1818: "Rayleigh Scattering"

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Re: 1818: "Rayleigh Scattering"

Postby Soupspoon » Sat Apr 01, 2017 11:30 am UTC

One128 wrote:
orthogon wrote:Why are people assuming Randall is siding with Blue Air Woman?

I don't know about people, but I'm assuming that because he made the same argument himself in What-if 141.

I think Simpler Explanation Woman is a stopped clock (with no minute hand!) in a similar vein to #1717's "am I sure I want you to agree with me on this issue, because that seems to auto-justify you in other ways that I'm less comfortable with..." type of thing.

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Re: 1818: "Rayleigh Scattering"

Postby TomPace101 » Sat Apr 01, 2017 11:54 am UTC

One128 wrote:Agreed, Randall messed up here. Air is not blue; it's colorless.

Simply taking the third panel reasoning one step further would reveal that the "air is blue" explanation is bogus:

"It's why the rising moon looks orange - because of all the... blue air in the way? Err... wait a second..."


Agreed. Rayleigh Scattering explains why the sunset is orange-red (low-frequency visible light): the direct light is what's left over after the higher frequencies have been scattered into other directions. The "air is blue" explanation doesn't do that. Rayleigh Scattering also explains the polarization of the light.

Furthermore, Rayleigh Scattering is not a quantum mechanical phenomenon. You can derive it from classical electrodynamics. Indeed, there's such a derivation in Classical Electrodynamics by J.D. Jackson, who in turn cites Lord Rayleigh's original papers, which were written before Quantum Mechanics was ever developed.

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Re: 1818: "Rayleigh Scattering"

Postby jc » Sat Apr 01, 2017 12:44 pm UTC

GlassHouses wrote:I thought leaves are green because they're chock full of chlorophyll, which is green. Why is chlorophyll green? Doesn't matter; what matters is that chlorophyll can drive photosynthesis, and when you've found a chemical with such an awesome superpower, you're not going to gripe about its color.


Good one! Except that there have been lots of explanations that chlorophyll itself isn't green. A water solution of pure chlorophyll actually has a faint maroonish color, a mixture of the blue and red ends of the visible spectrum. This was mentioned in passing by someone above ... lessee ... It was chenille. Chloroplasts don't reflect green light, they "glow green" as a side effect of the photosynthetic process.

What's apparently going on is that chlorophyll actually absorbs best in the green part of the spectrum, and the maroon color is the incoming light minus the photons that are absorbed. Chloroplasts have evolved a number of frequency-shifting compounds that intercept light of other colors , and release the energy as green photons. This increases the fraction of the incoming light that the chlorophyll molecules are able to absorb. But most of the green photons escape the chloroplast, making it look green to outside viewers. The process doesn't need to be efficient; it only needs to "pay for itself" by increasing the photosynthetic output by more than the cost of making these green-glowing molecules. Some of the "lost" green photons will hit other chloroplasts, so they're not entirely wasted.

But the other pseudo-explanations are often a lot more fun. ;-)

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Re: 1818: "Rayleigh Scattering"

Postby Copper Bezel » Sat Apr 01, 2017 2:15 pm UTC

orthogon wrote:These assumptions simply don't apply to air or glass.

Glass is green, though? = o
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Re: 1818: "Rayleigh Scattering"

Postby wumpus » Sat Apr 01, 2017 3:47 pm UTC

SpitValve wrote:
Copper Bezel wrote:It's not about people asking why colors happen, which is certainly a good kind of question! It's about how many people have this specific cliche as their programmed response to the question of why the sky is blue, often and misleadingly to the exclusion of saying that "air is blue", as if that weren't a true statement, exactly as played out in the comic. It's a case where throwing more terminology at an answer can reduce understanding and really might indicate a lack of understanding on the part of the speaker, too.


It's similar to Feynmann's response to an interviewer asking how magnets work. It's probably less complex than asking why your hand doesn't go through your table, but the magnet question comes up more often.


I remember reading "Cuckoo's Egg" by Clifford Stoll (interesting if you want to know how the internet was in the 1980s, otherwise tragically dated), where he mentioned his astronomy PhD orals consisted of the one question "why is the sky blue?" It took nearly everything Dr. Stoll learned in astronomy to answer that question [for sufficient levels of detail].

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Re: 1818: "Rayleigh Scattering"

Postby chenille » Sat Apr 01, 2017 5:11 pm UTC

jc wrote:Except that there have been lots of explanations that chlorophyll itself isn't green. A water solution of pure chlorophyll actually has a faint maroonish color, a mixture of the blue and red ends of the visible spectrum. This was mentioned in passing by someone above ... lessee ... It was chenille.

It wasn't me, and I'm not sure it's quite right either. Solutions of chlorophyll regularly do look green on account of the frequencies it absorbs. However the situation is more complex because you can get the solution to look red or pink from photons absorbed and re-emitted at a different frequency – maybe the glow you're talking about? I think the fair way to put is that chlorophyll has a typical green (absorption) color but also red fluorescence, and then careful lighting can bring out one or the other.

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Re: 1818: "Rayleigh Scattering"

Postby x7eggert » Sat Apr 01, 2017 7:38 pm UTC

jozwa wrote:So apparently plants reflect green light because there's more green light than any other color in sun's radiation, and so it would be too easy to get sunburnt if a pigment absorbed it.


