What does Niels Bohr’s complementarity principle say about light and electrons?

The complementarity principle, introduced by Niels Bohr in 1928, states that a complete understanding of atomic-scale phenomena requires describing them as both waves and particles.1

Regarding light and electrons, the principle asserts that these entities exhibit wave-particle duality. Depending on the specific experimental setup, they may behave as either waves or particles, but these two aspects cannot be observed simultaneously. Bohr argued that while these characteristics are incompatible in large-scale objects, they are complementary at the atomic level; neither description is sufficient on its own, and both are necessary to provide a full account of the phenomenon.12