That’s Liquid Oxygen. If you look very closely you’ll see that LOX is a light baby blue color very, very similar to the color of a clear blue sky. Even though oxygen isn’t the most abundant gas in the atmosphere, I often wonder if the main reason the sky is blue is that oxygen is blue.
Defending the liberal tradition in history, science, and philosophy.
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The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.
However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.
As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.
Kevin D for the win!
Half of the credit goes to the source I shamelessly copied and pasted my above response from. I knew the answer had to do with the scattering of light in the air, and that the wavelength of blue light was such that it scattered more than the other wavelengths, but I couldn’t articulate it well.
So, I searched and stole from here.
Damn. Â Missed it by an hour…
When I was stationed at Clark AB, Philippines, my dorm was located just across the street from a trailer-sized tank of LOX. During a group inprocessing meeting with the First Sergeant one of the guys asked if the LOX was too close and what would happen if it blew.
"Don’t worry, you’ll be dead so quick you won’t feel a thing."
I don’t see the blue that you apparently do, unless you are talking about the hottest area of the flame.
I could be very wrong (trying to think back to college astronomy, chemistry and such), but the color of fire also has a lot to do with its temperature, not necessarily its chemical composition, although that plays a large part. They use copper chloride, for instance, to make blue flame for fireworks.  Â
In terms of color/temperature of a fire, blue is the hottest (not because it’s burning more oxygen); white is next, then yellow, orange, red.  http://www.astrophysical.org/starclassification.php
The best simple explanation I ever saw for why is the sky is blue is: the sky is blue because air is blue. You don’t notice it at everyday distances, but it is.(And air is blue because of Rayleigh scattering, as Kevin says.)
The "scattering effect" is an explanation I’ve heard, I just always found it dissatisfying, because obviously not all atmospheres have blue skies. It also always seemed like saying “blueberries are blue because they reflect blue light.” Well, okay then….
Oh, Cardeblu, forgot your comment: watch the video again, and watch as the guy is pouring the stuff into the bucket before he lights the fire. To a lesser extent you can also see it in the vapor around his feet before he lights it.
That’s what LOX looks like: a pleasant light blue.
Ah, I see. You meant the actual LOX, whereas I was concentrating more on the effect of the fire.
Never mind…
Well, technicaly Dean, you’re correct. It’s more than "just" scattering (though I might argue that our experience encompasses very few atmospheres for comparison — Mars and maybe Venus are the only other examples I can think of, and I’m not sure we have useful atmosphere shots from Venus). What colors get scattered and to what degree will depend on a lot of factors. Chief among them are the composition of the atmosphere and the density. And then, of course, there’s the spectrum of the star itself.
My optical physics is weak (zach., where are you when we need you?), but I believe the relevant phenomenon are reflection, absorption, and transmission. And then different materials will have different degrees of reflection, absorption, and transmission across the spectrum, based on their spectral characteristics.
Basically, each element has natural "shells" where its electrons can "orbit" (we’re stretching both analogies and my memory here, so please be gentle). An electron can be made to "jump" from a lower shell to a higher shell, but only by absorbing a photon with just the right frequency to make up the energy difference between those shells.
But if that higher shell’s not a "natural" state for that electron, it "wants" to drop back to the lower-energy state. To do that, it must emit a photon of the same frequency as it abosrbed.
Now here’s the part I’m vague on. I can’t remember why, but some electrons quickly reemit the photons more or less backwards along the original path (reflection), or in a random direction (scattering); while some let the photons pass along the original path (transmission). While I don’t remember the cause, I know it varies based on chemical make-up.
So it’s entirely plausible that oxygen has high reflectivity in the blue range, and that this contributes both to scattering and to the blue color you see in LOX. I’m not a spectroscopist, so I’m having a hard time finding the answer.
If oxygen has high reflectivity in the blue range, that’s basically the same as saying that oxygen is blue, and what Jerry said (and what I was thinking) is fairly correct: air is basically blue, you just can’t see it unless you’re looking through a lot of it.
I imagine it would look different lit from different angles, though, which is why the scattering effect would still be useful information.
I’m just WAGging, here, so if someone has a firmer answer I’m all ears.
Oh, "air is basically blue" is a good summary; I’m just not sure how much of that is oxygen’s contribution. If I couldn’t find the reflection spectrum for oxygen (and believe me, I tried), then I’m completely outa luck when it comes to nitrogen. It may be that nitrogen contributes far more blueness. I can’t figure that out.