I heard (unconfirmed) that the plants are the second to use photosynthesis, before that there were organisms using the abundant green light. The plants used red and blue light because it was left.

https://www.nasa.gov/mission_pages/stat ... ady_to_eat

Blue and red wavelengths are the minimum needed to get good plant growth," Wheeler said. "They are probably the most efficient in terms of electrical power conversion. The green LEDs help to enhance the human visual perception of the plants, but they don't put out as much light as the reds and blues.

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Re: 1818: "Rayleigh Scattering"

Postby x7eggert » Sat Apr 01, 2017 7:52 pm UTC

cellocgw wrote:Not nececelery. There's a ton of IR re-emitted by warm things, and many insects (and snakes w/ 'thermalsensor pits') take advantage of this. Further, while the total wattage of solar IR is low-ish, the retina is a quantum detector (sorry, comic-woman figure!), and there are brazilions of IR photons showing up. The more likely explanation for our lack of IR sensitivity is that there aren't a whole lot of phospho-organic compounds with a bandgap small enough to respond in the IR.


Also we are warm-blooded. Especially our eyes are heated to enhance performance - we would see our own heat only.
Snakes are cold-blooded and use extra pits for IR detection.

cellocgw wrote:
Now, as to why some people are tetrachromes and can see in the nearUV -- but only after replacing the lens with a plastic substitute -- is an interesting question.


Red, green, intensity-rods and blue. Blue minus intensity = UV.

(Also there are people having extra cones for yellow. They might see pentachromatic.)

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Re: 1818: "Rayleigh Scattering"

Postby severach » Sun Apr 02, 2017 1:18 am UTC

I was always annoyed with the "Scattering" claim until I figured out what the explanations are missing. Scattering doesn't mean anything until you say where the light is scattered from and to.

Where does the light scatter to? To where observers see blue, the on average shorter paths.
Where does the light scatter from? From where observers see red, the on average the longer paths.

What causes blue sky at mid day? Raleigh Scattering.
What causes red sky at dawn and dusk? Someone over the horizon says "I'm in ur light scattering ur blues."

Solved easily with the first law of Thermodynamics.

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I can see the moon during the day, but only on days with very clear skies. Can't see it on the thick sky days even if it's up and in a viewable position away from the sun. Maybe skies were clear enough in the past that stars could be seen.

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Re: 1818: "Rayleigh Scattering"

Postby wbeaty » Sun Apr 02, 2017 7:30 am UTC

One128 wrote:Agreed, Randall messed up here. Air is not blue; it's colorless.


Nope. You've got the very misconception that this comic is designed to fight. Without air, the sky would be black. Ask yourself what material up there is colored blue? Not "sky." It's the thick air layer. Sunlight shines on air, reflected blue light hits our eyes: air is a blue material. But that's not the end of the story. Air is a colored material, but it's not a pigmented material.

Believing that air is colorless, where the blue color must have some other origin, that's the same basic mistake as believing that bluejay feathers are colorless, or that Morpho butterflies are colorless, or aerogel or opals are colorless. All of these are "structural colors," produced by wave interference, and there are no blue pigments in aerogel or blulejay feathers or Morpho wings. Grind up the material, destroy the structure, and we find a colorless substance; no pigments. It's a similar effect with air: the blue color of the air comes from wave physics. The blue color is perfectly real, same as blue human eyes. But don't assume that air is pigmented blue.

Incorrect question/response would be: Q: Teacher, since we all know that air is colorless, why is the sky blue? A: because complicated mathematics!! (Wrong from the start, air is not colorless.)

Correct question/response would be: Q: Teacher, why is the sky blue? A: because the air itself is blue (but a very dilute blue, a miles-thick layer is needed in order to see the color.) Q: OK then, why is the air blue? A: because of Rayleigh Scattering (also called Tyndall effect,) and the color of the air can change depending on the angle of light shining on it. If light shines on air from the side, it's blue. If light shines on air from behind, it's orange or red.

Even more correct: Q: Teacher, why is the sky blue? A: THERE IS NO SKY. "The Sky" is an illusory surface. There is no surface up there, and nobody painted it blue. Instead we're actually gazing into a thick cloud of brightly sunlit air. We may get confused, but it's because we believe that air is colorless. What's blue up there? "The sky" is the wrong answer. It's the atmosphere which has the blue color, it's the air-layer which is blue. (In other words, this childhood question cannot be answered unless we first debunk a common childhood misconception: kids' belief in the solid "sky" surface. When kids ask why the sky is blue, they're basically asking this question: Who painted the solid surface blue? What is "the sky," how did a solid bowl get up there in the first place? If the space shuttle goes up there, why doesn't it crash into the solid blue "sky?"

Also note that air behaves like glass, and like water: a thin layer looks totally colorless, but go and find yourself a thick layer and you'll see that it behaves like a strongly colored filter. The real trouble is that a mile-thick layer of air is far too thin to detect the color. Try a thicker 20mi layer instead.

Also: air is blue like bluejay feathers are blue, since the birds are Corvids, and their feathers are pigmented black, but also covered with a blue-reflecting photonic crystal (an interference filter.) Without the black background of the crow family, bluejay feathers wouldn't appear blue. The sky is very similar: it's backed up by a black background, the darkness of Olber's paradox. In other words, if bluejay feathers were white, then the interference filter wouldn't produce visible blue, and if outer space appeared white, then wave-scattering by air molecules could not create a visible blue color. (Observe the horizon on high-aerosol days, and it looks white. Watch it on extremely clear days, the blue of the sky extends all the way down to the ground.)

