Why is the air transparent and what makes most gases have low absorption?

[Relena]

Why are the atmosphere's main elements ( Nitrogen and Oxygen) transparent to visible light.

And what makes most gases have low absorption?

 

[Andy Resnick]

Gases scatter little light becasue they have low density. The spectral location of absorption lines depends on allowed atomic (electronic, vibronic, rotational) transitions, and for Earth's atmosphere, our eyes have evolved to take advantage of spectral reagions with little scattering.

 

[maverick_starstrider]

I think Andy has really hit on the explanation here. I don't think it's an oddity that atmospheric gases don't have absorption lines in the visible spectrum but rather that's one of the main reasons why the visible spectrum of our eye is what it is (as opposed to the IR region or the likes)

 

[Domenicaccio]

Isn't there also a reason related to the fact that a solid (at least crystalline solid) has energy bands while a gas is made of isolated molecules and therefore has energy lines? Could this mean that in white light a typical solid would absorb/block entire bands of wavelengths, while a gas would only block some monochromatic frequencies and therefore it would let most of the white light go through?

...or am I making this up? :)

 

[physixguru]

There are basically 2 reasons. For one, air is rarefied. The molecules are not very close together so that like waves (or particles) are not likely to collide with an air molecule along its trajectory. In other words, it has a very long mean free path.

Also, and probably more important, none of the molecules that make up air absorb the frequencies of light that are visible to the human eye. Therefore, they do not absorb light and re-emit it at a different frequency as colored things do and that makes the air transparent to visible light.

 

[kenewbie]

If your eye was built for a different wave-length, say at gamma ray level, you would be asking "why is aluminium transparent" instead.

Basicly, the frequency of a wave decides how it interacts with different materials. The range which we call "visible light" happen to pass through glass unaltered, while the range we call heat-waves does not.

This is one of the reasons why your car gets so damn hot in the summer: Light comes though the windows and bounces from the interior. It looses some energy during that bounce, and becomes a heatwave. Same thing, different frequency. Only, heat does not pass unaltered through the glass, so it is trapped inside and bounces back and forth, heating your car.

k

 

[Andy Resnick]

<snip>

This is one of the reasons why your car gets so damn hot in the summer: Light comes though the windows and bounces from the interior. It looses some energy during that bounce, and becomes a heatwave. Same thing, different frequency. Only, heat does not pass unaltered through the glass, so it is trapped inside and bounces back and forth, heating your car.

k

This is not true- simply allowing the IR radiation to get into the car and be absorbed by the car interior is sufficient to explain the behavior. One not need presume that visible light be 'down-shifted' to IR radiation, nor that IR radiation be 'captured' within the car interior. Neither occurs.

Finally, a small quibble- heat and IR radiation are not the same things.

 

[cmos]

This is not true- simply allowing the IR radiation to get into the car and be absorbed by the car interior is sufficient to explain the behavior. One not need presume that visible light be 'down-shifted' to IR radiation, nor that IR radiation be 'captured' within the car interior. Neither occurs.

Finally, a small quibble- heat and IR radiation are not the same things.

kenewbie's response, also works with the visible light, but like Mr. Resnick said, there is no redshift of that light into IR. The materials in the car may absorb some of the visible light converting that energy into heat. But as Mr. Resnick said, heat is not light; thus is gets trapped in your motorized greenhouse.

 

[DaveC426913]

This is not true- simply allowing the IR radiation to get into the car and be absorbed by the car interior is sufficient to explain the behavior.

I though the idea here was that windows are transparent to visible light but somewhat opaque to IR.

 

[Relena]

So if a hight intensity laser beam was adjusted to one the frequencies of oxygen visible spectrum, would light scattering be observable ?

 

[Gokul43201]

The best answer (in the sense that it provides the full overview) is the one given by Domenicaccio. Gases are transparent because their absorption spectrum is discrete rather than continuous. The reason for the discreteness is the inability to support collective excitations because of the weak interaction strength between molecules that are widely separated.

 

[Schirillo]

None of these answers make sense. If we see though air (i.e., it appears transparent) because it's molecules are less dense than light waves of the visible spectrum, how do we "know" that the light comes from the surfaces of objects. That is, how do we attribute the light to the surface and not the air in front of the surface?

