Attached Link: http://www.rockhounds.com/rockshop/gem_designs/refractive_index/

The 3dlapidary site has an explanation of refraction on it too, if anyone's interested. I think it's pretty much what ornlu said (though I might be wrong, I'm running on 3 hours of sleep and all the V's and n's and thetas are kinda running together in my mind), but they've got diagrams.