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

[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.