One128 wrote:"It's why the rising moon looks orange - because of all the... blue air in the way? Err... wait a second..."


EXACTLY RIGHT! Sometimes the blue of the air above us is actually the blue light that's been removed from the "sunset rays," ...and the resulting orange/red beam then goes on to be sunsets seen by everyone way downrange! If you see colored sunsets, it means that the people far over the horizon are looking up at the side of the exact same sunbeam, and seeing "blue sky."

After your students accept the idea that no "sky surface" exists, and that the color is coming from the air itself, then we can go to the next level, and show that air is behaving like the special ink seen on USA dollar bills: the ink has a color, but tilt the money, or move your head, and it has a different color. Two people 100KM apart, both looking at the same patch of air, can see totally different colors: one sees "blue sky," the other sees "red sunset." (The same thing happens with interference filters. The filter "is red." Tilt it, and the filter "is yellow." What substance "has" the color(s)? The filter, of course.)
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Re: 1818: "Rayleigh Scattering"

Postby hetas » Sun Apr 02, 2017 7:35 am UTC

wumpus wrote:I remember reading "Cuckoo's Egg" by Clifford Stoll (interesting if you want to know how the internet was in the 1980s, otherwise tragically dated), where he mentioned his astronomy PhD orals consisted of the one question "why is the sky blue?" It took nearly everything Dr. Stoll learned in astronomy to answer that question [for sufficient levels of detail].


So the answer to the color of the sky question is pHd level stuff.

You have to know your audience. There's not much point trying to give a pHd level answer to a child. I've learned quite a few things reading this thread and to me it seems that "air is blue" is a reasonable answer to the question of the color of the sky. The kid can think about that for a while and come back with follow up questions about sunset and sunrise.

Birds in wings on the other hand is a dirty lie.

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Re: 1818: "Rayleigh Scattering"

Postby One128 » Sun Apr 02, 2017 9:52 am UTC

wbeaty wrote:Nope. You've got the very misconception that this comic is designed to fight.


Not really. There are, however, numerous misconceptions in your response. I'll try to address them one by one.

wbeaty wrote:Ask yourself what material up there is colored blue?

Just because we see a color somewhere, doesn't mean that there is any material colored that way. Our eyes perceive light, not material.

If I blow fog into a room with a fog machine and shine red light on it, the room will appear to be filled with a red haze. The answer to the question "what material is in the room that is colored red" is of course "none". The fog itself certainly isn't red; it's colorless white. To say that the fog "is red", just because we happened to shine red light on it, would completely devoid that statement of any significance - such as a distinction from a genuinely red fog (i.e. one that would appear red under a white light).

wbeaty wrote:Sunlight shines on air, reflected blue light hits our eyes: air is a blue material.


First of all, what hits our eyes when we look at the sky isn't reflected light - it's scattered light.
Second, and more importantly: when light - even white - shines on air, it's not automatically the case that blue light hits our eyes. Often, red, yellow, or other-colored light hits our eyes. 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.

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).

wbeaty wrote:Believing that air is colorless, where the blue color must have some other origin, that's the same basic mistake as believing that bluejay feathers are colorless, or that Morpho butterflies are colorless, or aerogel or opals are colorless.

All of those other examples you cited do appear colored under a diffuse white light. But not air. Air is not blue under a diffuse white light. It's colorless.

wbeaty wrote:Correct question/response would be: Q: Teacher, why is the sky blue? A: because the air itself is blue (but a very dilute blue, a miles-thick layer is needed in order to see the color.)


No. Even if you take a miles-thick layer of air, it still won't appear blue. Not unless you light in a particular way to make it glow blue. (Or red.)

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, no matter how far away you look. Distant mountains won't be blue-hued; they will simply fade into a gray mist. You will see the true color of air: none.

wbeaty wrote:A: because of Rayleigh Scattering (also called Tyndall effect,) and the color of the air can change depending on the angle of light shining on it. If light shines on air from the side, it's blue. If light shines on air from behind, it's orange or red.


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?

If I shine light on my CD in one angle, it will appear to glow blue. If in another angle, it will appear to glow red. With another angle, green. Therefore, my CD is green? No.

wbeaty wrote:it's the air-layer which is blue.

Except that it isn't blue. Not unless it's lighted in a particular way. If it's not lighted in a particular way, it's not blue.

wbeaty wrote:The real trouble is that a mile-thick layer of air is far too thin to detect the color. Try a thicker 20mi layer instead.


Yes. You try that, next time your sky is completely overcast with a thick layer of clouds. Look at some landmark 20 miles away (visibility is often surprisingly good under an overcast sky), and report back to us with what color you saw. Was it blue? Was it red? Any color at all?

wbeaty wrote:Sometimes the blue of the air above us is actually the blue light that's been removed from the "sunset rays,"

Finally I can agree with something - the blue of the sky above is actually the blue light taken from sunshine (but only when the atmosphere is lighted in a particular way). It's not the color of the air in any meaningful sense.

(Just like the colors we can sometimes see on a CD are taken from the source of illumination - but neither of them is the color of the metal layer.)

wbeaty wrote:air is behaving like the special ink seen on USA dollar bills: the ink has a color, but tilt the money, or move your head, and it has a different color.


