# Refractive index

(I'm still reading the QED book by Feynman...)

What property of the material causes a specific refractive index for a particular medium? (in other words, from the FAQ section by ZapperZ, "So the lattice does not absorb this photon and it is re-emitted but with a very slight delay.". How is the delay different between different materials.)

Classically, it comes down to how easily a material can form electric or magnetic dipoles. In a dielectric, for example, when an electric field is applied, the bound electrons will move further away from their host nuclei, forming dipoles which will in turn reinforce that field. For most materials the polarization of the material is proportional to the applied field, and the constant of proportionality is wrapped up into the electric permittivity. Likewise, applied magnetic fields will often induce a proportional response in the magnetic polarization: this is wrapped up into a constant known as the permeability. Together, the permittivity and permeability determine the speed of field propagation through the material, which in turn gives the index.

Claude Bile