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Why is the sky blue?
- deleted 7mo ago[deleted]
- ranger_danger 7mo agoHere is a wonderful lecture with real-world demonstrations of the effect: https://www.youtube.com/watch?v=4a0FbQdH3dY https://www.youtube.com/watch?v=4a0FbQdH3dY https://en.wikipedia.org/wiki/Rayleigh_scattering 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?
- AndrewKemendo 7mo agoFs 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
- pfdietz 7mo agoAir 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.
- renewiltord 7mo agohttps://www.youtube.com/watch?v=4a0FbQdH3dY&t=2038 https://www.youtube.com/watch?v=4a0FbQdH3dY&t=2038 Direct link to timestamp 33:56
- dave_sid 7mo agoIt’s not. It’s raining here.
- dave_sid 7mo ago[flagged]
- Sharlin 7mo agoHN is in fact quite receptive to humorous comments. The bar on what's considered humorous is just higher than on Reddit. It's about the signal/noise ratio.
- dave_sid 7mo agoI’ll keep a close eye out for it.
- nh23423fefe 7mo agocomplaining in a self reply makes me downvote more
- dave_sid 7mo agoYou’re hard. Seriously hard.
- KellyCriterion 7mo agoInteresting 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
- Sharlin 7mo agoNot 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.
- adrian_b 7mo agoAlso the blood veins that you see as bluish through the skin are blue for the same reason, due to light scattered in their walls.
- varispeed 7mo agoI thought they are green.
- SAI_Peregrinus 7mo agoDefinitely more blue/purple.
- jjtheblunt 7mo agoIt's also the trick employed by Iridigm, which Qualcomm acquired in late 2004 (i was there then). https://en.wikipedia.org/wiki/Interferometric_modulator_display https://en.wikipedia.org/wiki/Interferometric_modulator_disp...
- RupertSalt 7mo agoObligatory xkcd: "Sky Color" https://m.xkcd.com/1145/ https://m.xkcd.com/1145/ Obligatory xkcd[2]: "Rayleigh Scattering" https://m.xkcd.com/1818/ https://m.xkcd.com/1818/ Others?
- margalabargala 7mo agoThe "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.
- zokier 7mo agoNot 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.
- lovecg 7mo agoThough 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?
- extraduder_ire 7mo agoThe transition metals are colourful because their outer shell of electrons (d-block) are about visible-light sized. Looking at a periodic table makes this pattern kinda clear.
- gh5000 7mo agoBut 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?
- justin_dash 7mo agoFor 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?
- teraflop 7mo agoBecause 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.
- photonic37 7mo agoYour 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.
- adornKey 7mo agoYou 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.
- 7mo ago
- jonahx 7mo agoGoing 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.
- jph00 7mo agoNo, 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.
- jonahx 7mo ago> 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/ https://ciechanow.ski/mechanical-watch/
- nemo1618 7mo agoLet'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 7mo agoIn 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… """
- ecshafer 7mo ago"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.
- leeoniya 7mo agoi 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 https://www.youtube.com/watch?v=36GT2zI8lVA
- archildress 7mo agoAnyone else immediately think of this commercial? https://www.youtube.com/watch?v=PbKsC4GCT5k https://www.youtube.com/watch?v=PbKsC4GCT5k *Since blue is the shortest wave length...*
- retroflexzy 7mo agoBack 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?_
- erikdkennedy 7mo agoYes! This is exactly why I wrote this article :) Any other questions give you the same disappointment?
- beders 7mo agoHow can light "bounce off" something if it doesn't have mass?
- tim333 7mo agoThe rest mass is zero but photons have momentum when moving.
- 1718627440 7mo agohttps://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi... The light "beam" we perceive is the result of infinite circular waves. The points were the light is not are points where they cancel each other out. We had that as part of the school curriculum, do you not have that, or did you forget?
- alejohausner 7mo agoAir is mostly nitrogen. Nitrogen gas is blue. There.
- TuringNYC 7mo agoBrilliant explanation and beautifully presented. I wish I had a technical writer who could write up our business case this well!
- erikdkennedy 7mo agoI'm the writer of the article, and happy to chat. Email is my username at gmail.
- liquidise 7mo agoYour blog layout, particularly on desktop, is brilliant.
- erikdkennedy 7mo agoMy 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?)
