Variations in Refractive Index of Materials It's quite a common fact that different types of glass have different refractive indices. Most sites I've found attribute these differences to variations in the 'density' of the glass, which is not very satisfying of an answer.
My questions:

*

*What is the underlying physical mechanism that determines the refractive index and its variations in glass?


*For other materials, such a Silicon\Gold\Silver\etc., does the refractive index have a fixed and known value (by model or in principle) under the assumption that the material in question is 'pure' (i.e. no foreign materials)?
 A: Heavy glasses are mainly heavy because the use elements such as lead with many nuclei. As a result, these materials also have more electrons per atom, especially in the higher (more loosely bound) orbitals.
The looser bound the electrons, the more they will react to the electric field wave. How exactly this increases the refraction index I don't know, but I will provide some handwaving argument that is hopefully useful to you. The electrons getting dragged around (i.e. a plasmon) have an effective mass and thus the plasmon has kinetic energy, which takes away the energy of the actual photon and reduces propagation speed.
I hope this is not too inaccurate, but hopefully someone else can expand
Tables on refractive index and loss for metals. Metals mainly have a large loss term because of unbound electrons which couple to phonons and dissipate energy. When you dope or mix metals, surely their refractive index will change. Metals also have a real index of refraction, which is strongly frequency dependent as can be seen in the link. Unfortunately, the simple picture I draw here doesn't provide any clue as to why this is so.
