If I make a cylinder like this:

cylinder { -y*2,y*2,1 material { Glass } photons { ... } }

and put it on a plane, I get a nice caustic highlight on the plane, but if I
make the cylinder hollow this way:

difference
{ cylinder { -y*2,y*2,1 }
  cylinder { -y*1.8,y*2.01,.8 }
  material { Glass }
  photons { ... }
}

I don't get any caustic highlight anymore.

  This may:
  a) just work fine. I don't get a caustic higlight on the plane because I
shouldn't, because this kind of object doesn't refract the light in that way.
  b) be a bug in the program.
  c) be because I haven't understood how photons work.

  Which one of those answers is correct and why?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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