From: heathjohn2@gmail.com   
      
   On Monday, February 5, 2018 at 12:30:39 PM UTC-5, questions...@gmail.com wr=   
   ote:   
   > 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.   
   > 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.   
   > 3. The amplitude of a plane wave is directly related to the intensity   
   >    of the light, i.e., how bright the star is.  The book "Astrophysical   
   >    Quantities", by Allen, has numbers for home many photons/second   
   >    per square centimeter of detector area we receive for a given   
   >    magnitude star, but I don't recall these offhand.  Converting to   
   >    an amplitude of a plane wave takes a little bit more algebra...   
   > -- jt]]   
      
   T= 1,000,000,000,000   
   n = .000,000,001   
   p = .000,000,000,001   
   Hz = cycles per second , frequency   
   Standard engineering stuff   
      
   The frequency of white light from a star is in the neighborhood of   
   500 THz and the size of the photons is around 500 n meters. The   
   energy of the individual photons is E=fh for Energy = Frequency   
   times h Planck's constant. We are half way to answering your question.   
      
   The next variable is a little tricky to measure. How much energy   
   is being receive from the star of choice per square meter. For   
   example the sun gives us 1300 watt per square meter. At 1300 watt   
   per second the photons , 500 n meters big , with energy E=fh are   
   over lapping to fit inside 1 square meter so you may consider this   
   to be more of a classical wave than individual photons.   
      
   However weak light from a given star , 1 p watt per second , with   
   photons 500 n meters big could have spaces between individual photons   
   within a square meter. In this case the white light is weak enough   
   to be quantifying into individual photons that are not over lapping   
   each other. You may consider this light to be particles , photons   
   , not a classical wave.   
      
   To address your question strong light will act like a wave. Vary   
   weak light will start to reveal its energy quantification into   
   photons. The answer is not yes or no. It depends on the energy   
   density per square meter of the light from the star.   
      
   [[Mod. note -- To briefly address two other widely-misunderstood points   
   (that haven't come up in this thread, but where there is often confusion):   
   * Interference and diffraction *do* occur even with very faint light,   
     including light that's so faint that the mean number of photons in   
     the apparatus is much less than one.   
   * Because photons are bosons, you can indeed do interferometry even   
     with incoherent light such as starlight; this is usually known as the   
     Hanbury Brown/Twiss effect:   
        https://en.wikipedia.org/wiki/Hanbury_Brown_and_Twiss_effect   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)