<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by JMB</i>
<br /><blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Thomas</i>
I don't think that you will observe refraction in the visible region of the spectrum at satellite level.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Yes, directly not, but if the scattering was incoherent, a blur of the images would be observed.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
Any scattering <i>is</i> incoherent at this height for wavelengths in the visible region and shorter wavelengths, so images will be blurred if the optical depth is large (which at such low densities is only the case for resonance scattering i.e. within spectral lines).
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by JMB</i>
<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Thomas</i>
The density of the atmosphere at let's say a height of 1000 km is just about 10^6 cm^-3. Phase-coherent effects at this height should thus require a wavelength of at least 10^-2 cm (which is in the far infrared).
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
You must look at the mechanism of the coherent scattering: individually, each molecule radiates a scattered field in all directions (almost), but the interference of the fields destroys them in all directions, forward and backwards, except in the direction of the exciting field, supposing that there are no fluctuations producing an incoherent scattering (Rayleigh or Raman).
An important incoherent scattering requires a high column density (compare the blue of the sky with the Sun)<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
The point is that Rayleigh scattering in the lower atmosphere <i>is</i> not important. The total scattered radiation we receive from a clear sky is at best about 10% of the direct solar illumination. In principle there should be none at all as the wavelength of light is much larger than the distance between two molecules in the atmosphere i.e. we should merely have a coherent forward scattering. But the number of molecules within a wavelength is not infinite but has a finite number with statistical variance. At ground level, the average distance between two molecules is 1/250 of the wavelength at 5000 A, so the relative statistical variance is 1/sqrt(250)=6*10^-2. Now the Fraunhofer criterion for a perfectly smooth surface (i.e. perfectly coherent scattering) says that the variance has to be less than 1/32 of the wavelength. However, 6*10^-2 is larger than 1/32 (by a factor 2) so there is no perfectly coherent scattering in this case, but a small contribution of incoherent scattering as well. It is this which (in my opinion anyway) is observed as the sky blue.
Thomas
www.physicsmyths.org.uk
www.plasmaphysics.org.uk
