.
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.
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.
At least my mental model of color before I learned the details in physics and chemistry was that of a property of surfaces, so your explanation wouldn't have served me. I would have expected, that the light becomes blue as part of the transition from "empty space" into "air", not as part of the "flight" through the air.
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"!
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.
"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.
I noticed once I took a ferry ride at night in a big city and one of the buildings had a company name written on it with bright blue lights and I couldn't read it; too blurry. I couldn't focus my eyes... And I have perfect vision. I had no issues with other colors. Not sure if it's something to do with my eyes specifically or human eyes in general or because of this effect associated with the blue color.
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.
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?_
At least there is an answer, a long answer, but we can explain it. Try answering to corollary question: why is the night sky black? We dont have any good answers to that one.
The big bang ... expansion ... something something ... dark energy....
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!)
Thoroughly enjoyed reading this too! Kudos and shout out to the author Erik Kennedy. One can definitely see the care and love. We hope to see more of this style the future
Kind of reminds me of worrydream (Brett Victor), better explained and neil.fun vibes (and many others I can’t recall right now)
The slider in the first demo under "What's so special about blue" is broken; the colors are correct when the slider is all the way to the left or right, but not when it's in the middle.
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.
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.
If the universe was infinite and eternal you’d expect the night sky to be white - all the gaps between stars would be filled in with stars further away.
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.
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."
Rayleigh scattering is elastic (only the direction changes), whereas Raman scattering is inelastic (energy, that is color changes in addition to direction) scattering.
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.
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.
> 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?
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.
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.
staplung|21 days ago
""" “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… """
SAI_Peregrinus|21 days ago
ecshafer|21 days ago
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.
Ferret7446|18 days ago
jstummbillig|21 days ago
SwtCyber|20 days ago
hackeraccount|20 days ago
munificent|21 days ago
> “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.
kazinator|21 days ago
Did English speakers say "this novel reads well" two, three hundred years ago?
erikdkennedy|21 days ago
Debates whether to update the sidenote with an explainer on ambitransitive and labile verbs
srean|21 days ago
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' ?
orlp|20 days ago
If something "carries" or is "a carry", it means it is so strong it metaphorically carries the rest of the setup with it. For example:
> This card carries.
> These two are the carries of the team.
tsoukase|21 days ago
The bell rang should become The bell was rung, either way it means The bell rang another bell.
suzzer99|21 days ago
KellyCriterion|21 days ago
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
Sharlin|21 days ago
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.
wasmperson|20 days ago
https://www.youtube.com/watch?v=NhjiIPohUyw
jjtheblunt|21 days ago
https://en.wikipedia.org/wiki/Interferometric_modulator_disp...
__MatrixMan__|20 days ago
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.
thoughtlede|20 days ago
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.
mncharity|20 days ago
So perhaps "clear with a blue tint"?
meindnoch|20 days ago
SwtCyber|20 days ago
jxdxbx|20 days ago
If you took a large column of air into space and shined white light through it, it would be blue.
dmonitor|20 days ago
mzhaase|20 days ago
ranger_danger|21 days ago
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?
pfdietz|21 days ago
AndrewKemendo|21 days ago
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
renewiltord|21 days ago
Direct link to timestamp 33:56
kazinator|20 days ago
> 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.
kazinator|21 days ago
thaumasiotes|20 days ago
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?
SwtCyber|20 days ago
justin_dash|21 days ago
photonic37|21 days ago
teraflop|21 days ago
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.
adornKey|21 days ago
michael1999|21 days ago
SwtCyber|20 days ago
seanalltogether|21 days ago
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.
articulatepang|20 days ago
I was confused about how a monotonic function (f^4) could possibly describe a resonance phenomenon (which ought to have a strong local extremum), and this is the answer.
interroboink|20 days ago
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%...
oxag3n|21 days ago
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?
16bytes|21 days ago
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.
zehaeva|21 days ago
(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.
mncharity|20 days ago
hakken306|20 days ago
On Earth, the sun's disc at noon appears yellow-ish due to being white minus some blue light scattered away by the atmosphere. But there is also the rest of the sky illuminating the ground, as you can still easily see in shadowed areas. That is blue light from the sky hemisphere.
It very approximately adds back up to white overall for something exposed to direct sun and the whole sky, depending on the sun's position in the sky.
awesomelybad|21 days ago
asqueella|21 days ago
piokoch|20 days ago
One day, long time ago, a now forgotten Polish physicist Marian Smoluchowski [1], was on the walk in the Alps Mountains (except being physicist he was a professional mountain climber/hiker), he looked at the sky and said - no freaking way, there is no dust over there.
So he started thinking and thinking (and Albert Einstein said, after premature death of Smoluchowski, that he was the best thinker he knew) and this moved him into consideration of fairly fresh and new branch of physics at that time - quantum mechanics.
As it turned out, that was it. He figured out that light is scattered on quantum fluctuation of the air density, what explained fully blue color of the sky as well as gave a birth to critical opalescence theory.
[1] https://en.wikipedia.org/wiki/Marian_Smoluchowski - if Smoluchowski hadn't died early, in 1917 from dysentery, he would have been better known, as he would have been awarded three Nobel Prizes (in physics and chemistry), as researches he did were awarded (other guys get the prize).
