I understand mathematically etc how light can doppler shift when looked at as a wave but can it be understood how light doppler shifts as a photon i.e. could just one photon be doppler shifted? For example; Say there is an atom with energy levels 1 and 2. One observer, stationary relative to the atom, observes a photon of energy level 1 excite the atom (to level 1 of course). Another observer is moving relative to the atom such that they observe the photon to have enough energy, due to the doppler shift, to excite the atom to level 2. Surely, then, they would observe the atom excite to level 2? However, this does not agree with the first observer. I feel that there must be something fundamentally wrong with my understanding of this process and that before I seek the answer to this particular question there is more to be understood about the process of Doppler shifting etc.