From: oliver.jennrich@gmx.net   
      
    writes:   
      
   > I understand that light from stars is coherent and can be treated   
   > as a plane wave. I wonder how can I calculate the amplitude A of   
   > such plane wave A exp[ct-kx] for a given star form its magnitude,   
   > bandwidth, distance and other parameters of the star.   
   >   
   > What is the typical range for A for a typical wavelength?   
   >   
   > Thanks   
   >   
   > [[Mod. note --   
   > 1. The light from a star is *incoherent* -- each of the huge number   
   >    of atoms in the star's photosphere is radiating independently, and   
   >    the light we receive is (that tiny fraction that happens to be   
   >    radiated in our direction) the incoherent sum of light from many   
   >    of those atoms.   
      
   It is useful to distinguish between *temporal* and *spatial* coherence.   
      
   > 2. The light from a star is coming from very far away, so to a *very*   
   >    good approximation it can be treated as a plane wave.   
      
   As it is, the light from stars show only (partial) *spatial* coherence -   
   the larger the angular diameter of the star, the less spatial coherence   
   there is. This is the principle behind stellar interferometry [1]   
      
   [1] see e.g.   
   https://www.eso.org/sci/facilities/paranal/telescopes/vlti/tuto/   
   utorial_introduction_to_stellar_interf.pdf   
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