Wave-particle paradox (wave-particle duality). The wave-particle paradox is the observation that light (and, in general, radiation and matter) sometimes behaves as a wave and at other times behaves as a stream of particles, even though these two descriptions appear to be contradictory in classical physics.
Wave behaviour of light is shown by phenomena such as interference, diffraction and polarization, which can only be explained if light travels as a continuous wave.
Particle behaviour of light is shown by phenomena such as the photoelectric effect and the Compton effect, which can only be explained if light is made up of discrete packets of energy called photons, each of energy \( E = hf \).
No single experiment shows both behaviours at the same time: light behaves as a wave while it is being propagated, but as particles (photons) when it interacts with matter (emission or absorption). Louis de Broglie extended this idea to matter, proposing that moving particles such as electrons also have an associated wavelength \( \lambda = h/p \), which was later confirmed by electron diffraction. This dual nature, not resolvable by classical physics, is the wave-particle paradox.
