I discuss some of the didactic problems in the introduction to quantum physics based on the concept of photon. I specifically argue that the derivation of the photon energy from the relativistic Doppler shift offers stimulating didactic opportunities. A detailed form of this derivation reveals the subtle link between relativity and quantization, and also helps in elucidating the meaning of the invariance of the speed of light
Tobias Kippenberg, Jérôme Faist, Marco Piccardo