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.
Why is the sky blue?
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Re: Why is the sky blue?
#112In 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…
"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…
It's reminds me of that scene from Fargo: "He was kinda funny lookin'" ... "Could ya be any more specific?"
Re: Why is the sky blue?
#113the pupil asked, why is the sky blue?
the master answered, because the sun is yellow, and the pupil was enlightened
Re: Why is the sky blue?
#114This 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!)
Re: Why is the sky blue?
#115I 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.
Re: Why is the sky blue?
#116Why is the sky black?
- at night (of course)
- there are ~1 septillion stars that are all shiny
Re: Why is the sky blue?
#117This post is so good! You are a hero.
Re: Why is the sky blue?
#118I 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?
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.
Re: Why is the sky blue?
#119Implementing an atmospheric shader in three.js is a fun way get an understanding of the interaction of the different scattering components, light, and observer’s position. Plus you get a pretty cool effect to play around with after you’re done.
The explanation that made it click for me a while ago was by someone who implemented a shader https://www.alanzucconi.com/2017/10/10/atmospheric-scatterin... — the explanations that don't end up producing an image all seemed to skip over one detail or another.
> the explanations that don't end up producing an image all seemed to skip over one detail or another.
Implementation can be wonderfully useful as both a test of, and a forcing function for, really understanding something. At least when ground-truth (ie tests) is available.
Re: Why is the sky blue?
#120Where I live, the sky is grey much of the time... Most of last week anyway!