Re: Sunsets

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

>  >Now if both red light and blue light were being transmitted by the sun
> > carrying equal energies then the red would loose less of its energy to
> > oscillating its neighbor and send more of its energy through the atmosphere
> > to run my solar calculator.  Whereas the blue light looses more of its
> > energy oscillating its neighbor and therefore sends less through for my
> > calculator.  Am I on the right track?
>
> I'm still confused by "oscillating its neighbor".  As the light passes
> through the air, the electrons in the molecules oscillate in response
> to the electric field.  I guess you could say that the light "oscillates"
> the molecules; if that's what you mean, then yes, it sounds like you've
> got the picture.
>
> Dan

I never cease to be amazed at how good we are on this list at taking
a simple question and making it really complicated. Now its my turn.
Rather than a classical explanation, let be propose a quantum
explanation.

First, the blue sky. We see it whether there is dust in the air or
not, so I would surmise that it is primarily due to scattering from
the molecules. Second, the blue sky is not very bright, which implies
that there isn't a huge amount of scattered light that gets to our
eyes from the blue part of the sky.

Here's my take on the process, from a pretty speculative viewpoint.
Those of you better versed in the quantum mechanics of molecular
interactions with photons, feel free to correct me. I would guess
that upper atmospheric ozone has an absorption band that lies across
the blue end of the spectrum. When a blue photon strikes the
molecule, it kicks it up into a higher energy state, maybe even
causes it to dissociate (but I suspect the dissociation requires
higher energy than blue light, since that is the process that removes
most of the ultraviolet from sunlight. Assuming that this is not a
dissociative process, we have a photon of blue light absorbed, thus
removed from the ray of sunlight passing by.

But, of course, the ozone molecule quickly decays back to its former
state, emitting a photon of the same wavelength--blue--as it
absorbed, but now the emission can occur in any direction, so the new
photons are emitted in all directions, with roughly equal
probability. Hence most of them don't go forward with the original
beam, and some fraction head toward the earth, and the eye of some
observer. Enough of these events and the observer sees a blue sky. I
don't expect it would take a huge number (atomically speaking) to
create this effect. This may even involve Raman scattering, but I
have no reason to suspect it either way. Also, it may be something
other than Ozone that is doing this, molecular oxygen or nitrogen,
perhaps.

Anyway, blue sky--look away from the sun and see that fraction of the
blue photons taken out of the beam and sent toward you (most of them
are sent elsewhere, but not along with the original beam). Pretty
much dust independent since the sky is blue even when the atmosphere
is dust free.

Now sunset. To get those rich, red sunset that poets rave about, we
need to take out a whole lot more of the blue light (as well as most
of the other short wavelength photons). Looking at the sun low on the
horizon will do part of it because we are looking through lots more
atmosphere then, but there's a lot of photons coming from the sun and
those include a whole lot of short wavelength photons that we need to
get rid of if we're going to see that red sky. Here is where the dust
comes in, as I see it. Remember, when we get a good red sunset,
almost half the sky can be tinged red, and the sun is actually dim
enough so that you can stare at it without problem. I would guess now
that not only is the dust taking out blue light, but all the rest of
it as well, and simply diffusing it all over the sky. After all, we
don't expect the dust to have the nice uniform size that the ozone
molecules (or whatever) have, so they may be effective at scattering
a wider range of wavelengths than the molecules. And since the dust
will be spread out over a goodly part of the sky, it ought to be
pretty good at scattering some of that red light back toward our
eyes. That's why such a large part of the sky looks red.

I'd be hard pressed to explain scattering from the dust quantum
mechanically, so I suspect its easier done classically. So why don't
the blue photons get scattered back to us the same way the red ones
do? They do, but I suspect the dust is better at scattering the red
light back to us and there isn't that much ozone (or whatever) to
re-scatter the blue. Thus the red dominates. But it does seem to me
that the dust is necessary to get the red.

Am I anywhere close to the currently accepted thinking in this?

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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