If a mode of light is in the single photon state in the reference frame of the emitter, what will the state look like in a reference frame where the wavelength is, say, 5% less or more? How about a state with, say, 5 photons? I saw some online discussions and some papers on arXiv (not necessarily peer reviewed) where it was suggested that the photon number is invariant. Unfortunately it was a while ago and I don't remember details. But It seems to me that you can conserve either photon number or energy but not both when you change to a different frame. What is a good reference that gives the correct way to find what |N> looks like in any inertial frame?