Fun fact: in most physically based ray tracers, the photons are shot by the camera, bounce on various objects, and finally hit a light (or a sky box), whose energy is then back propagated along the chain of bounces. In physics everything is reversed, the photons are generated by a light, and bounce on various objects (which change their energy by absorbing it) before ending up in a camera sensor. In fact, some advanced ray tracers follow this second model, but they are much much harder to build, and, in general, require higher computational power.

If you are looking for a good (you can build your own ray tracer) but not too hostile intro to physics based rendering I recommend "Advanced Global Illumination" by Dutre and others (http://www.amazon.com/Advanced-Global-Illumination-Second-Ed...). It is actually quite a fascinating computer science application, and you can produce beautiful images with it.