First, we render the objects with highlights turned off, or turned to zero. The result is a dull chalky look in our base color. Next, we render with highlights on and nothing else, once for each highlight. Now we read our two images into Photoshop. The amount of deflection depends on the materials in question and the angle the light hits the surface at.
Refraction indices are fairly abstract.
Simulating raytraced refraction effects is in some way easier than just raytracing them. By subverting a little bit more of CG rendering theory to our own use. Be sure to leave shadowing turned off. Open both of them in Photoshop. Because we had our lights set to pure red and pure green, and the chalk to pure white, the lighting for each side is in a completely separate channel.