In another thread, someone linked this site; one of the pages there says:
It strikes me that it might just be a semantic dodge (like saying that bananas aren’t yellow, they just reflect yellow light.
Anyway, how rigorously correct is the linked page?
Well, without going into how rigorous that page is, it is true to say that air is blue, at least when cooled so that it becomes liquid.
The blue colour comes from the oxygen: liquid oxygen itself has a definite blue colour. Here is a page with a picture of a test tube of liquid oxygen.
I do not believe that this is the reason for the blue sky, though. It might contribute to the colour, I suppose.
[url=http://radio.weblogs.com/0101365/2003/11/12.html]This page* seems to suggest pretty clearly that the blue colour of liquid oxygen is not the cause of the blue sky, though.
That’s really cool, r_k. I’d heard that liwuid oxygen is blue, but I’ve never seen a picture of it.
It’s beautiful.
That’s my page. I’m a research engineer at the UW, but my training is in electronics design and physics education is just a (major) hobby.
Yes, there is a semantic dodge here, but its in the textbooks. They say that air scatters blue light, yet they never simply come out and say what this implies: that air is a blue substance if observed against a dark background.
Actually, the whole point of the article is this question: how do you explain advanced physics concepts to little kids so that they come away with some useful knowledge? (Note that the page is about grade-school science textbook errors.)
As a former kid, I know that the younger kids think that air is invisible. The sophisticated ones can tell you that air is “transparent,” and point to the lens-like ripples over a hot fire. But if a kid is to understand where the blue color of “sky” comes from, these facts become misconceptions.
Air is not transparent, instead it is a weak scatterer like a very dilute fog. As a “fog” it behaves like bluish smoke or like a few drops of milk mixed in water: hold it up to a white light source, and the light source looks orange, but hold it up to a dark background and illuminate it from the side, and it looks bright blue.
Put this all in a sentence that a 4th-grader can understand: AIR IS NOT TRANSPARENT, INSTEAD IT IS BLUE, AND THAT’S WHY THE SKY IS BLUE.
Obviously air is not blue in the same way that paint or dye is blue. And that’s where the Rayleig scattering or Tyndall effect explanations finally come in. Yet these explanations are useless to most kids, since those kids are convinced that air is perfectly transparent, and the new explanations don’t alter their conviction.
For those who want to argue about this, here’s another question. Should we say that bluejay feathers ARE blue? Or do they only scatter blue light? After all, bluejay feathers generate the blue light via nanostructures which generate interference effects. There are no blue dyes present, the materials involved are transparent. The same question applies to opals: are they bluish and rainbow colored? Opals are made of regular arrays of transparent nanospheres in a transparent matrix. No blue dyes.
As for liquid oxygen, I suspect that the color comes from absorbtion bands (like blue dye) and has nothing to do with the blue of the sky. Liquid nitrogen appears water-clear. (Heh. But then I’ve never seen a twenty foot deep pool of the stuff.)
I’m sorry bbeaty, but I still don’t like it because by that definition, it’s equally correct to say “sunset is red because air is red.” So it’s both red and blue? It can be both?? Is it really productive to such an over-simplified explanation instead of introducing the concept of wavelength-dependent scattering properly? Or if that’s too complex, just saying “it’s a complex issue” for now?
I also don’t like the statement “Astronomy is not an experimental science” but maybe that doesn’t belong in this thread…
Sounds like you didn’t read the article being discussed?
Yes, in kid-speak we would say that air is blue AND red. Then we have to go looking for other materials which do this too. For example, some smoke is both white and orange (white when illuminated from the side, orange when we view light sources behind it.) Then there are the color-changing inks on modern currency. This could lead us into an investigation of “white,” of snow for example which is made of clear bits of ice, or white paint which contains nothing but tiny clear crystals. Where’s the white color? Replacing all of this with the phrase “snow is white because of optical scattering” gives kids a meaningless phrase to memorize. They didn’t learn any of the concepts because they never asked questions needing answering. It HALTS all their questions rather than encouraging them to learn.
