Quantum Theory

Albert Einstein (1905), building on ideas put forward by Max Planck (1900), successfully explained the photoelectric effect by treating light as interacting with matter in the form of photons -- discrete packets of energy. Each photon carries energy $E=h\nu$, where $h$ is Planck's constant ($h=6.625\times10^{-34}\ \text{J s}$) and $\nu$ is the frequency of the electromagnetic wave. Because light shows wave-like behaviour in its propagation (interference, diffraction, polarisation) and particle-like behaviour in its interactions with matter (the photoelectric effect), …