In 1917 Albert Einstein proposed a third way light could interact with an atom. If a photon with the correct energy struck an atom in the excited state then the atom would be stimulated to emit an additional photon and drop to the lower energy level. The two photons leave with the same energy (in terms of a wave— same wavelength) and in phase.

Today’s model is totally different from Bohr’s 1913 model. Bohr’s model could explain only the spectra of hydrogen and helium from which an electron was removed. With some modifications is could also explain the spectra of the alkalis like lithium. From 1913 through 1926 physicists tried to extend the model, with some successes, but the lack of a physics-based explanation of the postulates led to research aimed at a model that was not based on classical ideas like Bohr’s. One of the first steps was taken by the young German physicist Werner Heisenberg (1901-1976).