lighting: luminosity decay with distance?

Hello,

I'm rather new to VPython, using V5 w/Python3.1 and just wondering why
I'm not seeing luminosity decay when I move a simple sphere away from
a (local) light source.   It does nicely with reflectance across the
curved surface (decaying as the angle gets towards the tangent), but
nothing seems to decay as it goes farther from the light source.
Physics says it should decay at a factor r^3 (imagining photon
emission at a constant rate, it should strike farther distances with
less probability).  If implemented, the perceptual depth-cueuing to
the brain would be MUCH more effective than the (currently simple)
size adjustment.   But perhaps I'm just not perceiving it.(?)

Am I stuck with manually decreasing the (gray-scale) color of the
sphere as the objects moves away from the light source??

Cheers and such an easy-to-use package!

marcos

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