Photoelectric effect

In the external photoelectric effect, suitable illumination releases electrons from a material surface. A second electrode can collect them as a current. The emitted electrons originate in the material; the experiment does not mean that incoming light simply turns into electrons.

References: [1]

For a given surface, the standard experiment has a threshold frequency. Below it, increasing ordinary illumination intensity does not produce the expected emission. Above threshold, stronger fixed-frequency light mainly increases electron numbers, while maximum electron kinetic energy depends on frequency rather than intensity.

References: [1]

Einstein’s account assigns energy proportional to frequency to a light quantum. Emission requires the material’s work function; the remaining energy can become electron kinetic energy. A stopping-voltage measurement tests this balance by determining the maximum kinetic energy of the emitted electrons.

References: [1] [3]

Einstein’s 1921 Nobel Prize specifically recognized discovery of the law of the photoelectric effect. This distinguishes the award from his relativity work. Other light-sensitive devices, including solar cells, involve different arrangements of excitation and charge separation and should not all be described as surface electron emission.

References: [2] [1]