Hi All, this is puzzling me; we have a real part of the refractive index that governs refraction, ie. scattering, and we have an imaginary part, which describes the absorption of the light. The two are related, as you would expect, and a mathematical relationship exists between the two. The Kramers-Kronig relation. When you see graphs of the real and imaginary part of the refractive index, there is usually an abrupt change in the absorption at some photon energy, i.e. an absorption edge, where the photon now has enough energy to ionise the atom at that level. this is fine. If you look at the REAL part, there is a gradual change around this edge, this I have a problem with... Considering a photon with an energy just below the absorption edge, it will not be absorbed so strongly as it does not have enough energy to ionise this level. HOWEVER, it is being scattered very strongly as it is near a resonant frequency. Classically, I guess it kinda makes sense, its close to the frequency, but in terms of quantum mechanics, I dont get it, it does not have enough energy to be effected by that electron.. so why is the absorption spectrum so abrupt, and the scattering spectrum so smooth? many thanks!