Hi, Consider a person who holds a light source of frequency f. Consider an observer who sees this light. The energy of each photon as received by the observer is E = hf. The observer is now fixed. Now suppose that the person who holds the light source starts to move towards the observer, then according to Doppler effect, the observer receives the light with a frequency f2 which is different from f. Therefore, according to this observer, the energy of the photon changes. I tried to look about this issue on the internet, but they say that when moving with respect to the source, energy is not conserved. However, we can make the observer fixed, and make the source to move. In this case for the observer frame of reference the energy of the photon changes. How to resolve this considering the principle of conservation of energy? Thanks to any replies.