I'm far from an expert either! The important thing is that the sea of electrons in metals are able to interact with EM waves coherently, so phase is conserved,

I'm far from an expert either! The important thing is that the sea of electrons in metals are able to interact with EM waves coherently, so phase is conserved, so the classic ray optics happens, an that's why we can see things in a mirror, while in case of a white non-mirror surface we have the absorption-emission, and in that case the emitted photons are going everywhere, so we just see a big homogeneous surface. > I take it that this interaction between photons and plasmons is equivalent to the changing of energy states of electrons and the release of photons as electrons fall to lower states. Yes, except "better", because it keeps the phase. > band gap So there's the optical band gap and the electric. (The electric is required for charge transport, so it's sometimes also called transport gap.) So a cloth is an insulator, in an insulator the band gap is pretty big. The incoming photon does not lead to transport, but it leads to optical excitation. And then an emission of a photon with a lower energy. In some instances the relatively high energy of the incoming photon gets converted through various radiationless processes into other forms of energy. For example molecular rotation and vibration (usually photons in the infrared and microwave range do this to matter). These also lead to at least one quantum number change [as every interaction in a quantum theory must], but in case of UV or visible light the electron getting excited and jumping to a higher atomic orbital shell is in an atom that's bound in a molecule. The incoming EM field wave is not high energy enough to "penetrate into" the atom, it interacts with the whole sum of the EM field of the molecule. (And the energy basically gets transferred into the molecular bond: https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy#Principle_of_ultraviolet%E2%80%93visible_absorption https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_sp... ) See also: https://en.wikipedia.org/wiki/Hot_band https://en.wikipedia.org/wiki/Hot_band