I recently tried my hand at writing a ray tracer (something I haven't done
for 15 years!) and, in particular, I to write the shortest possible ray
tracer which was still comprehensible and interesting. The initial result
was a 222-line OCaml program which incrementally renders a sphere-flake via
OpenGL:

  http://www.ffconsultancy.com/free/ray_tracer/index.html

I then cut this program down to a 66-line OCaml program and ported it into a
97-line C++ program. These programs are compared on this page:

  http://www.ffconsultancy.com/free/ray_tracer/comparison.html

The cut-down versions don't do reflections, color or such a pretty
sphere-flake and output a greyscale PGM but they do still use hierarchical
spherical bounding volumes to accelerate rendering enough that they can
render scenes containing millions of spheres.