What is Refraction Refraction is the process whereby light enters a transparent medium and its direction of travel is altered. When the ray hits the surface it doesn't travel in a straight line but is bent at an angle. This angle depends on three things, the angle the ray hit the surface at, the refractive index of the medium it was initially travelling through and the refractive index of the medium it is going to travel through.
Refraction is due to the scattering of the light by the atoms that make up the material.
For most materials which have a regular interatomic spacing this means that the material has a single refractive index. There are some materials which due to an unusual interatomic spacing pattern have two refractive indices depending on from what direction the material is viewed. These materials are called birefringent and are the basis of a lot of non-linear optics. They are beyond the limits of this web site but further details can be found in the books in the bibliography for those interested. Snell's Law Snell was the person who first quantified the refraction effect, although it has been studied for centuries.