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Danish physicist Niels Bohr (right, 1885-1962) walking with the Swiss-German physicist Albert Einstein (1879-1955, left). Bohr suggested the quantised electron orbital theory of the atom, which stated that electrons could only occupy specific energy levels around an atomic nucleus. This explained the spectrum of hydrogen, and revolutionised chemistry and physics. Einstein explained the photoelectric effect, and put forward the theories of general and special relativity. Einstein received the Nobel Prize for Physics in 1921, Bohr in 1922. Photographed in 1934.
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