Re: Warum ist der Himmel blau?

Ralf Kusmierz wrote:

Ich hatte mal kürzlich frech behauptet, die Streuzentren seien die
statistischen Dichteschwankungen der Atmosphäre und dabei die
Vorstellung im Hinterkopf, daß deren charakteristische Abmessungen
deutlich näher an der Wellenlänge lägen und dadurch wesentlich
streuwirksamer wären als die reichlich winzigen Atome bzw. Moleküle,
bloß finde ich dafür keine Belege.

Anscheinend kann man das halten, wie man will. Vielleicht helfen die
folgenden Schnipsel aus einer Diskussion auf der HASTRO-L Mailingliste
ein wenig weiter:

=============================================================================

Subject: Re: [HASTRO-L] Blue sky and scattering (was:"golden thigh")
Date: Sat, 12 Apr 2003 20:09:22 +0100
From: Les Cowley <ast1@xxxxxxxxxxxxxxxxxx>
Reply-To: History of Astronomy Discussion Group <HASTRO-L@xxxxxxxxxxxxxxxx>
To: HASTRO-L@xxxxxxxxxxxxxxxx

Stephen Tonkin <mb@xxxxxxxxxxx> wrote

Well, well! I learn something new every day, and in this case it is
something of value. Up until about 15 minutes ago I had assumed that the
Einstein-Smoluchowski effect was only dominant in liquids and that the
particle separation in the atmosphere was too great. Thank you! (I like
the web site, BTW -- bookmarked.)

Do you know at about what altitude the Einstein-Smoluchowski effect
"takes over" from Rayleigh scattering?

Hecht "Optics 4th Edition" discusses this very point and concludes that
Einstein-Smoluchowski scattering is dominant in all but the upper atmosphere
where traditional Rayleigh scattering purely by air molecules is too weak to
contribute much to the sky colour at ground level.

Apologies if this is (partly) off topic but the reasons for the colour of the sky
are widely misunderstood and misleadingly taught.

Les Cowley

=============================================================================

Subject: Re: [HASTRO-L] [Fwd: Blue sky and scattering (was:"golden thigh")] (fwd)
Date: Mon, 14 Apr 2003 15:18:14 -0600
From: "Prof. E. F. Milone" <milone@xxxxxxxxxxx>
Reply-To: History of Astronomy Discussion Group <HASTRO-L@xxxxxxxxxxxxxxxx>
To: HASTRO-L@xxxxxxxxxxxxxxxx

I forwarded some of the initial comments about this to Andy Young, a leading
interpreter of astronomical phenomena involving atmospheric effects and here is
his response.
- gene milone


E. F. Milone, Professor and Director, Rothney Astrophysical Observatory
Department of Physics & Astronomy
The University of Calgary
2500 University Dr., NW
Calgary, AB T2N 1N4
Canada

Telephone: 403-220-5412
Fax: 403-289-3331
email: milone@xxxxxxxxxxx
URL: www.ucalgary.ca/~milone

---------- Forwarded message ----------
Date: Mon, 14 Apr 2003 13:37:19 -0700 (PDT)
From: Andrew Young <aty@xxxxxxxxxxxxxxxxx>
To: milone@xxxxxxxxxxx
Subject: Re: [Fwd: Blue sky and scattering (was:"golden thigh")]


Andy, Does this sound right to you?
- gene
--------------12514067C20320FA20237E9E
Content-Type: message/rfc822
[...]
MIME-Version: 1.0


Stephen Tonkin <mb@xxxxxxxxxxx> wrote

Well, well! I learn something new every day, and in this case it is
something of value. Up until about 15 minutes ago I had assumed that the
Einstein-Smoluchowski effect was only dominant in liquids and that the
particle separation in the atmosphere was too great. Thank you! (I like
the web site, BTW -- bookmarked.)

Do you know at about what altitude the Einstein-Smoluchowski effect
"takes over" from Rayleigh scattering?

Hecht "Optics 4th Edition" discusses this very point and concludes that
Einstein-Smoluchowski scattering is dominant in all but the upper atmosphere
where traditional Rayleigh scattering purely by air molecules is too weak to
contribute much to the sky colour at ground level.

Well, I suppose you're talking about "fluctuation scattering". But to
regard Rayleigh scattering at normal atmospheric densities to be due to
density fluctuations is basically equivalent to treating scattering by the
individual molecules whenever the density is low enough that the ideal-gas
law is applicable. The point is that if the density is so low that the
positions of the molecules are uncorrelated, the two descriptions are
equivalent.

I think this is a perverse point of view -- particularly as Rayleigh
himself concluded that most of the light from the blue sky could be
accounted for by scattering from individual molecules.

Apologies if this is (partly) off topic but the reasons for the colour of the sky
are widely misunderstood and misleadingly taught.

See my paper on the history and terminology of "Rayleigh" scattering in
Physics Today 35, 42 (1982). Some of the more extreme theoretical physics
types seem to delight in adopting an overly-complicated explanation for
everything, which I think is just dumb. That's part of the reason why
things like this are widely misunderstood: the obscurantists have had a
field day with elementary physics. And you can tell them I said so.

While it's true that the density fluctuations can be Fourier-analysed
into sound-wave-like components, and the resulting Brillouin scattering
can be observed, this is really a very small effect that is difficult to
detect (as a slight distortion of the Gaussian line profile due to
scattering by moving molecules). George Kattawar and I had some papers on
these effects in the atmosphere back in the very early 1980s.

-- Andy


=============================================================================

Subject: Re: [HASTRO-L] Blue sky
Date: Tue, 15 Apr 2003 01:52:16 +0400
From: "Michael L. Gorodetsky" <gorm@xxxxxxxxxxxxxxxx>
Reply-To: History of Astronomy Discussion Group <HASTRO-L@xxxxxxxxxxxxxxxx>
Organization: MSU, Faculty of Physics
To: HASTRO-L@xxxxxxxxxxxxxxxx

Dear all,
there is no physical difference between scattering on uncorrelated
thermodynamical fluctuations of density described by Einstein-Smoluchowsky
and Rayleigh scattering on particles much smaller than wavelength. The model
of Rayleigh of small scatterers in atmosphere published in 1871
is quite adequate. In both cases scatterers behave as oscillating dipoles
with intensity of scattered light inversely proportional to the forth power
of wavelength and squared cosine angular dependence. As the wavelength
dependence was discovered by Rayleigh and it is the reason of blue color of
light, the term Rayleigh scattering explaining blue sky seems quite correct.
In the same way scattering in fibers on frozen at glacification point
fluctuations of density is also called Rayleigh scattering though the
calculations also follow E-S.


--
-------------------------------------------------------------------
Thomas Schmidt e-mail: schmidt@xxxxxxxxxxxxxxx
.

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