 

[Schirillo]

None of these answers make sense. If we see though air (i.e., it appears transparent) because it's molecules are less dense than light waves of the visible spectrum, how do we "know" that the light comes from the surfaces of objects. That is, how do we attribute the light to the surface and not the air in front of the surface?

 

[DaveC426913]

What?

 

[DaveC426913]

What?10char[/color]

 

[sylas]

I though the idea here was that windows are transparent to visible light but somewhat opaque to IR.

That's true; but I think the more important factor for your car heating up is that the air gets trapped by the roof and sides. Outside, the air heated at the surface is continually replaced by cooler air from aloft, by circulation.

The question of the thread can be rephrased: why are our eyes particularly sensitive to wavelengths where there is the most light passing easily through the air? The visible portion of the spectrum is both the peak of the solar radiation, and also in a band that does not interact with atmospheric gases.

Cheers -- sylas

 

[Schirillo]

The question is really straight-forward. Light waves reflect off of objects that then stream into the eye by first traveling from the object through open space. How is it that we determine where the object is located? That is, why don't we "see" the stream of light waves as they travel in the open space (so that it appears like a colored volume rather than a distant opaque surface)? Instead we somehow decide that the stream of light originates at the surface of the object and then we eliminate our perception of that stream from the intervening space.

 

[sylas]

The question is really straight-forward. Light waves reflect off of objects that then stream into the eye by first traveling from the object through open space. How is it that we determine where the object is located? That is, why don't we "see" the stream of light waves as they travel in the open space (so that it appears like a colored volume rather than a distant opaque surface)? Instead we somehow decide that the stream of light originates at the surface of the object and then we eliminate our perception of that stream from the intervening space.

No, that's wrong. The reason we don't see the stuff in between is it has no effect on the light; it really DOES originate at the opaque surface. We don't see the light as coloured because it ISN'T coloured. If something is transparent... like the air... then light is passing through it without any effect. There's no special role for "eliminating perception". A machine measuring that light would also be measuring the surface that emits the light, just like the eye.

Cheers -- sylas

 

[Schirillo]

But the question remains, how does the eye (or a machine) "know" that the light originates at the opaque surface (and not closer to the eye)? Machines don't have depth perception. They only "see" in two-dimensions, without imposing some amount of empty (transparent) space between their receptors and the surface.

 

[sylas]

But the question remains, how does the eye (or a machine) "know" that the light originates at the opaque surface (and not closer to the eye)? Machines don't have depth perception. They only "see" in two-dimensions, without imposing some amount of empty (transparent) space between their receptors and the surface.

You can have depth perception with machines just fine; it's not magic.

Your question makes no sense. It's not about "knowing". The point is that the light ACTUALLY DOES originate at the surface -- whether you know this or not, and regardless of whether the light is received by a thinking being capable of "knowing" things, or a dumb machine that just makes measurements.

That's why it is incorrect to say that there's some kind of special "perception" process involved in eliminating the effects of stuff in between. There's nothing to eliminate.

There are certainly some odd things that can happen with perception. Here's my favourite; the human perception system sees blue and green spirals; but if you test the image more carefully, you can check that they are really the same color.

(Reference: The blue and the green -- Bad Astronomy Blog).

The point is, the invisibility of air is NOT a perception effect. It is because the air actually is transparent to visible light. This question is one of the physics of light and matter; not about perception and knowledge.

sylas

 

[Schirillo]

The question is not what actually exists (I know the light originates at the surface), it's what is perceived; just like the perception of transparency.

It does have to do with knowledge, since machines cannot perceive depth unless they emit a signal that bounces back off the surface (like bats using echolocation). There is no way to determine depth from just an incoming signal.

The spiral picture you've posted is simple simultaneous contrast, also known as induction. It can be explained by psychophysicists, my question cannot.

 

[sylas]

The question is not what actually exists (I know the light originates at the surface), it's what is perceived; just like the perception of transparency.

It does have to do with knowledge, since machines cannot perceive depth unless they emit a signal that bounces back off the surface (like bats using echolocation). There is no way to determine depth from just an incoming signal.

This is now just being silly.

Machines use exact same incoming signal we do. We get depth perception from that signal not by magic, but by primarily by having two eyes, for binocular vision. Machines can also have two light detectors, or two cameras, and pick up depth perception in precisely the same way, looking at the incoming signal at two different points. There's quite a lot of work doing just that.

