This is the pic I’m referring to. My questions are:
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Why is the atmosphere transparent to visible & radio wavelengths?
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Among the infrared & microwave windows, the atmosphere’s opacity fluctuate wildly. Why is it so??
Thanks!
Physics.
I’m not a science teacher but my understanding is that the composition of the atmosphere (nitrogen, oxygen, and a few others in much smaller percentages) determines what is absorbed, reflected, or passed through. Visible light doesn’t have any real special characteristics, but we evolved to see in those spectrums because that’s what was around. At some point the magnetosphere also comes into play, shielding us from some wavelengths as well.
I suspect that answer to your second question is just some specific details from the first answer, but I’ll let others more versed in the subject weigh in.
https://www.chegg.com/homework-help/definitions/atmospheric-em-transparency-2
I’m not an expert, but I have picked up a bit of knowledge on these matters. I’m sure if I make a mistake here, a real expert will come along and correct me.
The main atmospheric molecules that absorb EM radiation are H2O and CO2. Oxygen and nitrogen do absorb some, but not that much. Why they absorb certain wavelengths and not others has to do with the vibration modes of their chemical bonds. When a molecule absorbs a photon, a bond jumps from one vibration mode to a more energetic one. Later, it’ll emit a photon of the same wavelength by dropping back to the original mode. But that emission will be in a random direction. But the important part is that they can only absorb photons that happen to correspond to the exact difference in energy between the two vibration modes. Why those photons happen to lie between the IR and microwave part of the spectrum getting a bit beyond my knowledge.
As for the variability, that’s mostly because there’s a highly variable amount of H2O in various parts of the atmosphere.
One other thing. AIUI, there’s a general rule that the more atoms a molecule has, the more wavelengths it will absorb and thus contribute to atmospheric opacity (also global warming). Which is why methane (CH4) is a stronger greenhouse gas than CO2 and CFCs are stronger yet.
Thank you guys. The last paragraph gives me interesting related info. AIUI, aside from having more atoms, a molecule must be light enough to ‘float’ up to the higher atmosphere. Given the composition of CFCs, I guess that only those with less than 8 atoms will be able to do that, right?
Returning to the topic, I can’t see why various parts of the atmosphere having different amounts of H2O leads to fluctuations when the wavelength changes.
Also, why don’t O2 & N2 absorb radiation? Does the fact that they have homogeneous atoms have something to do with absorbability?
I misunderstood the question. I thought you were asking about how it varies depending on what part of the world you were in.
OK the reason for the variability is that H2O only absorbs certain wavelengths. Ditto for CO2. Most of the wavelengths for both molecules are in the IR region. However, they don’t cover every wavelength in that region. So there are some gaps in the spectrum where IR light can get through.
It should be noted that an absorption at a specific wavelength actually translates to an absorption in a small region around that wavelength. That is due to the motion of the molecules, basically because of the Doppler effect. Some molecules will be moving towards the source and will absorb EM radiation at a slightly shorter wavelength and some will be moving away and absorb those at a slightly longer wavelength. The hotter the gas, the broader the absorption lines. (Astronomers use that fact to determine the temperature of astronomical bodies, including nebulae.)
As your cite say, O2 does absorb some in the gamma-UV part of the spectrum, although I believe that it’s mostly ozone (O3) that does the job there. I don’t know where the absorption lines for N2 are. It must absorb some wavelengths, but perhaps they’re in the same place as other absorption lines.
The issue is not that O₂ and N₂ do not absorb radiation, but that they do so in the far-infrared and ultraviolet wavelengths rather than the visible/near-IR band discussed here, and not very intensely. The exact absorption spectra depend on temperature and pressure as well as the exact mix of gases, etc., but you can get an idea by looking up parameters in a database, e.g., HITRANonline line-by-line search: 1. Select Molecules
On the other hand, H₂O will totally absorb a lot of near-infrared wavelengths as you can see from your picture, while visible radiation is not very much absorbed by atmospheric gases.
