1818: "Rayleigh Scattering"

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Re: 1818: "Rayleigh Scattering"

Postby mathmannix » Wed Apr 05, 2017 7:20 pm UTC

Do children really ask "Why is the sky blue?"? I mean, sure this sounds like a typical question a precocious child might ask, and my own personal children are probably too young to wonder something like that, but in my experience, children generally accept things you tell them, until things stop making sense. Plus, there's no real reason to wonder "why" any thing is a color, it just IS that color, until you start getting into physics and Rayleigh and wondering why our eyes see colors. Maybe I was just always more accepting than questioning, though.

Also, I think my mom told me that the ocean was blue because it reflected the sky, and the sky was blue because it reflected the ocean. There's a nice symmetry there.
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Re: 1818: "Rayleigh Scattering"

Postby PinkShinyRose » Wed Apr 05, 2017 7:41 pm UTC

pscottdv wrote:
JohannesWurst wrote:3. An interesting question would be: Why are children interested in the color of sky and trees, but not in the color of other things, like tomatoes, or anything else? Why are they content with some answers, but not others?

(sorry, if this becomes a double post)


Here's another, possibly related, question: Why do children almost universally color tree trunks brown when the bark of almost all trees (and any other woody product left out in the sun for a long time) is grey?

I think most grey trees still have brownish grey stems, and a lot of trees are more greyish brown or just brown (except for the obviously white birch trees of course). I think the brown just stands out more than the grey.

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Re: 1818: "Rayleigh Scattering"

Postby orthogon » Wed Apr 05, 2017 9:13 pm UTC

mathmannix wrote:Do children really ask "Why is the sky blue?"? I mean, sure this sounds like a typical question a precocious child might ask, and my own personal children are probably too young to wonder something like that, but in my experience, children generally accept things you tell them, until things stop making sense. Plus, there's no real reason to wonder "why" any thing is a color, it just IS that color, until you start getting into physics and Rayleigh and wondering why our eyes see colors. Maybe I was just always more accepting than questioning, though.


I was wondering the same thing. My personal recollection is of its being a question that I became gradually aware of, and at some point, possibly as an adult, I became concerned that I didn't actually know the answer with any confidence, whereas I had the feeling that it was the sort of thing that anyone who considers themselves scientifically minded really ought to know. Finding out that it was "Rayleigh Scattering" was a bit of a relief, as this wasn't an effect I was familiar with or which had ever featured in any of my science education.
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Re: 1818: "Rayleigh Scattering"

Postby x7eggert » Sun Apr 09, 2017 2:07 pm UTC

One128 wrote:
wbeaty wrote:Air is a colored material, but it's not a pigmented material.


Air is indeed not a pigmented material, but it is also not a colored material. Not any more than you can say a CD is made of colored material, just because you can see colors on it when you light it in a particular way. Under diffuse white light, air doesn't have any color (and neither does a CD, discounting the sometimes fancifully dyed plastic).


Since you accept the existence of a sky while the sky is just whatever you see when you look at not-the-ground, the color of the sky needs to be whatever you usually happen to see, which mostly is the blue impression from Rayleigh Scattering. Just like a blue car isn't "undefined mixed color" because the dove sh*t on it, the sky is blue. Also the CD is silver, even though you see that rainbow on it (and also the printed name of the band).

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Re: 1818: "Rayleigh Scattering"

Postby orthogon » Sun Apr 09, 2017 5:37 pm UTC

x7eggert wrote:
One128 wrote:
wbeaty wrote:Air is a colored material, but it's not a pigmented material.


Air is indeed not a pigmented material, but it is also not a colored material. Not any more than you can say a CD is made of colored material, just because you can see colors on it when you light it in a particular way. Under diffuse white light, air doesn't have any color (and neither does a CD, discounting the sometimes fancifully dyed plastic).


