Light behaves as a wave. Light behaves as a particle. Both are true. That is wave-particle duality, one of the strangest and most important concepts in quantum mechanics. In some experiments, light diffracts and interferes like a wave. In others, it strikes detectors like a particle. The same is true for matter. Electrons, protons, and even atoms have been shown to exhibit wave behavior. The double-slit experiment is the classic demonstration. Fire electrons one at a time through two slits and they build up an interference pattern, as if each electron passed through both slits at once. But when you measure which slit each electron goes through, the interference pattern disappears.

Wave-particle duality was proposed by Louis de Broglie in 1924. He suggested that all matter has a wavelength, now called the de Broglie wavelength, given by λ = h/p, where h is Planck's constant and p is momentum. The idea was confirmed experimentally in 1927. Wave-particle duality is not a contradiction. It is a fundamental feature of quantum mechanics. Particles are not waves or particles. They are quantum objects that exhibit both behaviors depending on how they are measured. The distinction between wave and particle is a classical concept that does not fully capture quantum reality.

Wave-particle duality facts

• Proposed. Louis de Broglie, 1924.
• Formula. λ = h/p.
• Light. Behaves as wave and particle.
• Matter. Electrons and atoms also show wave behavior.
• Double-slit. Classic demonstration.

Wave-particle duality is not the same as complementarity. Complementarity is Bohr's principle that wave and particle descriptions are mutually exclusive but both necessary.