A few years ago I had an idea for a ray tracer that would simulate
each photon's quantum electrodynamic contribution to the resulting
image.  It would, of course, require vastly more processing time than
conventional radiosity methods, which simulate photons en masse.  But
such a ray tracer would generate images with diffraction patterns,
reflection, refraction, chromatic aberration, and so forth, without
their having to be programmed specially.

Here's the original note I wrote myself about the idea:

20000218

A ray tracer which simulated the QED level.  It would generate a
photon of a certain color, then let it travel at random, the compute
its contribution to the result.  After generating many many paths, the
unimportant ones would tend to cancel out, just like in the real
world. 

Such a tracer would provide accurate rendering of things like
diffraction gratings, pinholes, soap bubbles, x-ray crystallography,
prisms, lenses, etc.

Does it even know that photons travel in straight lines?  You could
make this an optional feature.  Disabling this knowledge would make
the simulation run slower, but would enable accurate simulations of
photons in gravity wells.