From: ckraftSPAMTRAP@west.net   
      
   On Wed, 20 Apr 2011 21:23:01 -0700, Louis Boyd   
    wrote:   
      
   >RichD wrote:   
   >> This may be a dumb question, I never studied acoustics....   
   >>   
   >> If you look into the wrong end of a telescope,   
   >> everything looks reduced, 'anti-magnified'.   
   >> Is there anything analogous acoustically?   
   >>   
   >> In both cases, the phenomena is described   
   >> by wave equations -   
   >>   
   >> --   
   >> Rich   
   >   
   >   
   >It should be simple  to demonstrate a Galilean telescope (one positive   
   >and  one negative lens)  which will magnify or minify an acoustic image   
   >  using an ultrasonic imaging  device and some  common optical lenses.   
   >They would only  need to be made of a material with a different velocity   
   >of propagation of sound  than a liquid they are submerged in.  The   
   >velocity of propagation of sound is somewhat  analogous to the velocity   
   >of propagation of electromagnetic energy to demonstrate refraction at   
   >material boundaries.   
   >   
   >"Magnify" doesn't have the same meaning in imaging and  non-imaging   
   >situations.   
   >   
   >An acoustic "telescope" doesn't have to be refractive.  A two mirror   
   >Cassegrain reflecting telescope  can  focus sound as well as   
   >electromagnetic energy.   I've experienced that working on microwave   
   >antennas.   
      
      
   I have an audio reflector, just the parabola, no subreflector.  I have   
   also worked around microwave antennas, and am trying to understand how   
   one would look backwards through a reflective telescope.   
      
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