Why is air transparent?

[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.