[elihu]

Not the person you're replying to, but in general refraction is pretty easy to do in a ray tracer. When the ray hits the object you use the normal vector, the incident ray direction, and the index of refraction of the material (or materials, if you're transitioning from one substance to another with different index of refractions) to calculate the new ray direction and continue from there. Usually there's some amount of reflection, too (the amount varying based on the angle of the incoming ray with respect to the surface), so you might spawn two rays: one continuing through the object, and the other bouncing off.

What's much harder to do is simulating the light that's refracted or reflected off of objects (called "caustics"), like the light on the bottom of a swimming pool. To do that in a physically correct way generally requires falling back on some kind of global illumination technique like path tracing or photon mapping.