IshKebab|21 days ago
erikdkennedy|21 days ago
"There's air in my room, it appears transparent. The sky is made of air, it appears blue. Why the difference"
jll29|20 days ago
That's because dogs only have two types of cones (color receptors) in their eyes, blue and yellow, not three as we do, so they see violet as blue.
Therefore, the sky is blue to us and to dogs, but for different reasons!
sixo|20 days ago
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.
snowwrestler|19 days ago
Visible light passes through the atmosphere, gets absorbed by the ground, and then re-radiated at infrared (IR) wavelengths. Atmospheric gases like water vapor, methane, and CO2 cause a certain amount of that IR light to shoot in every direction. So a portion of inbound solar energy gets trapped bouncing around inside the atmosphere for a while before it finally makes it out.
Basically the greenhouse effect is the sky glowing in infrared in addition to blue. We can’t see infrared, but we can feel it with our skin. It feels like warmth.
rob74|21 days ago
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!
erikdkennedy|21 days ago
(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)
hintymad|21 days ago
yawpitch|21 days ago
jxdxbx|20 days ago
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.
mncharity|20 days ago
mrb|20 days ago
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.
1718627440|20 days ago
mercutio2|20 days ago
b_brief|20 days ago
mncharity|20 days ago
jongjong|20 days ago
9dev|21 days ago
callmeal|20 days ago
aaroninsf|21 days ago
Prior to the great oxygenation event, Earth's sky was not blue; it was likely red-orange, carbon dioxide and methane being primary components.
mr_toad|20 days ago
actinium226|20 days ago
codeulike|21 days ago
jonahx|21 days ago
Jun8|21 days ago
retroflexzy|21 days ago
How does scattering work? Why does light scatter? _What does scattering even mean in the context of light?_
erikdkennedy|21 days ago
Any other questions give you the same disappointment?
sandworm101|20 days ago
The big bang ... expansion ... something something ... dark energy....
Night_Thastus|20 days ago
Thank you for making it. :)
(The blog post, that is, not the sky. If you made the sky - please let me know!)
wamatt|20 days ago
Kind of reminds me of worrydream (Brett Victor), better explained and neil.fun vibes (and many others I can’t recall right now)
thewakalix|20 days ago
createaccount99|17 days ago
archildress|21 days ago
https://www.youtube.com/watch?v=PbKsC4GCT5k
*Since blue is the shortest wave length...*
mvdtnz|21 days ago
unknown|21 days ago
[deleted]
tehjoker|20 days ago
deafpolygon|21 days ago
SwtCyber|20 days ago
librasteve|20 days ago
- at night (of course)
- there are ~1 septillion stars that are all shiny
mr_toad|20 days ago
Brajeshwar|20 days ago
trekz|19 days ago
raincole|20 days ago
greenwalls|20 days ago
numpad0|21 days ago
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.
ruicraveiro|20 days ago
trekz|19 days ago
unknown|21 days ago
[deleted]
TuringNYC|21 days ago
erikdkennedy|21 days ago
absynth|20 days ago
rayleigh scattering.
what's that?
light from sun bouncing around.
why?
because it does.
why is it blue though?
green was taken.
why?
do you want chocolate or raspberry icecream?
chocolate!
yay!
(No, this isn't the best way. But it was A way and that was enough. The kid became a physics lecturer.)
thot_experiment|20 days ago
reddalo|20 days ago
trekz|19 days ago
Darkphibre|21 days ago
nephihaha|20 days ago
unknown|20 days ago
[deleted]
zkmon|21 days ago
mr_toad|20 days ago
smegger001|21 days ago
brcmthrowaway|20 days ago
brcmthrowaway|20 days ago
manojlds|20 days ago
James_K|20 days ago
alexander2002|20 days ago
oxag3n|21 days ago
signa11|21 days ago
jabl|20 days ago
Rayleigh scattering is elastic (only the direction changes), whereas Raman scattering is inelastic (energy, that is color changes in addition to direction) scattering.
beardyw|20 days ago
jxdxbx|20 days ago
This is just silly. Some birds have blue feathers. There are various ways to be blue. Similarly, air is blue.
mr_toad|20 days ago
jonahx|21 days ago
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.
jph00|21 days ago
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.
jimnotgym|20 days ago
Sky is grey
dominictorresmo|20 days ago
kazinator|20 days ago
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.
goatlover|20 days ago
Why should only visible EM radiation have colors, but not radio, X-rays, etc?
jama211|21 days ago
RupertSalt|21 days ago
Obligatory xkcd[2]: "Rayleigh Scattering" https://m.xkcd.com/1818/
Others?
extraduder_ire|20 days ago
The liquid air article on wikipedia doesn't have any photos.
https://en.wikipedia.org/wiki/Liquid_oxygen
margalabargala|21 days ago
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.
halis|21 days ago
alejohausner|21 days ago
There.
pbhjpbhj|21 days ago
mjanx123|21 days ago
paraiuspau|20 days ago
fatih-erikli-cg|20 days ago
[deleted]
minneapoliced|20 days ago
[deleted]
FrogWizardMan|21 days ago
dave_sid|21 days ago
dave_sid|21 days ago
[deleted]
nemo1618|21 days ago
IceCoffe|21 days ago
adolph|21 days ago
So was blue intrinsically pretty and thus made into the sky, or considered pretty and thus imprinted in the minds of humans that way?