Black-body radiation : Planck's quantization :: Photoelectric effect : ?

Black‑body radiation led Planck to introduce energy quanta, showing that energy is exchanged in discrete packets. In the same way, the photoelectric effect prompted Einstein to propose that light itself is made of photons—discrete energy quanta that transfer energy to electrons. Einstein’s idea explained why light below a certain frequency, no matter how intense, cannot eject electrons, while above it, the energy of each photon determines the kinetic energy of emitted electrons. For example, shining a 500 nm laser (photon energy ≈ 2. 5 eV) on a metal with a 2 eV work function ejects electrons with about 0.