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… """
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.
Air is mostly nitrogen. Nitrogen gas is blue.
There.
Anyone else immediately think of this commercial?
https://www.youtube.com/watch?v=PbKsC4GCT5k
*Since blue is the shortest wave length...*
Back in my youth, after the Internet became common but before Wikipedia, I tried to discover the answer to this and came away disappointed again and again. Every article I could find simply stated "because light scattering", and barely much more.
How does scattering work? Why does light scatter? _What does scattering even mean in the context of light?_
For the sunset example then, a natural question (for me) is then why isn't the sky green in the transition from blue sky to red sunset sky?
Your intuition isn’t far off; there is an angle where the weight of green relative to the sum over wavelengths sees a local maximum. But it doesn’t dominate. In that transition zone, there is still an overlapping, transitioning abundance of redder and bluer wavelengths, adding with the green. Consequently, you see red, going into a red+green transition (== oranges, yellows), go into into a green+blue transition (== cyan), which already has few photons relative to the red and yellow zones, so it’s a dark/weak cyan, before it blends into the darker blue of the night sky.
Because the color of the sky is determined by a shifting mixture of wavelengths, not a single shifting wavelength.
Basically, the scattering process that "remove" blue from the spectrum also removes green, albeit to a lesser extent. There are some greenish and yellowish wavelengths in the sunset sky, but they're dominated by red, so the overall color appears red or orange.
In order for the sky to look noticeably green, there would have to be something that scattered reds and blues, without significantly absorbing green.
If you try to interpolate between sky-blue and orange using graphics software, the result depends on what "color space" you're using. If your software interpolates based on hue, you might see green (or purple) in the middle. But that's not physically realistic.
A realistic model is to interpolate each wavelength of the continuous spectrum separately. Interpolating in RGB color space is a crude approximation to this. And if you try the experiment, you'll see that the midpoint between sky-blue and orange is a kind of muddy brown, not green.
Let's be real. The sky is blue because God thought it was a pretty color, simple as. All this stuff about wavelengths and resonant frequencies and human color perception got retconned into the physics engine at some point in the past millennium, that's why all these epicycles are needed.
Our lord Zeus always thinks of everything
> thought it was a pretty color
So was blue intrinsically pretty and thus made into the sky, or considered pretty and thus imprinted in the minds of humans that way?
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.
Here is a wonderful lecture with real-world demonstrations of the effect:
https://www.youtube.com/watch?v=4a0FbQdH3dY
https://en.wikipedia.org/wiki/Rayleigh_scattering
I do have a question though.
The article says:
> blue and violet have the closest frequencies to a “resonant frequency” of nitrogen and oxygen molecules’s electron clouds
I thought it was more to do with the photon frequency matching the physical size of the air molecules? Or is that the same as its resonant frequency?
Air molecules are much smaller than the wavelength of visible light, by several orders of magnitude. This is why you can't resolve individual molecules in an optical microscope, and why photolithography with visible light doesn't go down to molecular feature sizes.
https://www.youtube.com/watch?v=4a0FbQdH3dY&t=2038
Direct link to timestamp 33:56
Fs is the frequency at which whatever your measuring is most efficient at vibrating
So it’s a combination of the composition of the thing and the environmental coupling with other vibrating things
Size and material composition are the primary factors
So for this case, the photon spectrum interact with nitrogen-oxygen mixture most efficiently at the frequency that reflects blue
I mostly studied sound frequency mixing with static objects (matching or cancelling the fs of room/space with the fs of a driver) but the principles of resonance hold across media
Obligatory xkcd: "Sky Color" https://m.xkcd.com/1145/
Obligatory xkcd[2]: "Rayleigh Scattering" https://m.xkcd.com/1818/
Others?
The "Rayleigh Scattering" comic is really spot on.
Air is blue. The reason air is blue is blah blah blah physics, see the article we're all commenting on, but at the end of the day air is blue. We don't demand the same elaborate physics questions for why a ripe banana peel is yellow.
Not really. If the explanation was "air is blue" then the naive expectation would be that sun would appear blue against blackish background, basically the image of sun is being filtered through the atmosphere; if sun is white and air is blue then white filtered through blue should be blue? But sun appears yellowish against blue background. So clearly something different is going on.
It’s not. It’s raining here.
Wow 3 down votes. Sorry for making a joke. This place is joyful.
HN 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.