Difference between revisions of "Colors of the world around us"

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tl;dr:
 
tl;dr:
* '''The sky''' is bluer when you look primarily at scattered light
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: ...due to Rayleigh scattering through nitrogen and oxygen. Their molecular size just happens to have a significant effect on blue.  
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* '''The sky'''  
: Since it's scattered, it's a diffuse effect throughout the whole sky
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: ...blue due to Rayleigh scattering through nitrogen and oxygen. Their molecular size just happens to have a significant effect on blue.
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: Since it's scattered, it's a diffuse effect throughout the whole sky. It's bluer when you look primarily at scattered light and other effects are less pronounced.
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* '''Sunsets''' are red, and more so around the sun, actually for much the same reason
 
* '''Sunsets''' are red, and more so around the sun, actually for much the same reason
: that is, light that has passed through more atmosphere has scattered blue to its side, leaving relatively more red
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: that is, when you look at a sunset, you are looking at the horizon, seeing light that has passed through more atmosphere - so has more of its blue scattered out, leaving relatively more red
 
: it helps that we're looking almost directly along the original light path; that's where the unscattered rays are visible
 
: it helps that we're looking almost directly along the original light path; that's where the unscattered rays are visible
: pollution also helps
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: pollution also helps - basically for more of the same effect
  
  
  
  
So, turns out it's not refraction.  
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It's sensible to theorize this might be refraction.  
  
If it were, direction would matter, and we'd see different colors depending on where we are, and probably dark or black from opposite the sun.
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It's not, though. If it were, direction would matter, we'd see different colors depending on where we are, and probably dark or black from behind us, opposite the sun.  
 
Yet it's blue everywhere, pretty evenly, and even photos from the space station reveal a blue sheen around earth.
 
Yet it's blue everywhere, pretty evenly, and even photos from the space station reveal a blue sheen around earth.
  
And most sunlight just makes it through undeterred.
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Also, most sunlight just makes it through undeterred.
If it didn't, then sunlight wouldn't be particuarly white,
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If it didn't, then sunlight couldn't be particularly white.
direct sunlight and hard shadows probably wouldn't happen.
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Also, direct sunlight and hard shadows probably wouldn't happen.
 
   
 
   
  
 
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Refraction does happen in the sky at all, e.g. within cloud or mist droplets doing their rainbow thing, but for you see very little of this, a few different reasons.
Refraction does happen in the sky, e.g. within cloud or mist droplets doing their rainbow thing, but for a few different reasons you see very little of this. There are a few cool related effects that do show up sometimes, though - see e.g. {{search|iridescent cloud}}, [https://en.wikipedia.org/wiki/Glory_(optical_phenomenon) glory] (similar to a rainbow but different mechanics).
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There are a few cool related effects that do show up sometimes, though - see e.g. {{search|iridescent cloud}}, [https://en.wikipedia.org/wiki/Glory_(optical_phenomenon) glory] (similar to a rainbow but different mechanics).
 
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Also, it ''is'' relevant that blue as a color is scattered more easily/further (see e.g. a [http://en.wikipedia.org/wiki/Prism_%28optics%29 prism], just to note that blue and purple get scattered most among the visible-light frequencies).
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The physics involves electric polarizability and a handful of other things we could spend a lot of time on, but it turns out that mainly, if the size of particle we're hitting is similar to a particle's size, interesting things happen.
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The physics involves electric polarizability and a handful of other things we could spend a lot of time on, but it turns out that mainly, if the size of wave is similar to a particle's size, interesting things happen.
  
When those two sizes are near each other, much larger, or much smaller,
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There are a few different effects happening at the same time, but one of them will dominate,
physics has different sets of equations to approximate the larger-scale effects that will dominate,
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basically depending on whether wavelengths are on the same order, much larger, or much smaller.
which is we can refer to these effects with a single, named formula.
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Physics has different sets of equations to approximate the larger-scale effects, which is we can refer to these effects with a single, named formula.
  
  
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For about the same size (often meaning colloids), [https://en.wikipedia.org/wiki/Tyndall_effect Tyndall effect] is significant (related to Rayleigh scattering)
 
For about the same size (often meaning colloids), [https://en.wikipedia.org/wiki/Tyndall_effect Tyndall effect] is significant (related to Rayleigh scattering)
 
: which is e.g. why fine smoke appears slightly blue.
 