Since you accept the existence of a sky while the sky is just whatever you see when you look at not-the-ground, the color of the sky needs to be whatever you usually happen to see, which mostly is the blue impression from Rayleigh Scattering. Just like a blue car isn't "undefined mixed color" because the dove sh*t on it, the sky is blue. Also the CD is silver, even though you see that rainbow on it (and also the printed name of the band).

wbeaty (and One128) was taking about air, not the sky. "Sky" is quite polysemous, since it can refer to the space above the Earth (where birds fly) to the notional object that we appear to see in the absence of clouds (which is blocked by the clouds) or to what we see when we look up including said notional object and any clouds. That the second of these is blue is, i think, far less controversial than that the air is blue. There isn't any blue material in the sky, but it looks blue consistently for a long time and from different angles, so to the extent that the sky is a thing, it's a blue thing. One could make essentially the same statement about the Screen of Death.

(* reads own post *) not sure what I'm saying here, but there it is.
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Re: 1818: "Rayleigh Scattering"

Postby ThatPITA » Mon Apr 10, 2017 4:47 pm UTC

This is a late response, but I get so confused: Rayleigh died in 1916, and scattering his ashes made the sky blue? What color was it before? Krakatoa scattered a lot of ash - here's what WikiPedia has to say about it:

The 1883 Krakatoa eruption darkened the sky worldwide for years afterwards, and produced spectacular sunsets throughout the world for many months. British artist William Ashcroft made thousands of colour sketches of the red sunsets halfway around the world from Krakatoa in the years after the eruption. The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration."[17] This eruption also produced a Bishop's Ring around the sun by day, and a volcanic purple light at twilight.
...

For several years following the eruption, it was reported that the moon appeared to be blue and sometimes green. This was because some of the ash clouds were filled with particles about 1 µm wide—the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green. People also saw lavender suns and, for the first time, recorded noctilucent clouds.[17]


What color was the sky during the Great Extinction of the Cretaceous Period? Did dinosaurs notice the color change?

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Re: 1818: "Rayleigh Scattering"

Postby speising » Mon Apr 10, 2017 5:50 pm UTC

I don't expect them to notice anything. I expect them to be *dead*.

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Re: 1818: "Rayleigh Scattering"

Postby Nork » Mon Apr 10, 2017 7:33 pm UTC

mathmannix wrote:Do children really ask "Why is the sky blue?"? I mean, sure this sounds like a typical question a precocious child might ask, and my own personal children are probably too young to wonder something like that, but in my experience, children generally accept things you tell them, until things stop making sense. Plus, there's no real reason to wonder "why" any thing is a color, it just IS that color, until you start getting into physics and Rayleigh and wondering why our eyes see colors. Maybe I was just always more accepting than questioning, though.

Also, I think my mom told me that the ocean was blue because it reflected the sky, and the sky was blue because it reflected the ocean. There's a nice symmetry there.

They do, or at least, my five year old did last year. It was a personal moment of nerd pride because I got to reinforce the "air is blue but you can't tell if there's not a lot of it" idea using Mountain Dew:
- Pour a small amount of Mt. Dew in glass, and ask what color it is ("It doesn't have a color")
- Pour a lot of Mt. Dew in glass, and ask what color it is. ("Now it looks like potty")
- Explain that you need enough of it in one spot to see the color, and that air acts the same way.
- Confidently drink Mt. Dew, knowing that science has been accomplished.


As an answer to the general "Do kids really ask X?" question, yes they do (and usually at the oddest times). I've had questions like "Why is the sky blue", "why does gravity work?", and "do worms have feelings?" At one point, I was driving down the road explaining the idea of a conscientious objector to my five year old, with no idea how we'd gotten there. The conversation started with her asking me why Snape hated Harry so much, and she kept focusing on the least convenient parts of my answers when choosing her follow up questions. Kids just sort of work that way.

