93641e31   
   From: timo@physics.uq.edu.au   
      
   On Sun, 15 Aug 2010, alex wrote:   
      
   > I am trying to work out why the difraction pattern is a cone rather   
   > than dots like from diffraction off a grating\lattice.   
   >   
   > Any analogies would be greatly appreciated.  Newton's rings look   
   > similar, could this be a clue?  Or is it from treating atoms a   
   > circular aperture to give cirlular diffraction rings?...   
      
   Some optical analogies are halos and coronae. Note that some (many!) of   
   these are produced by non-spherical particles, which will, individually,   
   not give rings. E.g., ice crystal halos, pollen coronae.   
      
   When you look at an ensemble of randomly-oriented scatterers, the ensemble   
   is effectively rotationally symmetric, even if the individual scatterers   
   are not. A cloud of ice crystals, pollen in the air, or a powder sample.   
      
   When the whole thing is rotationally symmetric, you get a rotationally   
   symmetric diffraction pattern.   
      
   --   
   Timo   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)