I think I am right in saying that when we read a book, (for example), a photon hits the page and excites an atom, the electron that has been excited then returns to its previous state and in the process emits a photon corresponding to some energy band, (this defines the colour we see?). My question is, how quick;y does this happen, and does this put a restriction on our human 'frame per seconds'? ie. if an atom can only excite and de-excite once every second then we would only see what changes every second? Obviously there is more than one photon and the human eye and brain have there own limitations but I am assuming that humans have infinite processing power. If atoms do have a limit on this does it vary atom to atom or electron shell to electron shell? Cheers!