Optical methods for the study of radio-frequency resonances

D URING MY FIRST YEAR as a student at the Ecole Normale Supérieure in Paris, our professor, Eugene Bloch, introduced us to quantum physics, which was seldom taught in France at that time. Like him, I was from Alsace and spoke German. He strongly advised me to read Arnold Sommerfeld's excellent book Atombau und Spektrallinien. When reading this book, I was particularly interested in the application of the principle of conservation of angular momentum in interactions between electromagnetic radiation and matter—an application that guided A. Rubinowicz to the interpretation of the selection rules for the angular momentum quantum number and for polarization in the Zeeman effect. On the assumption of light quanta, this principle led to attributing an angular momentum to the photons $\text{+h/2π}$ or $\text{−h/2π,}$ according to whether the light was circularly polarized to the right ${\mathrm{(\sigma }}^{+})$ or to the left ${\mathrm{(\sigma }}^{-}\mathrm{),}$ natural light being a mixture of the two types of photon.