Comically, you've actually demolished your own point when you talk about incoming signals. THAT'S what is important. The incoming signal DOES include depth information, but it does not include information about a transparent medium. That's why it is silly to say this is about "perception", when of course it is about the physics of how light and air interact, and what happens to the incoming signal; no matter WHAT is used to detect or measure it.

Your question cannot be tackled by psychology or brain studies, because this has nothing to do with "perception". That's the whole point. Transparency is not a "perception" effect, but about what can physically occur with an incoming signal, independent of what might be used to measure or perceive it. In other words: the question most definitely IS about what actually exists in the light signal.

 

[Schirillo]

It's not "silly". You can see in depth just fine with just one eye (just cover one and check), but a machine can't.

The incoming signal DOES NOT include depth information (if you exclude binocular receptors)...all it contains are wavelengths of light. These wavelengths are not colored, nor do they contain a "separate depth signal"

 

[Schirillo]

It's not "silly". You can see objects in depth with just one eye (cover one eye and notice that virtually nothing changes), however machines need stereo (or some form of triangulation). The incoming signal DOES NOT include depth information, it only contains wavelengths of light, not an additional signal as to where those wavelengths originated.

 

[sylas]

It's not "silly". You can see objects in depth with just one eye (cover one eye and notice that virtually nothing changes), however machines need stereo (or some form of triangulation). The incoming signal DOES NOT include depth information, it only contains wavelengths of light, not an additional signal as to where those wavelengths originated.

Pretty much everything you've said above is flatly wrong.

The idea that the incoming signal does not contain depth information is false. How do you think the eye figures it out? It figures it out because light signals have direction, which gives a different view in two eyes. This is depth information. You look at the angle from which a signal comes, in one eye and in the other (triangulation).

As I said before, there is quite a lot of work with robots that do depth perception using binocular vision, just like we do. They don't use magic; they just use the incoming signal.

You also get a much less effective depth vision with one eye, by changes in focus; which is pretty much the same thing but over the space of the retina rather than the space between two eyes. It is, of course, nonsense to say that covering an eye changes virtually nothing. It kills your depth perception, and that makes a huge difference, as anyone who has an eye out of action for any time swiftly discovers. You need stereo vision to get at the depth information available in light coming from some object.

Getting back to the point of the thread. The transparency of air is a physical aspect of light signals passing through air. The transparency of air is a property of light and air, not of the eye and mind.

 

[DavidSnider]

Pretty much everything you've said above is flatly wrong.

The idea that the incoming signal does not contain depth information is false. How do you think the eye figures it out? It figures it out because light signals have direction, which gives a different view in two eyes. This is depth information. You look at the angle from which a signal comes, in one eye and in the other (triangulation).

As I said before, there is quite a lot of work with robots that do depth perception using binocular vision, just like we do. They don't use magic; they just use the incoming signal.

You also get a much less effective depth vision with one eye, by changes in focus; which is pretty much the same thing but over the space of the retina rather than the space between two eyes. It is, of course, nonsense to say that covering an eye changes virtually nothing. It kills your depth perception, and that makes a huge difference, as anyone who has an eye out of action for any time swiftly discovers. You need stereo vision to get at the depth information available in light coming from some object.

Getting back to the point of the thread. The transparency of air is a physical aspect of light signals passing through air. The transparency of air is a property of light and air, not of the eye and mind.

He wasn't saying they use "magic", but rather, they build up a virtual model of the world based upon past and present information and use the whole model to judge things like distance rather than information that is based solely on what is currently hitting the eye.

 

[f95toli]

It's not "silly". You can see objects in depth with just one eye (cover one eye and notice that virtually nothing changes)

Have you actually done any work which requires ACTUAL depth perception with one eye? I do from time to time (my left eye is a bit dodgy and from time to time it doesn't work very well) and I can assure you that depth perception with one eye is nowhere near as good as with two eyes.
Yes, it can be done but it is to a large extent based on experience; if you know the size of an object you can judge the distance by looking at its relative size, i.e. the same way we perceive "depth" when watching a movie on TV.
E.g doing anything complicated under a microscope is nearly impossible for me since the "size" information does not work anymore.
Another "trick" which the brain does use automatically is to continuously re-focus the eyes (which gets very tiring after a while) which gives some limited information about depth.

