Transparency of atmospheric gases.

[Daecon]

I was talking to my housemate and he raised a point I wasn't certain of how to explain, about how Earth's atmosphere was transparent.

 

I mentioned about how the particles in gases were so widely dispersed compared to other particles, and he mentioned that other gases can block light...

 

So I commented that eyes must have evolved to take advantage of nitrogen's transparency to what we term "visible light" when he reminded me that oxygen and carbon dioxide are transparent to those same frequencies too.

 

So how do I respond to the fact that nitrogen, oxygen and carbon dioxide all being transparent to the same wavelength frequencies of the EM spectrum when other gases aren't?

[Daecon]

I guess what I'm asking is: Are "opaque" gases very rare, or what? I don't know much about their chemical properties.

[swansont]

Gases aren't going to be transparent in the UV, since ionization will occur. You start getting absorption in the IR, so gases cease to be transparent there, too.

 

 

Figure 1. Spectra of solar radiation outside the atmosphere and at the ground level showing the bands where there is

absorption due to water vapour and carbon dioxide in the mid-IR (vertical shading) and due to oxygen and water vapour

in the mid- and near-IR (horizontal shading), while oxygen and ozone absorb in the UV at wavelengths shorter than 0.4 µm (not shaded)

 

from http://sel18.hut.fi/304/Valomylly/bio.htm

 

There's also the effect of the peak of the blackbody radiation of the sun matching up pretty well with the 400-700 nm band.

 

Bromine and Chlorine are two gases that absorb in the visible. Iodine and sulfur have visible vapors as well, I think.