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Re: 1818: "Rayleigh Scattering"

Postby Reka » Mon Apr 10, 2017 10:11 pm UTC

ThatPITA wrote:This is a late response, but I get so confused: Rayleigh died in 1916, and scattering his ashes made the sky blue? What color was it before?

Gray, of course: haven't you watched any old movies? :p

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Re: 1818: "Rayleigh Scattering"

Postby somitomi » Tue Apr 11, 2017 8:39 am UTC

Reka wrote:
ThatPITA wrote:This is a late response, but I get so confused: Rayleigh died in 1916, and scattering his ashes made the sky blue? What color was it before?

Gray, of course: haven't you watched any old movies? :p

So how come it's blue in old paintings then? I know artists are bit of loonies, but did all of them just agree to pick that color instead of the boring gray?
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Re: 1818: "Rayleigh Scattering"

Postby speising » Tue Apr 11, 2017 8:40 am UTC

somitomi wrote:
Reka wrote:
ThatPITA wrote:This is a late response, but I get so confused: Rayleigh died in 1916, and scattering his ashes made the sky blue? What color was it before?

Gray, of course: haven't you watched any old movies? :p

So how come it's blue in old paintings then? I know artists are bit of loonies, but did all of them just agree to pick that color instead of the boring gray?

Come again?
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Re: 1818: "Rayleigh Scattering"

Postby Nork » Wed Apr 12, 2017 1:12 pm UTC

somitomi wrote:
Reka wrote:
ThatPITA wrote:This is a late response, but I get so confused: Rayleigh died in 1916, and scattering his ashes made the sky blue? What color was it before?

Gray, of course: haven't you watched any old movies? :p

So how come it's blue in old paintings then? I know artists are bit of loonies, but did all of them just agree to pick that color instead of the boring gray?

http://calvin-and-hobbes-comic-strips.b ... k-and.html

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Re: 1818: "Rayleigh Scattering"

Postby somitomi » Wed Apr 12, 2017 1:57 pm UTC

Nork wrote:
somitomi wrote:
Reka wrote:
ThatPITA wrote:This is a late response, but I get so confused: Rayleigh died in 1916, and scattering his ashes made the sky blue? What color was it before?

Gray, of course: haven't you watched any old movies? :p

So how come it's blue in old paintings then? I know artists are bit of loonies, but did all of them just agree to pick that color instead of the boring gray?

http://calvin-and-hobbes-comic-strips.b ... k-and.html

I choose to believe we were both intentionally referencing that piece of art.
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Re: 1818: "Rayleigh Scattering"

Postby Mikeski » Wed Apr 12, 2017 4:32 pm UTC

somitomi wrote:I choose to believe we were both intentionally referencing that piece of art.

You must have been, because it's the first thing your conversation made me think of.

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Re: 1818: "Rayleigh Scattering"

Postby ShuRugal » Thu Apr 13, 2017 9:35 pm UTC

ucim wrote:
ShuRugal wrote:what material is the prism made of?
Ordinary window glass.


Well, that doesn't narrow it down much. Your prism is most likely colorless or green, but could be carrying tints of a number of other colors depending on the manufacturing process used and the impurities in the glass. It could be a straw-yellow color, or it may have a bluish tint. Might also be reddish (though at this point, I could also be describing various grades of aviation gasoline. Is your prism flammable?)

ucim wrote:"Scattered" is a subset of "reflected" (as is "diffracted"), and "refracted" is a subset of "transmitted".


except, of course, that this is factually incorrect. Since scattering occurs at different wavelengths, and at a different angle for each wavelength, light which enters a scattering medium will exit that medium in a number of directions with an angle of incidence anywhere between 0 and 180 degrees. Some of the light will go out the same way it came in (though only a tiny potion), more will exit approximately perpendicularly (though, again, not much) and the majority will pass straight through.

"Scattered" is not a subset of any other light-matter interaction, it is its own, unique, phenomenon.

