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The photoelectric effect is a phenomenon in which electrons are ejected from the surface of a metal when light shines upon it. This effect was first observed by Heinrich Hertz in 1887 and later explained by Albert Einstein in 1905.

Einstein proposed that the observed effect was due to the interaction between light and electrons in the metal. He suggested that light was made of "quanta" of energy, later called photons, that interact with the electrons in the metal. When a photon of sufficient energy strikes an electron, it transfers its energy to the electron, causing it to be ejected from the metal surface.

Einstein's explanation further introduced the idea that each metal has a specific "threshold frequency" for the photoelectric effect to occur. This threshold is determined by the minimum energy required to overcome the binding energy of the electrons in the metal. In other words, only photons with frequency greater than or equal to the threshold frequency can cause the photoelectric effect.

Einstein's explanation of the photoelectric effect led him to formulate an equation that relates the kinetic energy (KE) of the ejected electron to the frequency of the incident photon. This equation is given by: \[ KE = h(\nu - \nu_0) \] Where: - \(KE\) is the kinetic energy of the ejected electron - \(h\) is the Planck's constant (approximately \(6.63 \times 10^{-34} Js\)) - \(\nu\) is the frequency of the incident photon - \(\nu_0\) is the threshold frequency of the metal This equation shows that the energy of the ejected electron is directly proportional to the frequency of the incident photon, and that the photoelectric effect occurs only if the incident photon's frequency is greater than or equal to the threshold frequency. By successfully explaining the photoelectric effect, Einstein's work provided significant evidence for the quantization of energy and laid the foundations for quantum physics.