User:Korovyev/Sandbox

Microscopic origin
The refractive index is the result of the scattering of electromagnetic radiation by a material. In a semi-classical picture, the electric field associated with a wave displaces the electrons in an atom, which oscillate radiating in various direction. The original and scattered radiation interfere and, for an homogeneous medium, only the component propagating in the same direction than the original wave survive (forward scattering). The net result is a phase shift of the incident wave proportional to the length of the material. This appears as a slowing down of the speed of in a medium.