Niels Bohr was a Danish theoretical physicist whose research helped establish quantum theory. Born in Copenhagen, he earned his doctorate at the University of Copenhagen in 1911. In 1913, he proposed an atomic model in which electrons occupy specific energy levels, explaining the spectrum of hydrogen. He received the Nobel Prize in Physics in 1922.

He founded the university’s Institute for Theoretical Physics in 1921, bringing together researchers who helped develop quantum mechanics. His principle of complementarity described how apparently conflicting accounts, such as wave and particle behaviour, can both be needed to explain observations. He debated the interpretation of quantum mechanics with Albert Einstein.

After escaping occupied Denmark in 1943, Bohr worked with the Allied atomic bomb project. He also called for international openness about nuclear technology and cooperation to prevent an arms race. He remained active in research and public affairs until his death in Copenhagen in 1962.