Aha! I found an article here:
http://www.sky-watch.com/articles/skyblue.html
To quote:
"The atmosphere around the Earth is largely made up of two colorless gases: oxygen and nitrogen. Red and blue light reacts very different from each other to oxygen. Because the wavelength of blue light is roughly the size of an atom of oxygen, blue light interacts with the oxygen and is scattered by it, while red light, with its longer wavelength, goes right pass the oxygen atoms. If the Earth had no atmosphere, the sun’s light would travel directly from the Sun in a straight line towards our eyes and we would see the Sun as a very bright star in sea of blackness. But because the Sun’s blue light is scattered by the oxygen in the atmosphere, blue light from the Sun enters our eyes from all sorts of different angles and we see the entire sky as blue. The atmosphere scatters violet light even more effectively, but our eyes are more sensitive to blue. Wherever we look towards the sky, some light is bouncing off an oxygen atom and entering our eyes, making the sky appear to be blue."
So, in short, it is the oxygen, and oxygen is basically blue.
I think the air is about 70% nitrogen and about 20% oxygen and about 10% everything else (that’s off the top of my head, nothing exact), but this seems to indicate that the nitrogen and others are not particularly relevant, the oxygen is what’s relevant and, indeed, oxygen is basically blue.
Woohoo! Score one for the Deanster!
Dean,
i think you’re fundamentally misunderstanding the phenomenon here. air is essentially colorless, as you can see if you take a bottle of it and look through it. the reason why the sky is blue has nothing to do with transmission or reflection properties of the gases. it has to do with the fact that the scatterers in the atmosphere are basically single atoms, which can be well described by rayleigh scattering (scattering from dipoles), which as kevin said, is much stronger for short wavelengths (blue) than long (red).
consider if there was no scattering: the only area of the sky that would be lit would be where you can see the sun. if you looked anywhere else, the sky would be black (due to it being unlit). the only way light can get to your eye from anywhere else in the sky is if the sunlight is scattered to that area of the sky, and then scattered from the sky to your eye. thus the area of the sky that’s blue consists of only scattered light, which is overwhelmingly made up of the short wavelengths. this effect has nothing to do with the gas involved, only with the size of the scatterer.
the article you found is basically wrong, or at best misunderstanding the physics involved.
zach., how is the size of the scatterer not a function of the gas involved? I’m confused.
If your lox is blue, you should throw it away–it’s supposed to be sort of orangish-red. It’s good with a shmear and a little bit bagel.
Dean,
You say oxygen is blue but the article you quoted says it’s colorless.
So, the explanation is still the one you don’t like. Blueberries are blue because they reflect blue. The sky is blue because oxygen atoms scatter (reflect) blue.
I don’t see why your so pumped when you just repeated the argument you didn’t like with different words.
Indeed, the very argument I posted.
Martin,
because the crucial criterion for rayleigh scattering to apply is that the scatterer be small compared to the wavelength. this is true for oxygen, nitrogen, and just about every other gas we commonly encounter. once this criterion is met, the scattering cross section of the scatterer versus wavelength (basically how strong the molecule scatters a given wavelength of light) follows a simple power law dependency, with the scattering strength going as 1/(wavelength^4).
Zach: Is it me who’s misunderstanding, or you? I read an article by a physicist that says the same things. Oxygen atoms scatter blue light, i.e. the blue light bounces off them and scatters, creating an ambient blue effect. It’s the oxygen primarily doing this, if it were other gasses then the rayleigh scattering would show different colors.
Blue light bounces off of blueberries, which absorb the other colors and cause the blue ones to go outward. That’s caused by a pigment in the blueberries that does that. That makes blueberries blue. Oxygen causes scattering of blue light waves (and others toward the ultraviolet) but not longer ones, the upshot of which is that the blue light is bouncing off those atoms.
No one said anything about either one transmitting light, as neither is the case. One is happening due to direct reflection, the other is happening due to a scattering effect, but what’s the difference between scattering and reflecting? The blue waves are bouncing off the oxygen atoms.
[shrug] It was just idle speculation, but there’s more than one article out there saying it’s oxygen that’s the culprit and it’s oxygen that makes the blue waves bounce around. Talk amongst yourselves on the terminology I guess.
(The air in the bottle is irrelevant, there would be no visible reflection or scattering in such a small amount, it would be like trying to measure the gravitational pull of a Chevy. Which has one, but…)
Kevin: The “explanation I don’t like” is because it doesn’t say enough. Saying blueberries are blue because they reflect blue light is like saying women are women because they are female. It doesn’t give any real answer. Everything blue reflects blue light, so the real answer is blue pigment in the berries. The real answer on the sky is that blue light bounces off of oxygen atoms. Not in the very direct way it would on a blueberry, so the scattering effect is part of the answer but is incomplete.
dean,
not pulling rank here, but this is the exact subject i am getting a doctorate in, and i can tell you definitively that it has nothing to do with oxygen specifically. reflection and scattering are also different processes. for example there is a defined angle of reflection for a given angle of incidence, while scattering is stochastic to a certain degree, though in aggregate it follows certain patterns.
if it’s really interesting to you, i can recommend a cheap book by h.c. van de hulst on the subject that’s available in a dover thrift addition for around 12 bucks. additionally if you have access to a library there’s a very readable book by bohren and huffman on the same topic.
