Wave-Particle Duality

Wave-particle duality: light and matter both exhibit wave-like AND particle-like properties depending on the experiment.

Light: behaves as wave in interference, diffraction, polarisation. Behaves as particle (photon) in photoelectric effect, Compton scattering, pair production.

Matter: behaves as particle in collision/Newtonian experiments. Behaves as wave in electron diffraction (Davisson-Germer, 1927), neutron interferometry, etc.

The two pictures are COMPLEMENTARY — you can't see both at once. Bohr's complementarity principle.

Photon energy E = hf, momentum p = h/λ. Matter wavelength λ_dB = h/p (de Broglie).

For everyday objects (e.g., a thrown ball), de Broglie wavelength is fantastically tiny (~10⁻³⁴ m) — wave nature is undetectable.

Wave-particle duality is a foundational pillar of quantum mechanics — it eventually led to the Schrödinger equation, where particles are described by wavefunctions.