I agree with this, too. So which color is this special ink - is it green, or is it golden red? It should be apparent that you can't say either. And yet, you've made that fallacy with air - you assigned it the color blue, even though it can just as well appear red. What basis is there for picking blue over red? None.

If we want to find out a color of a material regardless of lighting conditions, we can view it under diffuse white light. Under diffuse white light (shining uniformly from all sides), the ink on a bill (not a US bill, but another similar bill I have at my disposal) appears dirty brown (regardless of angle). Air, under diffuse white light, appears colorless. - This is the best we can come as to the inherent color of the material.

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Re: 1818: "Rayleigh Scattering"

Postby Soupspoon » Sun Apr 02, 2017 10:51 am UTC

wbeaty wrote:After your students accept the idea that no "sky surface" exists, and that the color is coming from the air itself, then we can go to the next level,

And then after all that, Truman Burbank, you find out the truth...
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Re: 1818: "Rayleigh Scattering"

Postby Angelastic » Sun Apr 02, 2017 11:03 am UTC

SuicideJunkie wrote:
The Snide Sniper wrote:I remember this topic mentioned in a What If? footnote.

That's good to know. #141, right?
I merely had an intense deja vu, and came to the thread only to find everybody talking about the topic as if it were completely new.
Thanks for pointing this out; I was sure this had been done before and wondered if he'd repeated a whole comic as some kind of lame April Fools joke.
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Re: 1818: "Rayleigh Scattering"

Postby Solarn » Sun Apr 02, 2017 1:42 pm UTC

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?

Why are so many people puzzled by magneticism but not by gravity?

I find the sky interesting because a small amount of air is not blue, but lots of it together are. Some things just have this property. Maybe children didn't learn that yet.

(sorry, if this becomes a double post)


My answers:

"Why are children interested in the color of sky and trees, but not in the color of other things, like tomatoes, or anything else?" Because there's so much more of the sky and plants than of anything else. If you let them continue, they will eventually arrive at "why are bananas yellow?"

"Why are they content with some answers, but not others?" Children are both extremely perceptive and extremely curious. They can tell if an answer is incomplete and will pursue it with dogged determination. But tell them an obviously bullshit magical explanation that's complete enough, and they will be satisfied.

"Why are so many people puzzled by magneticism but not by gravity?" We don't really experience gravity in our day to day lives. It's something that surrounds us so completely that we take no notice of it, like the fact that the air we breathe is not actually empty space, but a physical thing that we are immersed in. The same can't be said of magnetism.

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Re: 1818: "Rayleigh Scattering"

Postby PinkShinyRose » Sun Apr 02, 2017 1:58 pm UTC

One128 wrote:
wbeaty wrote:Believing that air is colorless, where the blue color must have some other origin, that's the same basic mistake as believing that bluejay feathers are colorless, or that Morpho butterflies are colorless, or aerogel or opals are colorless.

All of those other examples you cited do appear colored under a diffuse white light. But not air. Air is not blue under a diffuse white light. It's colorless.

I think you missed something here. I'm not sure how these scales return blue light, but if wbeaty is right that it depends on a black background that suggests angle does matter in these cases.

What this would suggest is that no material in the skin/feathers of these animals is blue, it is just the skin/feather as a whole that is blue. Just as with the sky, where no material has a colour, it's just that the sky as a whole shows colour depending on the direction of incident light.

The blue colour is an emergent property at a much larger scale than is the case with absorption or fluorescence.

PS I don't think I've seen a bluejay in real life, does the blue look shiny/somewhat metallic, like a peacocks blue?

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Re: 1818: "Rayleigh Scattering"

Postby One128 » Sun Apr 02, 2017 3:09 pm UTC

PinkShinyRose wrote:I think you missed something here. I'm not sure how these scales return blue light, but if wbeaty is right that it depends on a black background that suggests angle does matter in these cases.

What this would suggest is that no material in the skin/feathers of these animals is blue, it is just the skin/feather as a whole that is blue. Just as with the sky, where no material has a colour, it's just that the sky as a whole shows colour depending on the direction of incident light.

The blue colour is an emergent property at a much larger scale than is the case with absorption or fluorescence.


I don't think the level at which a color emerges is significant when it comes to whether something is of particular color. The question is simply whether we can see it to be that color at all. When we put a bluejay under diffuse white light, we can see that it is blue - no matter what the mechanism out of which the color arises. When we put air under diffuse white light, even large amounts of it, we do not see a blue color - we see no color at all.

In the case of the sky, the blue color is an emergent property of the particular way the air is lighted. If the air isn't lighted that particular way, the blue color does not emerge. This makes it not the property of the material itself, but the particular lighting of that material. The air itself does not have any inherent color independent of lighting. (While a bluejay does.)

PinkShinyRose wrote:PS I don't think I've seen a bluejay in real life, does the blue look shiny/somewhat metallic, like a peacocks blue?


I think so, yes. If you're familiar with peacock feathers, note that the pattern always appears blue on the inside and green on the outside, no matter what the lighting or angle. It's not like if you look at peacock feathers outside on a cloudy winter day, the colors just disappear.

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Re: 1818: "Rayleigh Scattering"

Postby wumpus » Sun Apr 02, 2017 4:59 pm UTC

wbeaty wrote:
One128 wrote:Agreed, Randall messed up here. Air is not blue; it's colorless.


