Wave-particle duality

Wave–particle duality

Light behaves as a wave (diffraction, interference) and as particles (the photoelectric effect). Matter does too: de Broglie proposed that any moving particle has a wavelength $\lambda=\dfrac hp=\dfrac h{mv}$. Electron diffraction through thin graphite proves it: electrons form rings like waves passing through a grating. Faster electrons have shorter wavelengths, so the rings get smaller.