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Posted by udit99 7 hours ago

Why is the sky blue?(explainers.blog)
327 points | 115 comments
staplung 5 hours ago|
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… """

SAI_Peregrinus 3 hours ago||
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.
reactordev 1 hour ago||
The real question is, is the sky blue for everyone? Some creatures can see ultraviolet. Some lack color at all…
ecshafer 4 hours 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 3 hours ago|||
i used something like this in unstructured technical interviews all the time.

"you type a phrase into google search, you press enter, get some results. tell me, in technical detail, what happened in that chain of actions"

the diversity of replies is fascinating, you learn a lot about a "full stack" candidate this way.

Feynman's classic "Why?" chain: https://www.youtube.com/watch?v=36GT2zI8lVA

SAI_Peregrinus 2 hours ago||
I'd probably spend at least 20 minutes just to get through how the keyboard works, much more if it's a USB-HID device.
dmd 2 hours ago|||
Hah - that is exactly what I did. Someone asked me this question and after 5 minutes in the weeds of the debounce on the mouse click they said "look all we wanted was to find out if you'd ever heard of DNS, let's move on, that was great".
leeoniya 1 hour ago|||
the good ones would usually follow up with, "how much detail do you _really_ want ;D"
HPsquared 42 minutes ago||||
It's reminds me of that scene from Fargo: "He was kinda funny lookin'" ... "Could ya be any more specific?"
jstummbillig 2 hours ago||
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.
brabel 2 hours ago|||
The same anticipation of great things happening preceded the arrival of widely available internet, but all we really got was cat videos initially, and doomscrolling more recently. I don’t have much hope for great things anymore.
reactordev 1 hour ago||
We got more than that. We got 24/7 surveillance.
Starlevel004 48 minutes ago||||
When is that going to be?
ryanmcbride 2 hours ago||||
Me too but I don't think these sorts of Solved Society endgames are likely to show up. Basically presents the same issue with a utopia.

Progression and regression are always going to be at war with each other. There will always be humans that want to hurt instead of help, there will always be humans who TRY to help but ultimately hurt. There will always be misinformation, there will always be lies, and there will always be liars.

The good news is there will also always be people trying to pull humanity forwards, to help other people, to save lives, to eradicate disease, educate, and expose the truth.

I don't think society will ever be solved in the way you're saying because there will always be hurtful people, but there will also always be good people to keep up the fight.

kakacik 1 hour ago|||
... and due to that, people will not appreciate all the knowledge, we will take it as air - invisible but cut the access in a myriad ways and its a catastrophe.

We value what we achieve with effort, I would say proportionally to energy put in (certainly true for me, thus I like harder efforts in activities and ie sport climbing).

munificent 4 hours ago||
Really cool article! Tangential:

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

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

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

* Intransitive: The bell rang.

* Transitive: John rang the bell.

"Scatter" is simply a labile verb:

* Intransitive: Blue light scatters.

* Transitive: Atmospheric molecules scatter blue light more.

kazinator 3 hours ago||
There are many verbs like this, and English is somewhat open toward using verbs that way, or becoming so.

Did English speakers say "this novel reads well" two, three hundred years ago?

srean 3 hours ago|||
I have always wondered about this. The verb for the first person is to 'see'. To a third person you 'show'

For the first person there is 'listen' (or 'hear'). Does English not have a corresponding word for the third person ?

What about Germanaic or Nordic languages ? Do they have a third person analogue of 'listen' ?

onestay42 3 hours ago|||
AFAIK listen used to be used therefor[sic] but it has fallen out of use nowadays. From wiktionary:

> Listen the watchman’s cry upon the wall.

Edit: formatting

srean 2 hours ago||
'Hear the watchman’s cry upon the wall' works the same way, no ?

I have clarified what I am looking for in a cousin comment.

smlavine 3 hours ago|||
"tell"?
srean 2 hours ago||
Ah! That's not bad but it's not the same thing. Good nevertheless.

I can 'show' (or point someone to a) a sight that I am not myself creating in anyway. The word I am looking for would mean to 'make you hear' in the same may to show is to make you see.

I showed him the distant tower.

I ??? him the faint sound.

ccozan 1 hour ago||
play?

I played him the faint sound.

tsoukase 2 hours ago|||
Labile verbs is a source of ambiguity of natural languages (only western ones?) that we are all accustomed to.

The bell rang should become The bell was rung, either way it means The bell rang another bell.

GuinansEyebrows 1 hour ago||
"the bell was rung" illustrates a cause (and introduces a question: who rang the bell?)

"the bell rang" illustrates an effect (the vibration and sound of the bell as it rings).

i think this is more an illustration of the ambiguity of the root word "ring", which can be an action by a subject upon an object, or to describe the behavior of the object itself.

erikdkennedy 4 hours ago|||
TIL!

Debates whether to update the sidenote with an explainer on ambitransitive and labile verbs

suzzer99 3 hours ago||
Now do clam steamers and shrimp fried rice.
KellyCriterion 5 hours ago||
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

Sharlin 5 hours ago||
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.

adrian_b 4 hours ago||
Also the blood veins that you see as bluish through the skin are blue for the same reason, due to light scattered in their walls.
varispeed 3 hours ago||
I thought they are green.
SAI_Peregrinus 2 hours ago||
Definitely more blue/purple.
jjtheblunt 4 hours ago||
It's also the trick employed by Iridigm, which Qualcomm acquired in late 2004 (i was there then).

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

amelius 2 hours ago||
I'm curious how they were able to patent a technique invented by nature millions of years ago.
jjtheblunt 1 hour ago||
the displays have an array of switchable mirrors individually addressable, unlike nature in this case.

(but sort of like chromophores in an octopus or cuttlefish, perhaps).

amelius 1 hour ago||
I see, but those MEMs mirrors were already invented.
moron4hire 54 minutes ago||
Inventions can be useful recombinations or applications of other inventions. They don't need to be wholly unique unto themselves. Indeed, the vast majority of them are not wholly unique.
thoughtlede 1 hour ago||
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.

nephihaha 3 minutes ago||
Where I live, the sky is grey much of the time... Most of last week anyway!
mrb 28 minutes ago||
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! It's technically correct to state this.

Then you can dive in and explain why. Specifically it's the color of gaseous nitrogen and oxygen.

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

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

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

Night_Thastus 33 minutes ago||
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!)

librasteve 15 minutes ago||
Why is the sky black?

- at night (of course)

- there are ~1 septillion stars that are all shiny

b_brief 42 minutes ago||
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.
hintymad 2 hours ago|
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.
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