Nope. You've got the very misconception that this comic is designed to fight. Without air, the sky would be black. Ask yourself what material up there is colored blue? Not "sky." It's the thick air layer. Sunlight shines on air, reflected blue light hits our eyes: air is a blue material. But that's not the end of the story. Air is a colored material, but it's not a pigmented material.

After your students accept the idea that no "sky surface" exists, and that the color is coming from the air itself, then we can go to the next level, and show that air is behaving like the special ink seen on USA dollar bills: the ink has a color, but tilt the money, or move your head, and it has a different color. Two people 100KM apart, both looking at the same patch of air, can see totally different colors: one sees "blue sky," the other sees "red sunset." (The same thing happens with interference filters. The filter "is red." Tilt it, and the filter "is yellow." What substance "has" the color(s)? The filter, of course.)


Things get even more complicated when you then have to explain why the Sahara looks yellowish from the ISS (especially straight down). If the air is blue, why doesn't *all* of Earth look blue?

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Re: 1818: "Rayleigh Scattering"

Postby ucim » Sun Apr 02, 2017 5:07 pm UTC

What color is a prism?

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Re: 1818: "Rayleigh Scattering"

Postby ShuRugal » Sun Apr 02, 2017 5:09 pm UTC

One128 wrote:<snip>



goddamn i wish i could gild this post.

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Re: 1818: "Rayleigh Scattering"

Postby ShuRugal » Sun Apr 02, 2017 5:14 pm UTC

ucim wrote:What color is a prism?

Jose


what material is the prism made of?

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Re: 1818: "Rayleigh Scattering"

Postby ucim » Sun Apr 02, 2017 5:34 pm UTC

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

Somewhere I have a microfiche whose label is green (in ordinary reflected light), but when put in a fiche reader, that label is yellow (in transmitted light). What color is it?

To oversimplify (which is sometimes enough for insight), only three things can happen to light when it hits something: It can be transmitted, reflected, or absorbed. "Scattered" is a subset of "reflected" (as is "diffracted"), and "refracted" is a subset of "transmitted". Often all these processes yield the same result: A thing absorbs non-green, reflects some of the rest, and transmits what's left. It's perceived as green from all angles. But sometimes that's not the case, such as my microfiche label. The answer to "what color is it?" isn't simple atomic. (And I'm ignoring the issue of how color is perceived and processed by the eye and brain, which is fascinating in itself).

In the case of air, light is reflected in different directions depending on its wavelength, and you are viewing the object from within the object itself. So, what you see depends on the angle between the part of the air you're looking at, and the light source.

In the case of a prism, light is transmitted in different directions depending on its wavelength. Strictly speaking, you are seeing the light source through the prism, but that's true of all transmitted light. The prism appears {whatever color of the rainbow you shadow}.

So... if a thing appears {color}, it's not always meaningful to say that the thing is {color}.

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Re: 1818: "Rayleigh Scattering"

Postby JohannesWurst » Mon Apr 03, 2017 2:44 am UTC

I agree with this, too. So which color is this special ink - is it green, or is it golden red? It should be apparent that you can't say either. And yet, you've made that fallacy with air - you assigned it the color blue, even though it can just as well appear red. What basis is there for picking blue over red? None.


In the case of the sky, the blue color is an emergent property of the particular way the air is lighted. If the air isn't lighted that particular way, the blue color does not emerge. This makes it not the property of the material itself, but the particular lighting of that material. The air itself does not have any inherent color independent of lighting. (While a bluejay does.)


So... if a thing appears {color}, it's not always meaningful to say that the thing is {color}.


Yeah. I think this is the problem. Some people would indeed say that white powder in red light is red, that a CD is commonly silver (shiny grey?) but sometimes other several other colors and in that sense air is blue. What basis is there for picking blue over red? Just that it is more commonly blue, I would say. I'm not saying anyone is right, but it becomes a problem of language instead of physics.

Is a ripe banana viewed through blue glasses green or yellow?

It's a language problem. I'd say both answers are valid in a way. I tend to green because who am I to say that transparent glasses are more objective than blue glasses (or certain angles at which I look at a prism). On the other hand I saw a TV show about optical illusions and they made the point that it is actually helpful that humans can't see objectively, but build a model of the world in their mind. (Think about the blue-black/white-gold dress.) For example In such a mental model a banana might still be yellow, because you know bananas are "normally" yellow.

----

What is an "emergent property"? Can you define that? Is maybe any physical property "emergent"? That is an actual question. I'm not claiming to be an expert.

(In object oriented programming there are attributes and methods. Is an emergent property like a method? But we can't know the source code of the universe.)

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Re: 1818: "Rayleigh Scattering"

Postby da Doctah » Mon Apr 03, 2017 3:19 am UTC

JohannesWurst wrote:What is an "emergent property"? Can you define that? Is maybe any physical property "emergent"? That is an actual question. I'm not claiming to be an expert.

(In object oriented programming there are attributes and methods. Is an emergent property like a method? But we can't know the source code of the universe.)


I'll take the first stab at it. An "emergent property" is anything that comes out of a system without having been put in. Artificial intelligence qualifies if you weren't trying to create something intelligent. In the original context of this thread, color is an emergent property when you use only monochromatic elements (I'm picturing the rainbows that appear in moiré patterns that aren't themselves colored, or the black-and-white disc that produces color when you rotate it).

