Wave-particle duality
Also called Duality
Wave-particle duality is the finding that light and matter each behave as waves in some experiments and as particles in others, with no single classical picture covering both. Which behavior appears depends on what the experiment measures.
Essential knowledge 15.1.A.1.ii puts it flatly: in quantum theory, fundamental particles can exhibit both particle-like and wave-like behavior. Neither picture is decoration on the other.
Evidence that light is a wave sits in Unit 14. Double-slit interference, diffraction, thin-film color and polarization all need a wave to explain, and none of them work with tiny bullets.
Evidence that light is particle-like sits in Unit 15. The photoelectric effect has a threshold frequency and an ejected-electron energy that ignores intensity (15.5.A.2), and Compton scattering behaves like a two-body collision solved with conservation of energy and momentum (15.6.A.2.i). The CED cites both as evidence for discrete, quantized packets.
Evidence that matter is wave-like comes from 15.1.A.4: particles demonstrate wave properties in variations of Young's double-slit experiment, quantified by the de Broglie wavelength.
The usable test is 15.1.A.4.ii. Quantum theory is necessary where the de Broglie wavelength is comparable to the size of the system. Above that scale the particle picture is sufficient, which is why duality never surfaces in a mechanics problem.