In my experience, thoughtless pursuit of “the single right answer” will get you good grades while ruining your ability to think. Too often the genuinely right answer is an essay, a network of connected concepts, not an answer that could fit on any test. If kids want good grades, they should definitely stop thinking and instead memorize their textbooks, and stay very far away from webpages like mine which might make them (gasp) point out their teachers’ errors, and worse, set them on the path to critical thinking and to science as a profession.
Heh. Don’t get me started. TOO LATE!
Sounds like you didn’t read the article being discussed?
Yes, in kid-speak we would say that air is blue AND red. Then we have to go looking for other materials which do this too. For example, some smoke is both white and orange (white when illuminated from the side, orange when we view light sources behind it.) Then there are the color-changing inks on modern currency. This could lead us into an investigation of “white,” of snow for example which is made of clear bits of ice, or white paint which contains nothing but tiny clear crystals. Where’s the white color? Replacing all of this with the phrase “snow is white because of optical scattering” gives kids a meaningless phrase to memorize. They didn’t learn any of the concepts because they never asked questions needing answering. It HALTS all their questions rather than encouraging them to learn.
In my experience, thoughtless pursuit of “the single right answer” will get you good grades while ruining your ability to think. Too often the genuinely right answer is an essay, a network of connected concepts, not an answer that could fit on any test. If kids want good grades, they should definitely stop thinking and instead memorize their textbooks, and stay very far away from webpages like mine which might make them (gasp) point out their teachers’ errors, and worse, set them on the path to critical thinking and to science as a profession.
Heh. Don’t get me started. TOO LATE!
And as I understand it, an individual hair of a polar bear is not white but rather is clear and about the same “color” as ice.
A case can be made for saying that air is not blue.
But if air is not blue, then to be consistent, we’d have to say that snow is not white (and neither is paper, wood, clouds, steam, white paint, plastic, etc.) In a similar way that white steam exists, blue air exists. Either both are “merely scatterers” and posses no color of their own. Or both are colored.
Or to reverse the reasoning, the existence of the concept called “white” sets a very strong precedent which justifies our calling air “blue.”
Is “blue” a property of the air? Sure, as much as any color is the property of a substance (after all, color is all in your mind, so to be truthful we shouldn’t say that ANY material posesses color.) On the other hand, the blue of the air is not fixed. When you’re observing a red sunset the red sky is not blue, and when you’re observing the blue sky the blue air is not red. The color depends on the viewing angle, so in the evening the same patch of air produces “blue sky” for people below it, and “red sky” for people downrange along the axis of the sunlight.
So, air is not perfectly transparent, instead it resembles the variable-color inks now used on US currency.
Well, first, I have seen a 20 gal. aquarium full of LOX and I can testify that it is definately a fascinating blue color… very much like the sky captured in a fishtank. (I kept my distance though… the first time those fools decanted that much LOX they used a styrofoam ice chest. Idiots.)
And second, to answer “the sky is blue because it really is blue” isn’t going to satisfy an inquiring mind. It may be an interesting conclusion to a scientist, but to them it’s a tautology, and even if they don’t understand that $32 word, they’re still going to be unsatisfied.
The easiest way I’ve been able to explain why the sky is blue to someone without getting into details about wavelength and scattering and lambda-to-the-fourth-power is to have them compare the color of the sun’s orb at noon to the color of the orb at sunset. The person needs to understand additive color theory, and know about how white light is comprised of the colors of the rainbow mixed together.
Show them that at sunset, the red and orange light is coming straight to your eye… the disc of the sun is orange, or even red, so the light must be coming straight from the sun to your eye. But during midday, the sun’s orb is nearly white. What’s happening? Aha - that blue light isn’t getting straight to our eyes, it’s going somewhere else. Make sure they grasp this point.
At this point, you can show or draw for them a globe illuminated by a point source, or a picture of the earth’s terminator from space, and show them where the people are that are seeing sunsets and sunrises, and where the sun is high in the sky. This isn’t necessary, but it does illustrate the geometry in case it puzzles your pupil. You can even illustrate the different atmospheric path lengths for sunlight at different times of day.