Also, remember that the difference between a 3D movie and an ordinary 2D movie is that the former conveys unique information to each eye; this is what makes it 3D.
If what you are saying were correct there wouldn't be any difference between 2D and 3D.

 

[VincentJL]

None of these answers make sense. If we see though air (i.e., it appears transparent) because it's molecules are less dense than light waves of the visible spectrum, how do we "know" that the light comes from the surfaces of objects. That is, how do we attribute the light to the surface and not the air in front of the surface?

If you are asking about how our brain deciphers the signals comming from the lightsensitive neutrons in our eyes... I have know idea (And I properbly got the description wrong too).

As to how we can see objects in the distance and "know" they aren't actually located 2 cms away, well we have stereo vision, and also we can move our heads around a bit to determine if something is close or far away. There is no real "origin" information encoded in the light, if it did come from somewhere in between you and the object, our eyes wouldn't be able to tell so. Consider movies, which slightly trick your brain into thinking something isn't just a flat projection of chaning colors, or even better consider holograms, which are even better at tricking our brains into thinking there is some object there.

 

[DaveC426913]

It's not "silly". You can see objects in depth with just one eye (cover one eye and notice that virtually nothing changes), however machines need stereo (or some form of triangulation). The incoming signal DOES NOT include depth information, it only contains wavelengths of light, not an additional signal as to where those wavelengths originated.

Schirillo has a point that can't be denied, though I'm not sure he's on the right track. There is is property of reflected light that hints at an object's distance: Focal length.

Our eyes will be in focus at a specific range of distance. An object six inches away will require a flexion of the eye's lens, whereas an object yards away will require the lens to relax. The brain does recognize this as a strong clue about the distance to the object.

Try this: put your hand one foot in front of your face and focus on it, closing one eye. The background of your living room will be out of focus. Now bring the living room walls into focus. There is most definitely an ability to tell nearby obects from distant objects.

(Of course, Schirillo is mistaken in that there's no reason why machines can't do this too...)



Another much more subtle and more conditional property of the imcoming light is contrast and colour. Due to the intervening air and impurities, distant objects such as mountains or skyscrapers will appear low in contrast and tinged in blue.

 

[sylas]

Schirillo has a point that can't be denied, though I'm not sure he's on the right track. There is is property of reflected light that hints at an object's distance: Focal length.

That's an odd remark, since you have just denied the point he was making in what you quoted. You quoted the extract where he says "The incoming signal DOES NOT include depth information", and then go on to explain, correctly as others have done also, why his point is wrong.

Cheers -- sylas

 

[DaveC426913]

That's an odd remark, since you have just denied the point he was making in what you quoted. You quoted the extract where he says "The incoming signal DOES NOT include depth information", and then go on to explain, correctly as others have done also, why his point is wrong.

Cheers -- sylas

Well, I'm not sure if I'm saying he's wrong or right about this particular comment; I'm just backing him up in his original assertion that the light rays (at least, collectively) do contain information about depth.

The depth information is conferable from the angle of divergence of the light rays which, through the diamter of the pupil, the action of the lens and finally the image on the retina, is decodable.

 

[sylas]

Well, I'm not sure if I'm saying he's wrong or right about this particular comment; I'm just backing him up in his original assertion that the light rays (at least, collectively) do contain information about depth.

The depth information is conferable from the angle of divergence of the light rays which, through the diamter of the pupil, the action of the lens and finally the image on the retina, is decodable.

I know that, and you know that. But he never made any "original assertion" about light having information about depth. In fact, he explicitly says just the opposite. He seems to think there's something special about "perception" that works for the eye without needing information in the light itself.

 

[DeShark]

I thought that air was quite transparent to almost all wavelengths. I think the reason (as already stated) is that the absorption band is discrete and not continuous and therefore the light passes through. As to why there is a discrete energy spectrum in small molecules (such as water, air, gas, etc) is to do with the separation of the discrete energy levels of individual atoms (a phenomena which arises from the solution of a central potential). In bringing together atoms (so that their potentials overlap), the band is split. With an infinite number (or just very very large) the band is split so much that it forms a continuum. So in solids, the spectrum becomes continuous and so it can absorb many different wavelengths and appears opaque. However, in gases and liquids, there are only a few atoms bound together so a discrete spectrum forms and most light passes through unhindered.

As for glass, I have no idea, but I do know that it is only transparent at certain wavelengths (which happen to correspond to the visible spectrum), I believe it's to do with the bonds being covalent, rather that ionic or metallic. Maybe someone else can help.