- PetitPrince 7mo ago> I feel like sticky nav and sidenotes aren't particularly unusual? Not unusual, but you used them with taste and restraint, like the rest of your layout and animations. That's something that HN comments like, I think. Notice the distinct lack of "OMG some fancy presentation trick ! Litteraly unviewable!" comments that often happens when an unusual layout is presented (and often with reason; but sometime to a fault). I guess the main praise your page UI is that it looks, well, like a page. But augmented I guess ? Personnally I really like the way you used a grid to separate the content from the nav. I like that you used both the left sidebar for nav and for the header number (re-using the same space for multiple purpose feels elegant, because those purpose are secondary to the content, if that makes sense). And I like that the grid anchors your eyes by fencing the different chapters along with the nav. (and now that I mention it, it feels weird that the headings are outside their chapters, but it didn't felt like that upon first reading).
- codeulike 7mo agoIn 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.
- jonahx 7mo agohttps://www.youtube.com/watch?v=yV-KiTAAcrY https://www.youtube.com/watch?v=yV-KiTAAcrY
- numpad0 7mo agoFunniest 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.
- signa11 7mo agodidn’t cv raman prove just that via his raman-effect for which he got the noble prize ?
- jabl 7mo agoKind 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.
- munificent 7mo agoReally 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.
- erikdkennedy 7mo agoTIL! Debates whether to update the sidenote with an explainer on ambitransitive and labile verbs
- thombat 7mo agoPlease do! It will fit the spirit of your splendid post, unpacking the complexity of a thing we take for granted (at least as native English speakers)
- kazinator 7mo agoThere 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?
- suzzer99 7mo agoNow do clam steamers and shrimp fried rice.
- rob74 7mo agoGreat 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!
- erikdkennedy 7mo agoThere 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)
- aaroninsf 7mo agoNot 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.
- mr_toad 7mo agoSurely it would have still been mostly nitrogen.
- kazinator 7mo agoIt'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.
- thaumasiotes 7mo agoThe 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?
- kazinator 7mo agoWell, 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.
- thaumasiotes 7mo ago> 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 model the article describes is: 1. The sun puts out a bunch of light. 2. Light that is lower-energy than blue light fails to be effectively scattered by the atmosphere, and follows a path from the sun to you. 3. Light at the levels of blue and up is effectively scattered by the atmosphere, and comes to you from a random direction. So if you look toward the sun, you receive light that has been depressed of its blue-and-up wavelengths, and the sun appears to be yellow. If you look away from the sun, you receive light that has been enriched in blue-and-up wavelengths, and the sky appears to be blue. Crucially, the sky looks blue because it is sending you more blue light than the background level. A cloud that isn't between you and the sun is getting its light from the sky background, which is blue. Why is the cloud not blue? It can disperse all the light it receives evenly, but that light is enriched for blue-and-up wavelengths.
- Jun8 7mo agoThis 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 https://www.youtube.com/watch?v=sJG-rXBbmCc&t=1674s
- oxag3n 7mo agoThe same reason it's polarized.
- jama211 7mo agoBrilliant, thank you
- deafpolygon 7mo agoVery well explained. I love the in-depthness of the article.
- deleted 7mo ago[deleted]
- FrogWizardMan 7mo agoto piss me off, goddammit
- halis 7mo agoNitrogen.
- oxag3n 7mo agoI 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?
- zehaeva 7mo agoI 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.
- mncharity 7mo agoIIUC, 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).
- 16bytes 7mo agoIs 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.
- mvdtnz 7mo agoSome 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.
- yawpitch 7mo agoThe 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.
- jxdxbx 7mo agoIf 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.
- yawpitch 7mo agoBy that logic the sky is near-Planck length ultraviolet, because if I put a sufficiently large quantity of it immediately beside a supernova it goes way past blue. Color is a property something has under certain conditions, it is not a property of what that something is under all conditions.
- mncharity 7mo agoPerhaps "transparent with a blue tint"?
- yawpitch 7mo agoI’ll allow “transparent with an occasional blue tint under the current conditions of its nearest star”.
- mncharity 7mo agoDoes that confuse sales staff when shopping for clothes? ... :) The general observation being that sometimes educational descriptions of things get hedges which wouldn't usually be applied in everyday life. Yes, the nice red shirt will look black under some lighting, like some meters underwater... but it usually isn't mentioned. Yet for example, colors of unfamiliar objects in education content can get an "appears" hedge -"it appears white", rather than the more usual, simpler concept of "if its light looks white, it's 'white'".
- awesomelybad 7mo agoImplementing 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.
- asqueella 7mo agoThe 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-scattering-3/ 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.
- mncharity 7mo ago> 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.
- 9dev 7mo agoSo, does that mean, and bear with me here, that… air is blue?
