Hi there, I've a few (probably very basic) questions about photon-electron (light-matter) interactions. Here we go: 1) How can an electron "understand" that the frequency of incoming photon is equal to its excited state and so absorb that. Is there any "virtual" resonance between the electron and its excited level so a photon with the same resonance frequency can interact with the electron & electron's excited state system? 2) Why (or how) do refractive index of a medium differs for light with different frequencies? It's probably related with energy conversation or something but I cannot get it. Mathematics of these questions are OK but I cannot understand these facts in a physical sense of manner. Thanks in advance.