From: boxman@voyager.net   
      
   On 3/24/2013 10:09 PM, W. eWatson wrote:   
   > I've searched the web for caustics and optics, but mostly see pictures   
   > of them. They seem to have some positive use. In some fashion they seem   
   > to magnify. They seem to be somehow related to critical curves, whatever   
   > they are. Both seem to be helpful in gravitational lensing or in other   
   > areas of optics. How?   
   In non-imaging optics (those that deal with concentration and   
   illumination rather then image forming) the caustic is defined as the   
   envelope of a one-parameter family of light rays.  By definition that   
   envelope is a curve that is tangent to every curve in the one parameter   
   family.   
      
   A simple example would be a set of parallel rays reflecting off a   
   parabolic reflector at some angle to the parabolic axis.  In this case   
   the family of rays are straight lines.  The caustic curve in this case   
   would be the optimal receiver shape to provide maximum collection.   
      
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