ABSTRACT
Einstein extended the quantization to light itself with the photon hypothesis with the explanation of the photoelectric effect. Bohr used Planck's ideas to explain the line emission of hydrogen atoms which gave an indication that the newborn quantum mechanics was giving a better description of experiments. In this chapter, the authors not follow a rigorous mathematical description. The postulates come in classes: the description of states, the description of physical quantities, the description of the act of measurement, the effect of the measurements on the state and the time evolution of a quantum system. Sommerfeld modifications of Bohr's theory was seen favorably because it gave a solutions to a few anomalous effects like the Zeeman effect and the Stark effects. As in the classical case of a two body problem, positive values of the total energy E correspond to free particles while negative values of E correspond to bound states. In quantum mechanics, electrons are indistinguishable.
