On Wednesday, April 15, 2020 at 2:50:01 AM UTC-5, Ned Latham wrote:   
   > I was thinking about parallel-sided glass blocks, and their   
   > fascinating demonstration of refraction of light on both   
   > entry and exit, and it occurred to me that I've never seen   
   > or heard anything about the beam itself except that it's a   
   > little wider on exit than on entry.   
   >   
   > So I wondered: has anyone ever looked at the exit beam in   
   > sufficient detail to detect a spectrum or dismiss the idea?   
      
   look up Fabry--Perot_interferometer on Wikipedia   
      
   Rich L.   
      
      
   [[Mod. note -- There's an e-acute accent on "Perot", but this got garbled   
   by some non-8-bit-clean software in the posting of this article. :(   
   The Wikipedia article in question is   
     https://en.wikipedia.org/wiki/Fabry%E2%80%93P%C3%A9rot_interferometer   
   -- jt]]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)