NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/physics/11399

Internet Message Format | 1992-07-22 | 3.5 KB

Path: sparky!uunet!gatech!mailer.cc.fsu.edu!sun13!ds8.scri.fsu.edu!jac
From: [e-mail] (Jim Carr)
Newsgroups: sci.physics
Subject: Re: Blue Sky
Message-ID: <[e-mail]>
Date: 22 Jul 92 20:38:24 GMT
References: <[e-mail]> <[e-mail]> <[e-mail]>
Sender: [e-mail]
Reply-To: [e-mail] (Jim Carr)
Organization: SCRI, Florida State University
Lines: 60

In article <[e-mail]> [e-mail] (John C. Baez) writes:
>In article <[e-mail]> [e-mail] (Matthew J. Bernhardt) writes:
>>[e-mail] (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
[e-mail]                           |  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)