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Re: sun's spectrum



Certainly light comming from the sun to my eyeball must be pretty well
collimated.

It certainly is, but the light incident on the material in the solar
atmosphere is not well collimated. Even within the last optical depth
on the way out the light is incident on it over over roughly 2 pi
steradians.

In absorption by an intervening nebula of transmitted starlight from a
distant source the incident light is well collimated. No light is
scattered into the direction of the transmitted beam in this case.
Description of the absorption spectrum seen here is much more
straightforward than it is for the solar atmosphere, where light is
also scattered into the line of sight.

I'm a bit annoyed by the tone in this group. A teacher asked for an
explanation of the absorption spectrum in the Sun and I gave him an
explanation in terms of absorption. It can be done. I did not judge it
appropriate either to send him off to Carroll and Ostlie to get the
straight scoop or to go into the intricacy of explaining opacity first.
It happens that I start my class on Chapter 9 in C & O on Friday; I
have to explain this to them, too, and I will do so differently, as is
appropriate for a bunch of physics and engineering undergraduates.

I've said before and I will say again, the central task of physics is
to describe Nature; we cannot explain Nature in any profound way. My
"explanation" certainly describes Nature adequately for the purpose,
and it is not wrong.

Now, with those ideas in mind, if you have a better answer to offer the
original questioner, please offer it. (Roger's answer was in my view at
too high a level for use in a high school setting, and it carped on my
misidentification of the chromosphere, even though I identified the
site of the absorption as being in the cooler part of the solar
atmosphere. I await the citation requested and an appraisal of the
Zeilik statement. Incidentally, I used that text once but I did not
like it. Five weeks in I really do like Carroll & Ostlie.)

Leigh