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Re: 1818: "Rayleigh Scattering"

Postby PinkShinyRose » Mon Apr 03, 2017 3:43 am UTC

JohannesWurst wrote:What is an "emergent property"? Can you define that? Is maybe any physical property "emergent"? That is an actual question. I'm not claiming to be an expert.

(In object oriented programming there are attributes and methods. Is an emergent property like a method? But we can't know the source code of the universe.)

It's a property that doesn't just follow from somethings parts, but only occurs because of how these parts interact.

I know too little about programming to come up with a good programing example.

In our example it's like how a butterflies skin blocks light, so light can only hit it's scales from one direction; thereby causing the skin with the scales to be blue, while the scales themselves may very well not be.

Or in the case of the sky: I was arguing the air itself is not necessarily blue, but when arranged as a huge ocean of air over us with a nearly point light source from outside it, the sky as a whole becomes blue.

PS: ninja'd, but I'm still posting since I'm also elaborating on what I was trying to say earlier.

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Re: 1818: "Rayleigh Scattering"

Postby chenille » Mon Apr 03, 2017 5:02 am UTC

JohannesWurst wrote:I think this is the problem. Some people would indeed say that white powder in red light is red, that a CD is commonly silver (shiny grey?) but sometimes other several other colors and in that sense air is blue.

But this is exactly where people normally distinguish the colors of particular instances from the colors of the materials. I'm happy to say the powder in the red room becomes red, but I doubt anyone would claim that salt or sugar are therefore red. Many birds have blue feathers, but it comes from structure, and the materials in them like keratin and melanin are not considered blue. A rainbow shines with the whole spectrum but the water that generate it is nearly clear (faintly bluish).

Likewise nobody has objected to saying the color of the sky – you know, when it isn't overcast or night or anything like that – is blue or azure. The question is, why would you attribute that to the air as a material, rather than to the way the atmosphere and lighting are put together? I've argued that's an obvious mistake, given that air acts opalescent rather than consistently blue, and in my opinion other people like One128 have done a great job explaining why in detail.

There actually is an exception: ozone is faintly bluish, and I've heard that's important to the color of the atmosphere as seen from space. But then let's be clear, that material color doesn't actually provide the blue color of the daytime sky to any significant degree. That comes from the way light is scattered by the air, making things blue in some contexts and red in others, as I think everyone agrees but I guess not everyone thinks is fair to share with children?

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Re: 1818: "Rayleigh Scattering"

Postby One128 » Mon Apr 03, 2017 6:04 am UTC

JohannesWurst wrote:Yeah. I think this is the problem. Some people would indeed say that white powder in red light is red, that a CD is commonly silver (shiny grey?) but sometimes other several other colors and in that sense air is blue. What basis is there for picking blue over red? Just that it is more commonly blue, I would say. I'm not saying anyone is right, but it becomes a problem of language instead of physics.

Is a ripe banana viewed through blue glasses green or yellow?


I agree that language can be ambiguous. 'Is' can be used to denote both inherent properties and temporary status or appearance. This isn't, of course, limited to colors. When I say 'my arm is missing', I could be stating that I am an amputee, or I could be complaining about the way a photograph of me is framed. - So how do we differentiate between the two? Context, of course.

If we hear "air is heavy with the scent of jasmine", we can understand that the speaker is describing some particular situation and not inherent properties of air in general. If we hear "air is blue", is the speaker talking about an inherent coloration of air, or about how air appears at the moment?

If we look at the comic, it's apparent that the latter interpretation is unsustainable. "Air is blue" in the sense of "air appears blue right now" does not make sense when substituted into the comic. It carries no explanatory powers whatsoever. If you look up on a clear day and ask, "Why is the sky blue?", and I answer, "Because when you look at the air above you right now, it appears blue", I don't think you're going to be very satisfied. This interpretation would be a true tautology and wouldn't explain anything at all.

On the other hand, "air is blue" in the sense of "air is an inherently blue material" does carry explanatory powers, and at least makes sense when substituted into the comic. Unfortunately, it's also false, because air is not an inherently blue material. But hypothetically, if it were, it would indeed explain the two things the comic mentions: why the sky appears blue on clear days, and why distant mountains appear blue on clear days. Alas, it fails to explain all those other things the comic does not mention: why the sky is red at sunset, why the stars and the moon don't appear blue like those distant mountains (particularly when near the horizon), or why distant objects don't actually appear blue when it's overcast. Those observations directly disprove the notion that air is an inherently blue material - or, as the comic puts it, that "air is blue". This meaning is clearly the one implied by the comic, so this is what we're discussing.

(As chenille mentions, strictly speaking air may have some very slight coloration due to components like ozone, and probably various local pollutants as well, but that's not causally related to the observed variations in sky color. Unless you live in Beijing, of course.)

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Re: 1818: "Rayleigh Scattering"

Postby orthogon » Mon Apr 03, 2017 8:33 am UTC

PinkShinyRose wrote:
JohannesWurst wrote:What is an "emergent property"? Can you define that? Is maybe any physical property "emergent"? That is an actual question. I'm not claiming to be an expert.

(In object oriented programming there are attributes and methods. Is an emergent property like a method? But we can't know the source code of the universe.)

It's a property that doesn't just follow from somethings parts, but only occurs because of how these parts interact.

