The Photoelectric effect

The photoelectric effect, in which electrons are emitted from matter after absorption of light, is a quantum phenomenon, discovered by Heinrich Rudolf Hertz in 1887. The theoretical description by Albert Einstein in 1905 was a major boost to the promotion of the quantum concept of wave-particle duality of light. Accordingly, light is absorbed in quanta (photons). Increasing the beam intensity increases the number of photons, and the number of electrons emitted. The energy of the emitted electrons depends on the photon energy, but not on the light intensity. All of the energy from one photon must be absorbed; part is spent to free the electron from the atom, whilst the rest goes into the electron's kinetic energy as a free particle.