In the early twentieth century, physicists believed that light was purely a wave, following the principles of electromagnetism established by James Clerk Maxwell. Yet a series of experiments revealed puzzling behaviour when light struck certain metals. When illuminated by light of sufficient frequency, these metals emitted electrons, producing an electric current. The effect, known as the photoelectric effect, could not be explained by classical wave theory. According to established physics, increasing the intensity of light should increase the energy of emitted electrons. However, experiments showed that only the frequency of light mattered — if the light’s frequency was too low, no electrons were released, no matter how bright the beam.
In 1905, Albert Einstein provided an explanation that redefined the nature of light. Building on Max Planck’s quantum hypothesis, he proposed that light is made up of discrete packets of energy, later called photons. Each photon carries an energy proportional to its frequency, given by Planck’s equation E = hf. When a photon strikes a metal surface, it transfers its energy to an electron. If the photon’s energy exceeds the metal’s binding energy, the electron is ejected; if not, no emission occurs. Increasing light intensity only increases the number of photons, not their individual energies.
Einstein’s model successfully explained all aspects of the photoelectric effect, resolving a major inconsistency between theory and experiment. It also revealed that light, long thought to be purely a wave, could behave as a particle — a revolutionary concept that became one of the foundations of quantum mechanics. The discovery confirmed that energy exchange in nature occurs in discrete units, not continuous flows, and that the particle and wave descriptions of light must coexist.
For this work, Einstein received the 1921 Nobel Prize in Physics. More importantly, his explanation of the photoelectric effect opened the way for technologies such as solar cells, photodiodes, and digital imaging. It also deepened the mystery of light’s dual nature, paving the path for later experiments, including the double-slit test, that revealed the strange and probabilistic world of quantum phenomena.