I know too little about programming to come up with a good programing example.


I think in programming it's known as a bug.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1818: "Rayleigh Scattering"

Postby Heimhenge » Mon Apr 03, 2017 8:49 am UTC

orthogon wrote:
PinkShinyRose wrote:
JohannesWurst wrote:What is an "emergent property"? Can you define that? Is maybe any physical property "emergent"? That is an actual question. I'm not claiming to be an expert.

(In object oriented programming there are attributes and methods. Is an emergent property like a method? But we can't know the source code of the universe.)

It's a property that doesn't just follow from somethings parts, but only occurs because of how these parts interact.

I know too little about programming to come up with a good programing example.


I think in programming it's known as a bug.


I guess you could say that a "bug" is an emergent property from a program, but a better example is the classic Game of Life by John Conway. Play it here, choose a pattern, and watch for emergence: https://bitstorm.org/gameoflife/

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Re: 1818: "Rayleigh Scattering"

Postby airdrik » Mon Apr 03, 2017 2:30 pm UTC

Virgin Atlantic must have been reading XKCD when they came up with http://www.virginatlantic.com/global/en ... nergy.html on Saturday (I'm kind of surprised it wasn't brought up before)

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Re: 1818: "Rayleigh Scattering"

Postby chridd » Mon Apr 03, 2017 3:26 pm UTC

One128 wrote:If we hear "air is heavy with the scent of jasmine", we can understand that the speaker is describing some particular situation and not inherent properties of air in general.
Actually that does sound to me like the person is talking about air in general; it would need a the to be talking about some specific instance of air ("the air is heavy with the scent of jasmine", "the air is blue"). Of course, in your example, I'd probably interpret it under the assumption that the person misspoke and meant to include the, unless there was context or clarification saying otherwise. (This is more spacial rather than temporal, though; if all air the the universe temporarily became heavy with the scent of jasmine, then "air is heavy with a scent of jasmine" would be true.)
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Re: 1818: "Rayleigh Scattering"

Postby SuicideJunkie » Mon Apr 03, 2017 4:32 pm UTC

Air is heavy with the scent of Jasmine. Therefore, air is lighter without the scent. Marketing has promised to deliver our new, lighter Air (tm) by next quarter, so get to work!

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Re: 1818: "Rayleigh Scattering"

Postby Quizatzhaderac » Mon Apr 03, 2017 8:56 pm UTC

JohannesWurst wrote:What is an "emergent property"? Can you define that? Is maybe any physical property "emergent"? That is an actual question. I'm not claiming to be an expert.

(In object oriented programming there are attributes and methods. Is an emergent property like a method? But we can't know the source code of the universe.)
For a computer model of a physical system with an emergent property it's like a method that needs to look at a broader context then the immediate object it is in. When we ask the sky object what we see when we look at a particular point in it, it needs to know about the color and relative location of the light source (the sun). The color "emerges" from the specific combination of the attributes in separate objects.

That's assuming the programmer understood and deliberately wanted to model a system with an emergent behavior and record it.

Within the context of programming "emergent behavior" is something that is very difficult to determine from the pieces of code. Most best practices are focused on eliminating accidental emergent behaviors.

An example of intentional emergent behavior in programming would be an MMORPG that that has coded mechanics specifically to create buy and sell orders at market. As a consequence of those mechanics, human behavior, and the laws of economics: equilibrium prices are created, even though nowhere is the code does the phrase "equilibrium price" exist.
The thing about recursion problems is that they tend to contain other recursion problems.

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Re: 1818: "Rayleigh Scattering"

Postby JohannesWurst » Tue Apr 04, 2017 12:33 am UTC

One128 wrote:If we look at the comic, it's apparent that the latter interpretation is unsustainable. "Air is blue" in the sense of "air appears blue right now" does not make sense when substituted into the comic. It carries no explanatory powers whatsoever. If you look up on a clear day and ask, "Why is the sky blue?", and I answer, "Because when you look at the air above you right now, it appears blue", I don't think you're going to be very satisfied. This interpretation would be a true tautology and wouldn't explain anything at all.


I know, I said myself that this would be a tautology, so maybe I get what you are saying.

There is one new information though: The sentence "Air is blue", reminds someone that air is even a thing. That's something little children don't know and what is unintuitive for adults.

Maybe the puzzling thing is "How can emptiness have a color?". Then "The sky isn't actually empty." would be a good enough explanation of the phenomenon.

I remember that I was a bit shocked to learn as a kid that we basically dive at the bottom of an ocean of air. Or that you can extinguish a candle by covering it with a glass.

----------------

Someone mentioned emergent properties with the game of life. Is "inherent" the opposite of "emergent"?

Elsewhere I read that our universe could be simulated with Wolframs rule 110. (xkcd 505)

That's what I mean with "Isn't everything emergent?". In rule 110 the color of bananas isn't "inherent" anywhere surely? There aren't even bananas or any physical objects "hardcoded" in a 110 cellular automaton. Is "hardcoded" the same thing as inherent?
Maybe the ultimate reason why bananas are yellow isn't rule 110, but something with the four fundamental forces or string theory. In any case I can't imagine that "Bananas are yellow because their material is yellow." or even "Bananas are yellow." is "hardcoded".