: which is e.g. why fine smoke appears slightly blue.
: since this is a fairly narrow effect, we don't see this a lot. It's also subtle enough that you may assume you're just seeing it wrong.
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: since there is a narrow band where this applies, we don't see this a lot.
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: It's also subtle enough that you may assume you're just seeing it wrong.
  
 
For particles '''much smaller''' than the wavelength of light, there is [http://en.wikipedia.org/wiki/Rayleigh_scattering Rayleigh scattering].
 
For particles '''much smaller''' than the wavelength of light, there is [http://en.wikipedia.org/wiki/Rayleigh_scattering Rayleigh scattering].
  
(There are some further related effects, like Raman scattering)
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(There are some further related effects, like [https://en.wikipedia.org/wiki/Raman_scattering Raman scattering])
  
  
  
For the sky, the largest components by far are nitrogen and oxygen ([https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Composition together 99%]), and since both of those are roughly a factor thousand smaller than the wavelength of visible light, Rayleigh scattering is the one that applies here.
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For the sky, the largest components by far are nitrogen and oxygen ([https://en.wikipedia.org/wiki/Atmosphere_of_Earth#Composition together 99%]), and since both of those are roughly a factor thousand smaller than the wavelength of visible light, Rayleigh scattering is the one that applies most.
  
 
Rayleigh scattering is [http://en.wikipedia.org/wiki/Elastic_scattering elastic scattering], which means that the energy (so wavelength, color) is preserved, and you can only change its direction.{{verify}}
 
Rayleigh scattering is [http://en.wikipedia.org/wiki/Elastic_scattering elastic scattering], which means that the energy (so wavelength, color) is preserved, and you can only change its direction.{{verify}}
There's also a lot of atmosphere, meaning that if light scatters even just a little, you get a pretty diffuse effect.
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Each such scattering is very subtle, but since there's a lot of atmosphere.  
  
That last bit is also why the reason the sky is fairly bright in the first place, and relatively evenly so.
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This is also why it is quite a diffuse effect, why it looks fairly uniform throughout the sky,
In contrast, the atmosphereless moon has a pitch black sky,
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and also why the sky is fairly bright in the first place. {{comment|(In contrast, the atmosphereless moon has a pitch black sky, shadows are always crisp, and any light that don't hit anything fly by undisturbed)}}
shadows are always crisp, and any rays that don't hit anything fly by undisturbed.
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Note that there are other contributing effects, Rayleigh scattering is just the largest.
 
  
For example, Ozone is slightly blue by itself - but during the day the intensity of the Rayleigh effect makes that almost negligible. It's only right around sunrise and sunset, when there is little light, that its contribution matters to a noticeable degree{{verify}}
 
 
 
Also, people paying attention to the spectrum will be wondering why the sky isn't a blue-violet mix.
 
It is. Just not noticeably, mostly because our eyes are not as sensitive to violet as to blue ''and'' that the sun puts less of it out (remember it's at the edge of visible).
 
  
  
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Some footnotes:
 
  
Scattering happens in all of the visible spectrum, but the size of these particular molecules mean it happens significantly more for blue than for red, perhaps a factor four.
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Further notes
(This relates to that visible light is a pretty large range, 400nm-700nm, and that the wavelength of red is approximately twice that of blue - this turns out is a big difference in this context). It's also not linear - on the red end the difference between color is smaller, making blue stand out a little more than it would if it were linear.
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* there are other contributing effects, Rayleigh scattering is just the largest.
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: For example, Ozone is slightly blue gas by itself - but during the day the intensity of the Rayleigh effect makes that almost negligible. It's only right around sunrise and sunset, when there is little light, that its contribution matters to a noticeable degree{{verify}}
  
That amount of difference is noticeable, and therefore blue on a while - but not overpoweringly so, which is also why the sky is a pale rather than a deep blue.
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* People paying attention to the spectrum will be wondering why the sky isn't a blue-violet mix.
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: It is.
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: Just not noticeably, mostly because our eyes are not as sensitive to violet as to blue ''and'' that the sun puts less of it out (remember it's at the edge of visible).
  
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* Scattering happens in all of the visible spectrum, but the size of these particular molecules mean it happens significantly more for blue than for red, perhaps a factor four.
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: (This relates to that visible light is a fairly large range, 400nm-700nm, and that the wavelength of red is approximately twice that of blue - this turns out is a large difference in this context).
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: It's also not linear - on the red end the difference between colors is smaller, making blue stand out a little more than it would if it were linear.
  