- callmeal 7mo agoOxygen actually - liquid O2 is blue. So to me a 100mi deep mix of 20% O2 may as well be blue :)
- zkmon 7mo agoOkay, why does visible light have that range of frequencies?
- smegger001 7mo agobecause they are the frequencies that pass though water most readily, and we are made of mostly water
- jet98 7mo agothat'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?
- nostrademons 7mo agoThe 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=0b3b97fa6688 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-infrared/ https://a-z-animals.com/animals/lists/animals-that-can-see-i...
- hintymad 7mo agoThis 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.
- IshKebab 7mo agoIf 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?
- erikdkennedy 7mo agoIt 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"
- Darkphibre 7mo agoI 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."
- manojlds 7mo agoSo are there animals that see a violet sky?
- thoughtlede 7mo agoI 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.
- jxdxbx 7mo agoYes, 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.
- rcxdude 7mo agoNo, 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.
- tehjoker 7mo agoThis 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.
- thot_experiment 7mo agothe pupil asked, why is the sky blue? the master answered, because the sun is yellow, and the pupil was enlightened
- b_brief 7mo agoGood 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.
- Night_Thastus 7mo agoThis 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!)
- wamatt 7mo agoThoroughly 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)
- mrb 7mo agoI 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.
- mercutio2 7mo agoI'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"!
- 1718627440 7mo agoAt 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.
- 7mo ago
- librasteve 7mo agoWhy is the sky black? - at night (of course) - there are ~1 septillion stars that are all shiny
- mr_toad 7mo agoIf 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.
- librasteve 7mo agothis! guess this is the definitive proof that the visible universe is finite
- alexander2002 7mo agoThis post is so good! You are a hero.
- nephihaha 7mo agoWhere I live, the sky is grey much of the time... Most of last week anyway!
- jxdxbx 7mo agoIf 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.
- mr_toad 7mo agoAt the end of the day all colors are due to electrodynamics.
- brcmthrowaway 7mo agoHow does a 3D raytracing engine accomplish this?
- brcmthrowaway 7mo agoIf the sun is emitting pure white light, why is it yellow?
- minneapoliced 7mo ago[flagged]
- kazinator 7mo ago> 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.
- kazinator 7mo ago> 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.
- goatlover 7mo agoI 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?
- mncharity 7mo ago"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.
- sixo 7mo agoHere'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.
- interroboink 7mo agoI 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%20light%20scattering.pdf http://users.df.uba.ar/bragas/Web%20roberto/Papers/sobelman%...
- fatih-erikli-cg 7mo ago[dead]
- actinium226 7mo agoYes! He answers why the sky isn't violet, love it!
- articulatepang 7mo agoOne 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.
- reddalo 7mo agoThis blog seems promising; I just wished it had an RSS feed so I could see when new articles come out.
- trekz 7mo agoIt looks like it does have a feed: https://explainers.blog/feed.xml https://explainers.blog/feed.xml
- beardyw 7mo agoOddly, no mention of rainbows.
- jimnotgym 7mo agoLooks out window Sky is grey
- ruicraveiro 7mo agoHi, I'm interested in what's next from your blog. It would be really nice if you'd include an RSS feed in it.
- trekz 7mo agoLooks like there's an RSS feed at https://explainers.blog/feed.xml https://explainers.blog/feed.xml
- piokoch 7mo agoThe 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 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).
- James_K 7mo agoThe sky is blue because air is blue.
- Brajeshwar 7mo agoThis is the kinda website I’d love to subscribe and follow. Unfortunately, this is the only article for now. And it has no RSS (neither Newsletter).
- absynth 7mo agoWhy is the sky blue? 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.)
- paraiuspau 7mo agoWhy is water wet? Why did Judas rat to Romans while Jesus slept? -- gza
- jongjong 7mo agoI 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.
- SwtCyber 7mo agoThis is the kind of post you send someone when they ask a "why" question and you want them to come back smarter instead of just satisfied :)
- dominictorresmo 7mo agowow that was a very good article! I always wondered why the sky is blue
- jll29 7mo agoAlthough 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!
- greenwalls 7mo agoAwesome graphics. Looking forward to more articles.
- thewakalix 7mo agoThe 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.
- sandworm101 7mo agoAt 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....
- deleted 7mo ago[deleted]
- snowwrestler 7mo agoThe 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.
- createaccount99 7mo agoIs the source for the blog, open? What do they use to make the interactive charts? Vibe coded new stuff, or based on some existing framework?