A banana doesn't appear yellow under every* possible circumstance.
Air doesn't appear blue under every possible circumstance.
As Randall wrote in What If? 141:
Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons


I can imagine very well, that it is practical to make a distinction between different ways colors can arise, though.

----------------

Please tell me if I'm being annoying. I appreciate your previous explanations. When I talk with someone about the color of air next time, I will be able to take the opposite stance of whatever his opinion is, thanks to you all! :twisted:

*: I hope I used "every" correctly here. In the German language it's sometimes switched around with "any".

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Re: 1818: "Rayleigh Scattering"

Postby airdrik » Tue Apr 04, 2017 2:34 pm UTC

Compare 435: Purity. As you move from right to left, it is emergent properties from the field to the right that form the basis of the field to the left. Physics forms the baseline of Chemistry, but nothing in Physics defines anything about the materials and reactions described in Chemistry. Thus Chemistry arises from emergent properties of the physical system. Similarly Chemistry forms the baseline of Chemistry, but nothing in Chemistry defines anything about how biological systems work. Thus Biology arises from emergent properties of the chemical systems. Etc.
While we may say that all systems are emergent over the basic laws of physics, it isn't useful to only study physics and model everything as interactions of fundamental particles; much like just because all programs eventually get compiled down to machine code where all behaviors become emergent (nowhere in the machine code does it know anything about displaying a checkbox) doesn't mean all programmers should do all of their programming using machine code. Instead we study Chemistry and the laws and behaviors around that as though those are inherent to Chemistry, not as though they are emergent from the laws of Physics. Similarly we study Biology and the laws and behaviors around that as though those are inherent to Biology, not as though they are emergent from the laws of Chemistry.

As for why Bananas are yellow (or any other color), there are a few different answers to the question depending on the perspective taken (and this makes this question a more interesting one than the color of the sky which has less reasons). From a physical perspective, bananas appear yellow because their surfaces reflect more yellow light and absorb more blue/purple light. From a Chemical perspecitve, bananas are yellow because their skins are composed of materials which have the aforementioned physical properties. From a biological perspective, bananas are yellow because over the course of evolution and natural selection, yellow appears to be the generally preferred color for such a fruit. That last reason can be further expounded upon in a number of ways including relation to colors of other fruits in the regions where bananas grow, visual perception of animals in those regions, the chemicals involved in producing the yellow color which may produce some other benefit, etc. From another biological perspective, bananas appear yellow because our eyes and other systems involved in visual perception evolved to focus in on a specific range of useful colors because they provide some optimal range of perception and acuity; and within that range of colors, bananas appear in the portion of that range that our brains interpret as yellow. From a linguistic perspective, Bananas are yellow because "yellow" is the word that was chosen to describe the color of bananas and other similarly-colored objects (which is probably the least-useful explanation you could give). There's also the theological perspective: Bananas are yellow because $deity decided they should be that color, "Because Allah loves wondrous varieties!" (most likely for the aforementioned biological reasons, using a combination of physics, chemistry and biology to direct bananas to be the color they are).

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Re: 1818: "Rayleigh Scattering"

Postby HES » Tue Apr 04, 2017 2:44 pm UTC

airdrik wrote:From a biological perspective, bananas are yellow because over the course of evolution and natural selection, yellow appears to be the generally preferred color for such a fruit.

Although in the case of the banana, the selection wasn't so natural.
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Re: 1818: "Rayleigh Scattering"

Postby orthogon » Tue Apr 04, 2017 3:49 pm UTC

airdrik wrote:Compare 435: Purity. As you move from right to left, it is emergent properties from the field to the right that form the basis of the field to the left. [...]

Wow. This describes succinctly how everything is physics, and yet at the same time everything isn't physics. I'd never thought of emergent properties being the concept that squares that circle, but it so is.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1818: "Rayleigh Scattering"

Postby pscottdv » Wed Apr 05, 2017 11:20 am UTC

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?

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Re: 1818: "Rayleigh Scattering"

Postby pscottdv » Wed Apr 05, 2017 11:35 am UTC

Solarn 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?

Why are so many people puzzled by magneticism but not by gravity?

I find the sky interesting because a small amount of air is not blue, but lots of it together are. Some things just have this property. Maybe children didn't learn that yet.

(sorry, if this becomes a double post)


My answers:

"Why are children interested in the color of sky and trees, but not in the color of other things, like tomatoes, or anything else?" Because there's so much more of the sky and plants than of anything else. If you let them continue, they will eventually arrive at "why are bananas yellow?"

"Why are they content with some answers, but not others?" Children are both extremely perceptive and extremely curious. They can tell if an answer is incomplete and will pursue it with dogged determination. But tell them an obviously bullshit magical explanation that's complete enough, and they will be satisfied.

"Why are so many people puzzled by magneticism but not by gravity?" We don't really experience gravity in our day to day lives. It's something that surrounds us so completely that we take no notice of it, like the fact that the air we breathe is not actually empty space, but a physical thing that we are immersed in. The same can't be said of magnetism.


I like your answer for what makes a child content with an answer, but I think you got the explanation of why they find magnetism interesting wrong. We experience gravity all the time so it feels normal to us. Magnetism is outside our usual experience so it feels weird. Children are interested in things that feel weird whether it be magnetism or toy slime.

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Re: 1818: "Rayleigh Scattering"

Postby hetas » Wed Apr 05, 2017 5:41 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?


Trees are brown in children's books?


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