In classical physics, wave and particle are opposite and irreconcilable concepts. A particle is localised: it sits at a point, has a trajectory, collides and bounces, exchanging energy and momentum in a sharp way. A wave, instead, is spread out in space: it broadens, overlaps with other waves, produces interference and diffraction, and carries energy in a continuous way. An object is either one or the other — not both.
Quantum mechanics demolishes this alternative. Light, and more generally all matter, behaves sometimes as a wave and sometimes as a particle, depending on which experiment we put in front of it. This is not a gap in our knowledge waiting to be filled, nor a compromise of “a bit wave and a bit particle”: they are two complementary descriptions of the same object, each correct within its own experimental context.
The wave-like character of light was known since the nineteenth century: interference (Young’s experiment), diffraction through slits and gratings, polarisation. These are all phenomena that only a wave can produce. The corpuscular character instead emerges in the photoelectric effect and in the Compton effect: light exchanges energy with matter in discrete, quantised packets, the photons, each with energy . Neither aspect is more “true” or more “fundamental” than the other: they emerge depending on the question we put to nature.
Principle — Wave-particle duality
Light, and more generally all matter, shows a dual nature:
- sometimes it behaves as a wave — producing interference, diffraction, polarisation;
- sometimes it behaves as a particle — exchanging energy in discrete, quantised packets (photons).
Neither description is more fundamental than the other: they are two faces of the same phenomenon, which emerge depending on the experiment.
The modern formulation of the principle is due to Einstein (who in 1905 attributed a corpuscular nature to light to explain the photoelectric effect) and to de Broglie (who in 1924 extended the symmetry to matter: if wave-light can act like a particle, then a particle with mass must also be able to act like a wave). The three atoms that follow trace exactly this path: the photon as a quantum of light, de Broglie’s matter wave, and the experimental confirmations — Compton and the single-electron double slit.
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Topics: Quantum physics Concepts: Wave-particle duality · Photon · Interference
Related exercises: Connecting Malus’s law and a single photon · Double slit, which intensity graph · Detector at the slits and interference