The evolution of our eyes to make the air transparent is not relevant as far as I can see. We use glass because it's transparent, not the other way round. Unless someone can prove me wrong?

 

[sylas]

I thought that air was quite transparent to almost all wavelengths. I think the reason (as already stated) is that the absorption band is discrete and not continuous and therefore the light passes through. As to why there is a discrete energy spectrum in small molecules (such as water, air, gas, etc) is to do with the separation of the discrete energy levels of individual atoms (a phenomena which arises from the solution of a central potential). In bringing together atoms (so that their potentials overlap), the band is split. With an infinite number (or just very very large) the band is split so much that it forms a continuum. So in solids, the spectrum becomes continuous and so it can absorb many different wavelengths and appears opaque. However, in gases and liquids, there are only a few atoms bound together so a discrete spectrum forms and most light passes through unhindered.

Actually, when you look at the whole spectrum, air is opaque at most wavelengths.

There are windows of transparency around visible light, and around radio frequencies... not surprisingly, we've latched on to these wavelengths for use in the atmosphere. Natural evolution has given us eyes that use the visible wavelengths in which the atmosphere is transparent, and radio communications of course uses those wavelengths where the atmosphere is transparent as well.

But for most of the rest of the spectrum, the atmosphere is opaque.

Here is a diagram, from a site considering what wavelengths can be used by astronomers wanting to look out into space through the atmosphere.

Source: http://fuse.pha.jhu.edu/~wpb/spectroscopy/atm_trans.html, at the John Hopkins University, courtesy of http://fuse.pha.jhu.edu/~wpb/.

The evolution of our eyes to make the air transparent is not relevant as far as I can see. We use glass because it's transparent, not the other way round. Unless someone can prove me wrong?

Evolution is extremely relevant to why our eyes work with wavelengths where the atmosphere is transparent AND where there is a lot of ambient light available to be used.

But you are right about glass. We did not evolve eyes in order to use the properties of glass. We evolved eyes that could see through air (and water, if you dig into it). Having done so, we now have a technological interest in solid materials that are transparent in the wavelengths for which are eyes are sensitive. We picked on glass as a useful material because of this property.

Cheers -- sylas

 

[DeepQ]

The topic of this thread is an excellent question, but overall the replies contain numerous false statements, so beginners need to be careful drawing conclusions from this discussion.

For example, it should not be concluded that all gases have no color in our visible spectrum. Fluorine is yellow-brown, chlorine is greenish, bromine is red-brown, iodine is violet, etc. The colors of most of the planets in our solar system are due to their atmospheric gases.

That also leads to the importance of distinguishing between "air" and "atmosphere". For example, many of the bands in the Johns Hopkins Atmospheric Transmission chart posted earlier are due to other layers and effects of the atmosphere, not simply air. For instance, the opacity below 8m wavelength is due to ionospheric reflection. Here on the Earth's surface our "normal air" is transparent at that wavelength, not opaque.

I actually liked Schirillo's questions... and instructors/experts should be careful to understand what people are really asking and not quite so harsh in reply.

It's important to recognize that the world we see as humans is a virtual image, constructed by the mechanism of our eyes. What we see is just a transformed representation of reality.

Light is radiated from surfaces in many directions, it's all "just a blur", but it strikes across the cornea and lens surfaces of the eye where it is bent toward a focal point. This is a spatial transformation that allows us to see an image. But, it's not the format of the light as it passed through the air. If you want to see a closer representation of that, grab a convex lens, almost any magnifying glass will do, and look through it across the room. Move the lens until the image expands, appearing to fill the entire lens. You are recollimating the light at the back surface of your eye's lens. The world looks quite different in that domain, yet all the information contained in that light is still there. Your eye simply has no way to "translate it".

Anyway, it would be great to get back to the original question. More should be said.

For example, some readers may want to know more about why light is absorbed or re-emitted as it passes through various materials? This is a great question! How does it change color? Is energy lost in the process? Can we treat the vacuum of space itself like air of very low density (but over huge light paths)? If so, does it also have a color? Could this affect what we observe of extremely distant stars? Might such effects throw-off our computations related to the accelerated expansion of the universe? If so, should we recompute our dark matter results? Etc.

One thing leads to another. From the start, our basic understanding needs to be solid.