Measuring the separation of the sodium D-doublet with a Michelson interferometer

Revisiting a method proposed by Fizeau in 1862, in this paper we measure the separation of the Na-doublet (the wavelength difference Δ λ between the two emission D-lines of the sodium spectrum) with a didactical Michelson interferometer. We describe the setup, how the measurements have been done and develop a mathematical model in order to explain the principal features of the collected data. Discussing the limits of this model, we suggest further experimental and theoretical extensions of the experience, also focusing on the didactical aspects to show how this experiment could bring advanced modern physics topics into high schools.