> ... the diffraction grating model itself    
   > depends on assuming a constant speed of light, c, as    
   > per Einstein's second postulate.   
      
   Given that diffraction effects can be demonstrated in other situations, such   
   as with water waves, claiming the model is based on a cosmological constant   
   seems meaningless to me.   
      
   At any location the result of interference between light from two points on   
   the grating will depend on the phase difference: for equation-writing purposes   
   that difference is often based on distance by assuming a constant speed. The   
   real world isn't as    
   constrained: what if the grating is in a medium with refractive index   
   variations?   
      
   On the other hand, if the phase difference itself is changing with time, then   
   the diffraction pattern will get washed out, so there is an underlying   
   assumption that the frequency is constant (or at least is changing "slowly"   
   compared with the sampling of    
   the diffraction patterns).   
      
   Hope this helps   
      
   Henry   
      
   On Saturday, 5 October 2019 09:17:53 UTC+1, RichD  wrote:   
   > On another board, during some discussion of relativity,    
   > it was claimed that a test of relativistic Doppler shift    
   > could be performed via frequency combs (a recent tool)    
   > and diffraction gratings; i.e. v = fλ   
   >    
   > Then, someone objected, dismissing the argument as    
   > circular, since the diffraction grating model itself    
   > depends on assuming a constant speed of light, c, as    
   > per Einstein's second postulate.   
   >    
   > I don't recall this.  Anyone here wish to comment?   
   >    
   > --   
   > Rich   
      
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