See also
http://irina.eas.gatech.edu/EAS8803_Fall2009/Lec6.pdf
and previous threads on the topic
To expand on this: Every way that a molecule can wiggle is a way that it can absorb energy. For a molecule with only two atoms, like nitrogen or oxygen, you can rotate in two different directions (the third direction you can rotate takes almost no energy, and even the two that do take energy, take the same amount of energy), or you can expand/contract, and that’s it.
For water, meanwhile (a bent molecule of three atoms), you can rotate (at different rates) in all three directions, and you can have one bond stretching while the other contracts and vice-versa, and you can have both bonds stretching/contracting at once, and you can change the angle of the bend. So there are a lot more ways to wiggle a water molecule, and so water absorbs a much wider variety of wavelengths.
Illustrating what Chronos said
Thanks again. With the new info, my understanding now is that with such composition of our atmosphere having H2O, CO2, N2 & O2… there happen to be absorption lines overlapping heavily from 10^-13 to 10^-7, 10^-5 to 10^-3 and 10 to 1000m wavelengths. The infrared and microwave spikes are just lone lines, not having ‘friend lines’ nearby. By pure coincidence, the radio and optical windows have no such lines.
Questions:
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Is it pure coincidence? In the words, in another planet with a different composition of gases, organisms will evolve different eyes than us, right?
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Why is it that there’s no animals on Earth which have ‘radio-eyes’? The atmosphere is transparent there, too.
I don’t know if it is a “pure coincidence” that these types of common molecules are colorless at optical wavelengths; it is a consequence of their electronic properties.
Even on Earth, there is a huge abyssopelagic zone where some species feature adaptations like bioluminescence and specialized eyes, so you don’t even need to extrapolate to xenobiology to observe different types of eyes. Look at an arthropod, for that matter.
As for radio-eyes, the first thing that comes to mind is that even UHF and SHF waves are still relatively long, as in centimetres, so there is a question of how useful they would be for “vision”, plus the problem of receiving and/or generating such signals. Compare echolocation, though, for hints at the possibility of different types of senses.
Presumably yes. The visual spectrum of light is just a label we put on what we can see. There’s nothing intrinsically different between the visual spectrum and other wavelengths.
The Wikipedia only lists a handful of colourful gases: Ozone, Fluorine, Chlorine, Bromine, Iodine, Chlorine dioxide, Dichlorine monoxide, Nitrogen dioxide, Trifluoronitrosomethane, and Diazomethane.
There is actually a little bit of a coincidence here, in that the window of transparency of the atmosphere corresponds to the peak of the solar spectrum. Even a creature that did have, say, radio-wave eyes wouldn’t find them as useful, because the Sun just isn’t nearly as bright in radio as it is in what we call “visible light” (in fact, it’s not even the brightest radio source in the sky).
My first guess is that there’s less radio-wavelength photons for critters to work with:
The spectrum diagram in your OP shows a an atmospheric transmissivity window in the radio range, but the sun just doesn’t put out much energy in those wavelengths. The diagram I linked to only runs out to wavelengths of 2.5 microns, which is in the “infrared” section of the diagram you linked to. At those wavelengths, solar irradiance is about 1% of what it is in the visible wavelenghts. I can’t even find a solar irradiance diagram that reaches into radio wavelengths, which are on the order of 1 meter, but it seems like the solar irradiance at radio wavelengths is likely to be very, very low.
Some animals can sense infrared:
Some can sense ultraviolet:
But given that the visible wavelengths of sunlight are the most abundant in terms of power, it makes sense that the vast majority of plants and animals (outside of that that live in caves or abyssal ocean depths) have evolved to utilize visible light.
And pit vipers mostly aren’t seeing infrared produced by the Sun and reflected off of the the things they want to see; they’re mostly seeing the infrared that’s produced directly by their prey’s body heat. Which also means that they don’t care if the light can make it through miles of atmosphere; they mostly just care that it can make it through the few feet that is their striking distance. At thicknesses of a few feet, air is transparent to almost everything.
I feel we’re lucky to have an atmosphere transparent right at the peak of our star’s power range. This should be included as a criterion in the search of (intelligent) life.