So then if the blue light isn’t getting to your eye, where is the blue going? The blue of the day sky is the blue light from the sunsets and sunrises at other parts of the world where the sun is at the horizon. It’s bouncing around off the atmosphere… the blue reacts different to the air, and bounces around a lot. Introduce the word ‘diffusion.’
To the blue light, the air is milky like dishwater or fog, and so it bounces around, diffuses, eventually bouncing off the air over your head into your eye. But to the red and orange light, the air is more clear, so it can pass straight thru, even at sunset and sunrise. It still bounces around a little, and so that’s why the clouds and sky are orange and red only at sunrise and sunset.
That usually either makes them think for a while… or complain it’s too hard to understand… or else, if they really weren’t interested, just say “oh, OK.” (Or, in the case of my fiancee, think “my sweetie’s so SMART, he can explain why the sky is blue!” 
)
You can tell by someone’s reaction to that lesson whether they are really inquisitive about nature or not.
PS - Yes, the photons from the midday blue sky don’t travel a quarter way around the globe, but this explanation illustrates the logic, not technical accuracy.
Variable color inks??? Viewing angle??? This seems like a roundabout way to give a non-explanation to something that’s not very hard to explain the right way. You don’t have to get into the physics of scattering, wavelengths to the fourth power, and all that, just to note that the air bounces around blue light more than other colors. And when you’ve finished with this explanation, the kids will have an appreciation for why the sky is blue but the setting sun looks orange, other than “Mr. Beaty said so.”
Somewhere in there I also got the answer to something I was curious about, which is why the sky is a lighter blue sometimes and darker others, becuase from what I understand, the amount of moisture int he air causes it to diffuse even mroe, and have more to go through to get to my eyes. Right?
The variable color inks is an analogy for the folks here, not intended for the kids.
One thing that’s probably not clear. I’m not a grade-school teacher. I’m relying on my vivid memories of my world as an eight year old, and also on explanations tried out on my own daughter. The standard explanation didn’t work on her. She came away with the idea that there’s a giant prism somewhere which sends out blue sunlight. (A sensible theory when one knows that the only way to separate the white light is to use a glass prism.) The idea that white light is made of colors is almost too sophisticated for fourth grade, and we certainly cannot build on that concept until it has had a long time to soak in (until later grades.)
Your explanation is perfectly obvious, even trivial, to you and me. It MIGHT be obvious to a thirteen year old. What happens when you try it out on much younger kids, say fourth-graders or even below? Doesn’t work. “Daddy, why is the sky blue?” They don’t get it. Saying that air bounces the blue light around is a VERY sophisticated concept for grade school, and if you just give them that sentence, they’ll just have to memorize it for the test, since they cannot understand it. What do we do? Tell them that the explanation is too complex for them to understand, and they should wait for seventh grade? Like hell. Just craft an explanation which they CAN understand, but (and this is important) which doesn’t give them misconceptions which they must unlearn later on.
Why can’t we just tell them that blue light is bounced around? For one thing, they know, deep down in their souls, that air is totally transparent, so any descriptions which talk about light bouncing around will make no sense at all. That’s the whole point of the excercize with the jar full of clear water from a brown muddy river. It’s a hands-on demonstration that clear stuff can add up to become colored stuff. Another point: they believe in a thing called “The Sky.” Why is “The Sky” blue? They think it’s a kind of solid ceiling, and they’re asking who painted it that color. That’s what it looks like when you go outside, no? Nobody has ever told them that “The Sky” is not a solid ceiling. The answer to “why is the sky blue?” does not involved bouncing blue light. It involves first telling kids that “The Sky” doesn’t exist. It involves telling them that air is not invisible; and that when you shine some light on it, you can see the air. It’s blue foggy stuff.
On the other hand, I’m not certain that things like the jar full of river water are simple enough. It’s a metaphor, an analogy, and below a certain age the kids can’t handle such things at all. I’m fairly certain I could handle them in fourth grade. I was very aware of the strange fact that a jar of water from an opaque muddy pond was clear. I just needed someone to tell me that air has a similar effect: a thin layer looks completely transparent, but a thick layer is a misty fog, a fog which is blue rather than white.