Well, I took a few minutes and tried to tackle this in my livejournal. It’s mostly reiterating what other people have said, but maybe a different phrasing will help?
I also have a poorly drawn picture to help illustrate it.
The link should appear below this, thanks to that commentluv feature here, if it’s still working.
Elisha Feger’s last blog post..Why is the sky blue?
Zach: I understand that scattering and reflection aren’t the same. I just thought they were similar. Anyway, so it’s not the oxygen?
The air is not blue, and the oxygen is not blue. The blue color of the sky is because blue-wavelength photos from the sun are reaching your eyeball from everywhere, whereas the other wavelengths from the sun are either absorbed by the atmosphere or go straight through (so youd need to look directly at the sun to see them).
Rayleigh scattering is just one kind of scattering, another is Compton scattering (which is not important here, but is important when you go get a CT scan). scattering just means that a certain photon of a certain energy (ie, color) gets bounced off of something. Or diverted in other ways, or even totally absorbed and re-emitted again ina random direction (which amounts to the same thing).
Oxygen is NOT BLUE. Oxygen is a molecule. Colors, like blue, are a property of photons. The color of a photon before it interacts with (scatters off of) a given oxygen molecule may be blue or may be not blue; the color of the photon after it interacts with (scatters off of) oxygen may also be blue or not blue. It depends on a lot of things.
there’s a huge difference between scatering and reflecting, and that difference is that scattering can change the wavelength/energy/color of a photon, and reflecting doesnt.
the air in the bottle analogy is very very important and relevant, because it is still scattering blue the way the atmosphere does. the reason it looks clear to you is because you are outside it, it doesnt envelop you. If you were much smaller and inside the bottle then you wouldnt be able to tell where the bottle boundary was, and the air above you (inside and outside the bottle alike) would look blue.
Other atmospheres are not blue because other atmospheres dont have the same proportion of oxygen and nitrogen. The density, temperature, and chemical composition affect the rayleigh scattering. This means that its not necessarily the blue light that will get scattered, even if the amount of oxygen is the same in that atmsphere as earth’s.
Zach: Anyway, you’re getting your degree in light scattering? What specific application? I’m curious.
Aziz: That all makes sense. At this point all I’m concentrating on is the assertion (made on more than one site I googled) that it’s primarily the oxygen that causes the scattering to look blue in our atmosphere. It’s possible these sites are just generalizing too much. I seek merely to understand, Sensei, even if this started as no more than idle speculation.
By the way, I will pick a nit: oxygen does indeed have a color, depending at minimum on what state it’s in. When it’s in its liquid state, it is most definitely blue, and by interesting coincidence it turns out to be very close to the color of a clear blue sky. The video above shows that, but the picture here shows it even better:
http://en.wikipedia.org/wiki/Liquid_oxygen
Also, is oxygen a molecule, or an atom?
Anyway, I accept that this is all probably coincidental, as I defer to you and Zach’s greater knowledge of the physics involved. But it’s interesting nonetheless.
In the atmosphere, Oxygen comes in O2 (what you require to not suffocate) and O3 (Ozone). When people say "Oxygen" and they’re talking about the air, they mean O2.
And I will counter pick your nit: The oxygen in the atmosphere is not in the liquid state, it’s in the gaseous state. Molecular oxygen gas, again the thing that’s actually in the atmosphere and thus present to scatter light, is colorless.
http://en.wikipedia.org/wiki/Allotropes_of_oxygen
Let me just quote a little:
Dioxygen Main article: Oxygen The common allotrope of elemental oxygen on Earth, O2, is known as dioxygen. Elemental oxygen is most commonly encountered in this form, as about 21% (by volume) of Earth’s atmosphere. O2 has a bond length of 121 pm and a bond energy of 498 kJ/mol.[1] Oxygen itself is a colourless gas with a boiling point of -183°C. It can be condensed out of air by cooling with liquid nitrogen, which has a boiling point of -196°C. Liquid oxygen is pale blue in colour, and is quite markedly paramagnetic : liquid oxygen contained in a flask suspended by a string is attracted to a magnet.
Alternately, ignore the astrophysicist. I tried to pay as little attention to Electromagnetic Theory as I could get away with and still get a B. :p
Elisha Feger’s last blog post..Why is the sky blue?
Dean,
my research is in the biomedical applications of Mie and Raman scattering. Rayleigh scattering is an approximation to Mie scattering that can be made in the limit of small scatterer sizes.
But no, it’s not the oxygen. Or it is the oxygen, but not solely the oxygen. All the airborne gases and particulates contribute here. If the atmosphere were 100% N2 the sky would still be blue.
