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Re: [Phys-l] Greenhouse gases

This is another case of bad test questions.

D is correct, but NOT what makes gasses green house gasses. It's an artifact of their character.** What makes a gas green house is its absorption in the mid and far IR and its transmission in the near and visible. This is because, not the mechanism, which was your principal question, most of the sun's energy is in the vis. and near IR; not the UV.

Regarding your mechanism question, its dealt with in most optics texts, e.g. Jenkins and White.

Again not the basic mechanism you desire, but that of simple global warming (greenhouse) is found in wiki's Greenhouse effect page.

Especially note:

**Note O3/2 are anti-greenhouse gasses as they absorbs UV and transmits vis. and IR. The absorption above 190 nm is due to O3, as O2 completely absorbs below 190, and is transparent above thru the vis.


On 2009, Jun 11, , at 04:32, Brian Whatcott wrote:

Casting around for context, I recall that greenhouse gases attract that
name by
acting like a blanket while passing light, and I recall the warnings
that it is too easy to become sunburned on an overcast day.
On the basis of these possibly irrelevant, even doubtful data,
I need to vote for D

Brian W

Ken Fox wrote:
I am not at all sure what the mechanism for the process in this question is.
This comes from a recent IB Physics exam, aimed at a Honors HS or early
college level student. I'd like to be able to explain it on that level.

Greenhouse gases
A reflect IR radiation but absorb UV radiation.
B reflect UV radiation but absorb IR radiation
C transmit IR radiation but absorb UV radiation
D transmit UV radiation but absorb IR radiation

I apparently don't know what happens to photons when they interact with any
kind of material, (metals, dielectrics or gases) to ultimately just be
absorbed, transmitted or reflected. Is there a brief answer?

Thank you

Ken Fox
Forum for Physics Educators

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