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Re: 1818: "Rayleigh Scattering"

Postby ShuRugal » Thu Apr 13, 2017 9:47 pm UTC

Nork wrote:They do, or at least, my five year old did last year. It was a personal moment of nerd pride because I got to reinforce the "air is blue but you can't tell if there's not a lot of it" idea using Mountain Dew:



So, you gave your child an answer which is well and truly proven to be wrong (air is clear, regardless of how much of it you stack up, unless you fill it with something that isn't) and used an example of something which has a property that air does not as evidence.

This is why homeschooling should not be a thing. If you, as self-proclaimed nerd, thought this was a correct thing to teach your child, what garbage must the truly dull homeschool parents be filling their kids' heads with?

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Re: 1818: "Rayleigh Scattering"

Postby ucim » Fri Apr 14, 2017 12:35 am UTC

ShuRugal wrote:Well, that doesn't narrow it down much. Your prism is most likely colorless or green, but could be carrying tints
You're missing the point.

ShuRugal wrote:except, of course, that this is factually incorrect. Since scattering occurs at different wavelengths, and at a different angle for each wavelength, light which enters a scattering medium will exit that medium in a number of directions
Scattering is not a subset of specular reflection, but it is a subset of reflection. Specular reflection is a subset of reflection. Not all reflection is specular.
Spoiler:
...though some of it can be spectacular.

(I know... ouch!)
As to "the majority that will pass straight through", that majority is transmitted, not reflected.

Now I know there is more going on, but remember what I said at the top of the post you are quoting from - to wit: "To oversimplify (which is sometimes enough for insight)..." The point was to have people think about what color "means" in the context of "what color is it?". When you look at a bird's feathers, you are probably comfortable saying that they are blue (or whatever color they appear), even though the color is not from a pigment but rather from a trick of diffraction. On the other extreme, you are clearly not comfortable saying that a prism is blue (because you happen to be standing in the blue part of the rainbow it's creating). Somewhere in between you might scratch your head and realize that an optical trick isn't enough to disqualify something as creating a color, but some optical tricks are more equal than others.

And I won't even get into the example of my brown car, which looks bright red in every kind of light except the orange one in the parking lot. Should I call it a "red car that looks brown" or a "brown car that looks red"? The difference is actually quite striking, and is why LED theatrical lighting will have a hard time replacing gels.
Spoiler:
It can do neat stuff gels can't, but gels can do neat stuff LEDs can't. The audience doesn't look at the light, it looks at the stage in the light. An important transformation takes place when the light is reflected. An object can appear a different color in two different lights that, to look at them directly, would be perceived as being the identical color.
And then you get into how the rods and cones react to light, and how the brain pre-processes the differences between the signals of the various cones... all of which is fascinating (and RELATEDTM), but takes us away from the point of the original question.

So, I stand my my oversimplification. Explanations are like art: They are lies that shows us truth. The map isn't the territory
Spoiler:
...except, I suppose, in one rather inconvenient case.
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Re: 1818: "Rayleigh Scattering"

Postby Nork » Mon Apr 17, 2017 7:28 pm UTC

ShuRugal wrote:
Nork wrote:They do, or at least, my five year old did last year. It was a personal moment of nerd pride because I got to reinforce the "air is blue but you can't tell if there's not a lot of it" idea using Mountain Dew:



So, you gave your child an answer which is well and truly proven to be wrong (air is clear, regardless of how much of it you stack up, unless you fill it with something that isn't) and used an example of something which has a property that air does not as evidence.

This is why homeschooling should not be a thing. If you, as self-proclaimed nerd, thought this was a correct thing to teach your child, what garbage must the truly dull homeschool parents be filling their kids' heads with?

