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Arnold Sommerfeld (left, 1868-1951) and Neils Bohr (1885-1962), theoretical physicists. Bohr was one of the greatest physicists of history. It was he who first suggested that the orbital angular momentum of an electron in an atom had to be quantized (multiples of a certain fixed value). He used this theory to explain the emission and absorption spectra of hydrogen, finding excellent agreement with observations. Sommerfeld developed this idea, using elliptic orbits instead of Bohr's circular ones, and by showing how relativistic effects would be seen in the fine structure of atomic spectra. Sommerfeld and Bohr are seen here together in 1919.
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