From: jac@ds8.scri.fsu.edu (Jim Carr)
Newsgroups: sci.physics
Subject: Re: Blue Sky
Date: 22 Jul 92 20:38:24 GMT
Reply-To: jac@ds8.scri.fsu.edu (Jim Carr)
Organization: SCRI, Florida State University

In article <1992Jul22.145723.24741@galois.mit.edu> jbaez@riesz.mit.edu (John C. Baez) writes:
>In article <BrrMJB.Brv@acsu.buffalo.edu> mjb@acsu.buffalo.edu (Matthew J. Bernhardt) writes:
>>andrew@rentec.com (Andrew Mullhaupt) writes:
>>
>>>Why is the sky blue?
>>
>>	I'll bite.  The composition of the Earth's atmosphere (four fifths
>>nitrogen, one fifth oxygen) is such that incoming light is refracted to make
>>the sky appear blue.

BZZZZt.

>As far as I know, refraction has nothing to do with it.  Rather, blue
>light is scattered.  In physics I recall learning how a free charged

ding!

>particle is more apt to scatter light of higher frequencies, and this
>was claimed to be the explanation for why the sky is blue.  Later I
>vaguely recall someone  saying that this was a misleadingly
>oversimplified account.  If anyone REALLY knows this stuff I'd be glad
>to hear it.  

Oversimplified, but not misleading.  And not charged particles.  What is
required for Rayleigh scattering is the presence of molecules with an 
electric dipole moment.  Neutral, but polarizable.  This is enough to 
produce a blue sky and a red sunset, and also explains why the light in 
the sky is polarized.  If there were no scattering, the sky would be black.

(Rayleigh scattering is inversely proportional to the FOURTH power of the 
wavelength, so the comparatively small difference between the wavelengths 
of blue and red light leads to a large difference in scattering probability. 
The blue light is scattered more, so you see blue when you look at light 
coming from an angle away from the sun, and you see the unscattered red 
light when you look at light from the sun -- especially when that light 
has to go through a lot of atmosphere when arriving obliquely at sunset.)

(The polarization is due to the way the scattering is produced by the 
absorption and reradiation of the light by the dipoles.)

This is oversimplified because the air also contains aerosols that also 
contribute a similar effect.  This is the reason we had such beautiful 
sunsets after the Mt. St. Helens and Mt. Pinatubo eruptions.  This effect 
is easily seen in the classic experiment where you put a drop of milk in 
an aquarium filled with water while shining a bright white light in 
one end.  You see blueish scattered light and reddish transmitted.

>I've always wondered why there are so few blue animals.  I am also
>reminded of George Carlin's routine beginning "There is no blue food!"

George did that routine before blue-corn became popular enough to 
show up as corn flakes as well as tortillas, or squid-ink pasta made 
it into american grocery stores.  Interesting that there are blue birds 
and flowers but not blue animals; where is a biologist when we need one! 

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)
Xref: sparky sci.physics:11426 sci.astro:8248 sci.philosophy.tech:2867