Portrait of the German physicist Arnold Sommerfeld (1868-1951). He worked on a variety of problems including gyroscopes, X-ray and electron diffraction and radio waves. However, Sommerfeld is better known for his theory on the atomic spectra. He developed the Bohr's theory of the atomic structure by replacing the idea of circular electron orbits by elliptic orbits, with the atomic nucleus at a focus of the ellipse. In doing so he also included the effects predicted by the theory of relativity elaborated by Einstein. Sommerfeld's atomic model was the first attempt to untangle the atomic structure by combining quantum and relativistic effects.