The rest of things look white because fairly direct sunlight, which is only slightly yellowish, is still much stronger than the blue light that scatters from the atmosphere. On overcast days things are still white, because then you don't see any blue, you see white light scattered through by the much-larger-particled clouds.
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* That amount of difference is noticeable, but not overpoweringly so, which is also why even through dozens of kilometers of relatively dense atmosphere, the sky is only a pale rather than a deep blue.
 
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* only the sky looks blue, not objects, because most light makes it through anyway
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: the light that makes it through is also much stronger than the amount of blue light that scatters down
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: also why things are still white on overcast days (when you can't directly see the blue sky)
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: also, if a strong light source is close enough to white, we easily adapt to consider it white, regardless of its tint
  
  
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'''On sunsets'''
 
'''On sunsets'''
  
This also implies that in the direction of the light beams itself, there is less blue left.
 
You get more yellow, to orange, and even red when you're looking more directly at the source.
 
  
For example, the sun is yellower when seen from within the atmosphere than from outside it,  
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In most directions, the above effect works out as blue, because you only see the small amount of scattering, while the light that's scattered from is stronger, and passing by.
because some of the blue is scattered away so, relatively speaking, you see more of the rest - but also still a lot of scattered blue.
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However, when you are looking light that is pointed in your direction,
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you see light with blue scattered away. You don't see that blue, you see mainly what's left,
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which is relatively yellow, perhaps orange, and even red.
  
Sunsets are orange because of this effect.
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The effect is stronger around sunrise and sunset mostly because the light had to go through more atmosphere to get to you, meaning it scattered away more blue in the process.
The effect is stronger around sunset mostly because the light had to go through more atmosphere to get to you, meaning it scattered away more blue in the process.
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With just nitrogen and oxygen, this amount of scattering usually gets you orange. To get deep red, you need fairly unusual circumstances.
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The effect also happens during the day, but less pronounced as it's not passing through nearly as much atmosphere, and more importantly, the sun is way too bright to look at in the first place. (also, any clouds in the path will make it look more white just through context)
  
  
This mostly points to microscopically dispersed small particles in air (which is the definition of an aerosol),
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There is also a brief period where the sunlight is lighting the underside of clouds
but there are different kinds of aerosols.
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Man-made aerosols, primarily pollution, are almost always a mix of a ''lot'' of different things,
 
so for a good deal is larger particles.
 
It turns out most pollution and smog and such is too large to be a good Rayleigh scatterer, some of it is going to be a Mie scatterer mixing in white so making for grayer washed out colors, and some of it just blocks more light.
 
So actually, smog usually works against a red sunset, for for it.
 
  
Less pollution in your path is also the largest reason why on mountains and in airplanes you'll see nicer sunsets.
 
  
Temperature affecting how and how long pollution hangs around also explains why winter gives somewhat nicer sunsets.
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Actually, with just nitrogen and oxygen, this amount of scattering usually gets you orange.
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To get deep red, you need fairly unusual circumstances.
  
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Most relevant are microscopically dispersed small particles in air (which is the definition of an [https://en.wikipedia.org/wiki/Aerosol aerosol]), but there are different kinds of aerosols.
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Man-made aerosols, primarily pollution, are typically a mix of a ''lot'' of different things,
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which means it easily includes larger particles. As a result, most pollution and smog and such is too large to be a good Rayleigh scatterer, some of it is going to mix in some Mie scattering making for grayer washed out colors, and some of it just blocks more light so look washed out and darker (and if local, will look like / be smoke).
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So actually, pollution smog often works against a red sunset.
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It will only help create a specific color when it has fairly uniform
  
 
Some natural aerosols (think forest fires, volcanic eruptions) are actually more uniform,
 
Some natural aerosols (think forest fires, volcanic eruptions) are actually more uniform,
so can be more selective. It also helps that those two disperse a lot of it into a lot of atmosphere.
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so can be more selective for a color.
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It also helps that those two cases disperse a lot of it into a lot of atmosphere.
 
If it's in the right range, it can add more to Rayleigh scattering than to others, and some have specific colors.
 
If it's in the right range, it can add more to Rayleigh scattering than to others, and some have specific colors.
 
(see the NOAA link below for more details)
 
(see the NOAA link below for more details)
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Less pollution in your path is also the largest reason why on mountains and in airplanes you'll see nicer sunsets.
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Temperature affecting how and how long pollution hangs around also explains why winter gives somewhat nicer sunsets.
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Revision as of 14:49, 12 September 2021

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

The sky