There are oxygen atoms and oxygen molecules. Obviously the element oxygen is an atom. But gaseous oxygen occurs as O2 and sometimes O3 (ozone), which are molecules.
Interesting article on the photon effect in re: LOX. It tends to happen more with the liquid phase of oxygen because of the density, but it is an intrinsic property of oxygen in general.Â
"The blue colour of liquid oxygen is believed to be due to a single photon simultaneously elevating two electrons to excited states on two separate ground state oxygen molecules. The absorption of the photon, combined with 1-3 vibrational quanta, creates a series of relatively-strong absorption peaks in the red, yellow and green regions of the visible spectrum, thus resulting in the perceived blue colour of the liquid. The temporary loss of the blue
colour after passing through an activated silica gel column, and the slow return of colour, suggests the transition is relatively infrequent.
The transition can occur when two ground state oxygen molecules collide and the resulting double molecule state ( which is not strongly bound ) contains a singlet component. The three-body ( 2 oxygen and one photon ) process is more likely to occur in the liquid ( as more collisions will occur between the oxygen molecules ), but it is an intrinsic property of the oxygen molecule, and not a bulk property of the liquid."
So, oxygen is blue, just not the way most blue materials are blue?
Zach, i like to joke that I am a Compton guy. Of course, since I am an MRI guy, I’m really doing RF, and dont muck about with that ionizing stuff at all. What I’d love to get into would be Terahertz, actually.
Dean, Oxygen in the atmosphere is O2, which is a molecule. pwned!
also, your nit remains unpicked. Oxygen does not have a color. Oxygen molecules in liquid state at a certain range of temp and pressure does have a color,but its not something intrinsic to oxygen the atom nor the molecule O2 itself. The color is purely a function of the way in which the atoms are arranged and their environment.
OXYGEN IS NOT BLUE. “liquid oxygen” is “oxygen molecules in liquid phase at a certain range of temperature and pressure”.
However, “it’s primarily the oxygen that causes the scattering to look blue in our atmosphere.” is mostly true a statement. The reason the sky is blue is because blue light from the Sun is scattered off of oxygen molecules in the atmosphere.
I retire from the field defeated, with a salute to the victors.
BTW, are large bodies of clear water blue for basically the same reason? I’m imagining so but while I’m asking…
dean,
my recollection is that water is blue due to blue-green algae. oxygen dissolved in water is still in its gaseous form.
oh man. Water is really complicated. short answer, everything above blue gets absorbed, but water is very unique in a lot of ways.
heres a casual reference on it i found from googling:
http://www.dartmouth.edu/~etrnsfer/water.htm
which gives you an idea of the complexity.
Erm, well, if you go to one of the great lakes or the atlantic or pacific and pull a random glass full of water you’ll see that it’s mostly clear with impurities, the color effect of blue only happens when you’re looking through or at a whole lot of it. So I wondered if the light scattering phenomenon was part of it.
dean,
very possible. dissolved oxygen would likely rayleigh scatter, as well as any other isolated particulates. there is also the issue that it may be reflecting the sky, which, as we’ve gone to great pains to discuss, is blue. probably a combination of all of these things (and probably more!). the ocean is a much more complicated optical system than the sky, and i admit to some ignorance on this point.
no, zach its nothing to do with dissolved particulates, even distilled water is blue, if you look thru enough of it. see my link.
andnote that heavy water is not blue, but transparent .
andnote that heavy water is not blue, but transparent .
Now that’s cool info.
Posting in epic thread!
aziz,
who knew this was such a fascinating subject?? i remember once needing to check out a book from the library: a two-volume tome simply titled "Water."
heh I bet that 2vol tome was just the cliff notes. Water is such an astounding subject you could well have a complete science devoted to it.
since the signal I detect in MRI comes exclusively from water protons, I will confess to being biased, and also since I kind of like being alive I am further biased still – but water is just the coolest. even when its the warmest.
heh 2 oh.
aziz,
most definitely. I was doing research on the hydrogen bonding network and how it might relate to otherwise unexplainable backgrounds in some Raman spectra I was acquiring, and even knee-deep in so much information about water I still had to look up other papers going beyond what was in the book!
Water is mind boggling. I’ve done some study on atmospheric spectra, and water lines are one of the most noticeable signals in town – despite how little of it there is compared to other gases.
Elisha Feger’s last blog post..Why is the sky blue?
So did anyone ever answer why Mars’ sky isn’t blue? Because that actually seems like the puzzling part now.
Elizabeth,
here’s one answer:
http://www.webexhibits.org/causesofcolor/14C.html
they claim that the martian atmosphere has large amounts of suspended dust whose absorption and scattering give it its "butterscotch tan" color.
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