Yeah, that was an embarrassing moment of "I've read this thing on the internet and haven't bothered to fact check it yet because it sounds neat", that had to get rectified later. In the end, she got lessons about how to think through a problem on your own, how to double check your work, and that you need to admit you're wrong when you are. Eventually the correct answer may have been incorporated, but I'm not sure - she's five and when she asks these questions, she's basically saying "I like the attention I get when you're explaining something to me". She doesn't really try to remember the lessons - she mostly just seems to remember which types of questions get the most enthusiastic responses from me.

That said, I'm not sure how my story fits into the homeschooling/public schools debate. If I ever have to home-school, I imagine I'll treat it more seriously than I treat her random questions of the day.

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Re: 1818: "Rayleigh Scattering"

Postby wbeaty » Wed May 03, 2017 9:23 am UTC

One128 wrote:There are, however, numerous misconceptions in your response. I'll try to address them one by one.


Heh, and fail!

First, red illumination is not an analogy for the sky. Instead, shine white light in a fog-filled room with black-painted walls, and observe a bright blue color. The fog is the origin of that color. The fog "is blue." The blue color may be a consequence of pigments, or of interference, photonic crystals, rayleigh effect, etc., but the color is located in the fog, not in the light.


One128 wrote:First of all, what hits our eyes when we look at the sky isn't reflected light - it's scattered light.


Yes, get our terms correct: scattering is a subset of reflection: scattered light is always reflected light, but reflected light isn't necessarily scattered light.

One128 wrote:In the most generic case of diffuse white lighting - one that we generally use as a standard for evaluating color - we would simply see white light. There is no basis for calling the air blue - any more than calling it red, yellow or white.


All your misconceptions hinge on your error above.

On the contrary: for light scattered by air, the scattered color is blue, regardless of source diameter. In other words, for diffuse white lighting, the perceived color of air is blue, not white. This may be difficult to demonstrate using a 10KM cube of atmosphere, but we can use a miniature version which displays Rayleigh color: use a piece of aerogel.

Aerogel under collimated illumination is blue, aerogel under diffuse illumination is blue. Only under transmitted light does aerogel appear red-orange. But note well: to perceive the blue color, the aerogel (and the atmosphere) must be observed against a dark background. If somehow we should combine the transmitted and the scattered light, for example by placing the aerogel against white paper, then the observed colors will vanish. In "the blue sky" case, this doesn't occur, since the layer of air is backed up by a black background.

One128 wrote:Air is indeed not a pigmented material, but it is also not a colored material. Not any more than you can say a CD is made of colored material


But a CD *is* a colored material! Like soap bubbles and glass interference filters, the color of these materials is structural. Google: structural colors. The CDROM aluminum itself is colorless broadband reflecting. The rainbow colors are a consequence of the diffraction grating. Like cdroms and holographic foil giftwrap, dichroic inks are colored materials, and their color varies with observation/illumination angle.

One hangup I detect in most people: equating pigment with "having color." So, if it's not pigmented, therefore it cannot "have color?" This is wrong. Materials with structural colors (such as soap films, CDs, bluejays, human irises, aerogel, the sky...) all "have color" without having pigments. Destroy the structure, grind up the photonic crystal layer or the thinfilm stack, and unlike with pigments, the colors vanish. Tear the iris muscle out of a blue eye, place it on a white surface, and you'll not detect any blue at all. There was no blue pigment: the color was entirely structural.

Perhaps this will illuminate the problem: water films, soap bubbles, oil films, they are rainbow-colored materials, yet water itself is colorless. The word "water" doesn't imply "thin film." Only water with the thinfilm structure is a colored material. An oil slick is rainbow colored, while the oil itself is not. On the other hand, the word "aerogel" does imply a structure: although silica is colorless, yet silica aerogels are blue materials (except when viewed with transmitted light.) The color isn't in the silica molecule, it's in the gel structure at larger scale.

One128 wrote: just because you can see colors on it when you light it in a particular way. Under diffuse white light, air doesn't have any color (and neither does a CD, discounting the sometimes fancifully dyed plastic).


