From: "James F. Tims" <p00168@psilink.com>
Subject: Re: Why the sky is blue - again
Organization: Semper Excelsior
Date: Tue, 22 Dec 1992 05:48:06 GMT
X-Mailer: PSILink (3.2)

>DATE:   Mon, 21 Dec 1992 21:13:00 GMT
>FROM:   zellner@stsci.edu
>
>>  All that stuff in Jackson essentially reduces to treating air molecules as 
>>  classical harmonic oscillators, excited by the incident light, and radiating 
>>  as dipole radiators.
>
>>  The frequency of light is well below the natural frequency of the air 
>>  molecules. When sufficiently outside the resonant region, a harmonic 
>>  oscillator's amplitude is approximately constant .....
>>
>>  The Rayleigh scattering case is below resonant frequency ...
>
>As is so often the case in these newsgroups, someone gives the correct analysis
>early on, and after that things go off into never-never land.  Except for a few
>narrow "telluric" bands in the visible, air molecules have their rotational
>resonances in the microwave domain, their vibrational frequences in the 
>infrared, and their electronic resonances in the UV.  So one way to answer the
>question "Why is the sky blue?" is simply to say "because air molecules don't
>absorb or radiate at visible wavelengths; they just scatter."  
>
>>  I'm still confused.  Sorry.  I'm really trying to understand this.
>>  Some of the posts seem to confirm my vague recollections that the sky is
>>  glowing, although my reasons for it were incorrect. ... I can't figure out 
>>  whether the atmosphere is a lens or a radiator; whether the photon 
>>  originally came from the sun (no glowing) or whether it came from the air 
>>  molecules (glowing); whether a different atmosphere would be a different 
>>  color because of its spectral signature or because of its optical proper-
>>  ties; whether the air molecules in the viewed part of the sky are even
>>  receiving their fair share of red (!glowing) or whether the red is being 
>>  absorbed and re-radiated (glowing).  
>
>None of the above!  The daytime sky is not glowing at visible wavelengths!  
>(Sure, there are some "dayglow" and "nightglow" emissions, but they are 
>overwhelmed by simple blue-sky scattering in the daytime.)  Rayleigh scattering
>is not remotely resonant scattering, and has nothing at all to do with spectral
>line absorptions or emissions.  It is a purely classical effect, having nothing
>to do with quantum mechanics, and would look much the same for any clear
>molecular atmosphere.  Radio waves scattered by a cloud of baseballs would 
>behave just the same.  Under high spectral resolution the blue-sky spectrum
>would show all the solar Fraunhofer lines, modified only by very minor effects
>such as Doppler broadening due to the earth's atmosphere. 
>
>In fact I believe the result is very general, that waves of ANY kind, when 
>scattered by obstacles much smaller than the wavelength, will scatter according
>to the Rayleigh law.  Along those lines -- I have heard that Lord Rayleigh's
>original analysis was extremely ingenious, being based more on raw mathematics
>than any physics, but I have never seen it.  Can anyone enlighten us about how
>it went? 
>
>Ben
>

OK.  Let's see if I'm getting anywhere.  All the light hits
the atmosphere and starts bouncing around, the odds of it bouncing
being some factor of how many molecules there are to hit, how big they
are, and so forth, and on the wavelength; the odds of bouncing get better 
the shorter the wavelength gets.  I assume the Rayleigh equations 
calculate the actual odds, given a molecular size, density, and a 
wavelength.  (Holding the speed of light constant.  8^)  

Blue takes enough extra bounces that a lot more of it bounces into the eye
than do the lower frequencies. The lower frequencies do in 
fact get back to the eye, but compared with the blue, they're small change.  
When looking through a lot of the atmosphere toward the rising horizon, 
more of the blue has scattered, leaving relatively more red, which has
miraculously managed to come through almost unscathed -- unscattered. 
The unbouncy unbouncy red rubber ball of song.  8^)

Does this mean that UV-sky is "brighter" than the blue?  Maybe that
warning that says DON'T LOOK DIRECTLY AT THE SUN ought to read
DON'T LOOK UP DURING THE DAY WITHOUT PROTECTIVE LENS.  Oh.  They
already have done that, haven't they?  Sunglasses. Right?  

  ,...,.,,
 /666;    ',    jim tims
////;    _~ -   p00168@psilink.com
(/@/----0-~-0
 ;'  . `` ~ \'
  , `    ' , >
;;|\..((   -C---->>  Let me guess.  UV is short enough to where the molecule
;;| >-  `.__),;;     size starts to matter, and O3 is more opaque than O2.