From Doppler Effect to Gauge Invariance

A novel introduction to gauge invariance is provided by basic pedagogical arguments. A standard problem on Doppler effect leads to a deep insight into the nature of the field theory which describes the photon. The gauge invariance emerges as a nontrivial symmetry which is necessary for the Lorentz invariance of local field theories, and some general properties of photons are recovered without any explicit use of Maxwell equations. The consequent local U(1) symmetry of all charged particles is then highlighted.