Yes, good point: the colors of CDs and of reflection holograms require collimated light. But the blue color of the sky does not. If we place air (or a hunk of aerogel) against a black surface, then illuminate it with a diffusing hemisphere, it will still appear blue. But, if we replace the black background with white, then the blue color disappears, same as with bluejay wings and blue irises in human eyes. The blue structural color requires that we prevent the orange transmitted light from getting back to the observer.


One128 wrote:You can verify this next time you are under a completely overcast sky (which gives out diffuse white light). The air won't appear blue


Might that be because 10-20KM distances are required? Maybe not, because many other structural colored materials behave that way too. In general, if a material splits light into brightly colored transmitted and reflected/scattered beams, and then if we then set up a situation in which we recombine the transmitted and scattered beams, then the observed colors will vanish. The structure was changed. Peel the photonic crystal layer off a black-pigmented bluejay feather, and paste it on a white feather instead, and the feather appears white. The split blue and orange rays were recombined. For bluejays to "be blue," the black background is required, it absorbs the transmitted orange light. I conclude: bluejays are blue. Apparently you conclude that bluejays are colorless, because the color requires specific illumination, and the materials involved have no pigments?

One128 wrote:This should have rung a bell - clearly, if the air appears red when we shine light on it in a particular way, there is no basis for calling it blue, as opposed to red. What reason is there for calling it blue, and not red? Because we like shining from the side more than we like shining from behind?


Air is blue. Also, air is red. Air is a dichroic material, much like color-changing inks and paints. Or like rainbow-colored soap bubbles.

Find a car with dichroic paint. The car looks bright red. What there has the red color? Not the illumination. The color is produced by the dichroic paint. Now change your angle, and the car is bright green. What material there is green? The dichroic paint.


One128 wrote:Under diffuse white light (shining uniformly from all sides),


Ah, that's different.

If we observe the transmitted light combined with the reflected light, then all structural colors vanish, and only pigments remain. Your recipe for light shining uniformly from all sides is a good way to detect pigments, and separate them from structural-color materials such as bluejays and Morpho wings and solutions of suspended nanodots.

If you then use this to deny the existence of structural colors, then I think you've left the path of simple truth and scientific thinking. Bluejay feathers really "are blue," even if the color requires a black absorptive background, it isn't based on pigments, and hence disappears under certain types of illuminations.
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Re: 1818: "Rayleigh Scattering"

Postby chenille » Fri May 05, 2017 10:37 pm UTC

wbeaty wrote:Peel the photonic crystal layer off a black-pigmented bluejay feather, and paste it on a white feather instead, and the feather appears white. The split blue and orange rays were recombined. For bluejays to "be blue," the black background is required, it absorbs the transmitted orange light. I conclude: bluejays are blue. Apparently you conclude that bluejays are colorless, because the color requires specific illumination, and the materials involved have no pigments?

I'm...actually ok with saying a bluejay feather is not blue if you take out the melanin layer that forms an integral part in generating its color. That's not because I don't believe in structural colors, it's because it stopped acting like a blue feather. It might be blue again in certain circumstances, but it's no longer so obvious to me that I should ascribe that color to the object itself.

wbeaty wrote:Air is a dichroic material, much like color-changing inks and paints.

Exactly, and that's what's wrong with the glib answer. It's not like how grass is green because of chlorophyll or the Statue of Liberty because of copper oxide, it really is something more unusual. Saying the sky is blue because air is blue is not a good explanation of its color, but a potentially misleading oversimplification. An actual explanation needs the context: the sky is blue because air is blue when it has a black background with non-transmitted light.

Personally I think that's different enough from most people's intuition - children or adults - on colors to warrant further comment, and I find the scattering behind it is simple enough for just about everyone to understand. I mean, when someone asks why a car changes from red to blue, they're plainly after more than "because it has color-changing paint". And if you want more people to appreciate structural colors, I'm not sure why you'd be down on the opportunity to share this simple example of one.


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