What principle did Niels Bohr introduce to describe wave-particle duality?

Niels Bohr introduced the principle of complementarity to describe wave-particle duality. The principle says that quantum objects can display wave-like or particle-like behavior, but the experimental setup determines which aspect becomes observable. Both descriptions are necessary for a complete account, even though they cannot be fully revealed in the same measurement arrangement.

Bohr presented this idea publicly in 1927, during the development of the Copenhagen interpretation of quantum mechanics. Experiments such as the double-slit experiment show the tension clearly: an interference pattern reveals wave behavior, while which-path detection reveals particle-like behavior and destroys the interference pattern.

Complementarity is often confused with Heisenberg’s uncertainty principle. The two ideas are closely related but not identical. Uncertainty concerns limits on jointly defined quantities such as position and momentum; complementarity is the broader conceptual framework about mutually exclusive experimental descriptions. Superposition, meanwhile, describes how quantum states can combine before measurement.