The photoelectric effect is a fundamental quantum phenomenon where electrons are emitted from a material when light strikes its surface. This effect was first observed in the late 19th century and played a crucial role in the development of quantum physics.
Light is composed of discrete energy packets called photons. Each photon carries energy equal to Planck's constant times the frequency of light. When a photon strikes an electron, it can transfer its energy. However, the electron needs a minimum energy, called the work function, to escape from the material's surface.
A key observation of the photoelectric effect is the existence of a threshold frequency. Below this critical frequency, no electrons are emitted regardless of how intense the light is. Above the threshold, the kinetic energy of emitted electrons increases linearly with frequency, following the equation: kinetic energy equals h times frequency minus the work function.
The photoelectric effect reveals a crucial distinction between classical and quantum physics. Increasing light intensity increases the number of emitted electrons but does not change their kinetic energy. Only increasing the frequency increases the kinetic energy of individual electrons. This behavior cannot be explained by classical wave theory, which predicted that brighter light should give electrons more energy.
Einstein's quantum explanation of the photoelectric effect earned him the 1921 Nobel Prize in Physics. His work proved that light has particle properties, establishing the concept of wave-particle duality. This fundamental discovery led to numerous modern technologies including solar cells, image sensors, photomultiplier tubes, and photodiodes that are essential in today's electronic devices.