Wave-particle duality is the principle that light and matter can display either wave-like or particle-like behaviour, depending on how they are observed experimentally. It does not mean that a quantum object is literally a classical wave and a classical particle at the same time.
This is an Additional Higher Level (AHL) concept in E.2 Quantum physics.
The Reasoning
Classical physics treats waves and particles as separate models. Waves spread out and produce diffraction or interference, whereas particles transfer energy and momentum in localized interactions. Quantum experiments show that neither classical model alone provides a complete description.
| Quantum object | Wave-like evidence | Particle-like evidence |
|---|---|---|
| Light | Interference and diffraction patterns | The photoelectric effect and Compton scattering show energy and momentum transferred by photons |
| Matter | Electron diffraction shows that particles have a wavelength | Electrons are detected as localized impacts and possess momentum |
For light, each photon has energy
The photoelectric effect supports the particle model because one photon transfers energy to one electron. Increasing intensity increases the number of photons, but electrons are emitted only if the frequency exceeds the threshold frequency.
