I recall being taught that electrons in various "orbitals" or "energy states" absorb and emit photons of a specific frequency. I also learned that relative speed changes the observed wavelength of photons, due to the Doppler Effect. So, how is it that some electron can "absorb" a photon of a specific frequency if there is always going to be some movement relative to the electron and from where the photon was emitted? What is the "margin of error" in the frequency of a photon that a specific electron wil still absorb it? In other words, assume that some bound electron in atom A1 moves from state S to S' and emits a photon of wavelength X. There's another electron A2 in state S' which absorbs X and switches to state S. I don't see how this can happen. I'm saying that A2 and A1 will nearly always have some motion relative to each other, and thus X leaving A1 is different from X arriving at A2 (unless there is some margin of error where the electron can absorb frequencies in the range Y to Z, and X is within this range). Could anyone provide insight into this? I must be misunderstanding something.