complementarity principle, physical principle enunciated by Niels Bohr in 1928 stating that certain physical concepts are complementary. If two concepts are complementary, an experiment that clearly illustrates one concept will obscure the other complementary one. For example, an experiment that illustrates the particle properties of light will not show any of the wave properties of light. This principle also implies that only certain kinds of information can be gained in a particular experiment. Some other information that is equally important cannot be measured simultaneously and is lost.
See W. Heisenberg, The Physical Principles of the Quantum Theory (1930, repr. 1949); N. Bohr, Atomic Physics and Human Knowledge (1958); B. L. Cline, Men Who Made a New Physics: Physicists and the Quantum Theory (1987).