Check this scene:

//-------------------------------------------------------------------------
global_settings { max_trace_level 3 }

camera { location <0,10,-10> look_at <2,1.5,0> angle 35 }
light_source { <100,100,0> 1 }

plane { y,-1 pigment { checker rgb 1, rgb .5 } rotate y*30 }

sphere { 0,1 pigment { rgbf <1,1,1,.9> } interior { ior 1.5 } }
sphere { <2,2,0>*.8,1 pigment { rgbf <1,1,0,.9> } interior { ior 1.5 } }
sphere { <4,4,0>*.8,1 pigment { rgbf <0,1,1,.9> } interior { ior 1.5 } }
sphere { <6,6,0>*.8,1 pigment { rgbf <1,1,1,.9> } interior { ior 1.5 } }
//-------------------------------------------------------------------------

  The shadow is green, as it should.
  Set the max_trace_level to 2. The shadow is still green (and the objects
are black, as they should).
  Set it to 1. The shadow is still green.

  How can this be possible? Povray has to pass through 8 surfaces (ie.
calculate 9 rays) from the floor to the light source to see what's the
color of the shadow. The max_trace_level doesn't seem to affect this.
  What's the max_trace_level for the lighting calculation? How can povray
calculate the shadow so fast (I can't notice any slowdown in the shadow area).
Or is the shadow ray calculated in another way I'm not aware of?

-- 
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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