光电效应

Low-frequency light produces photons with insufficient energy to overcome the binding energy, or work function, of electrons in the metal. Even intense low-frequency light contains many photons of the same low energy, so no electrons are ejected. Only photons with energy exceeding the work function can free electrons from the metal surface.

Einstein showed that each photon carries energy directly proportional to its frequency, expressed as E = hν, where h is Planck's constant. Higher-frequency light produces higher-energy photons. This quantized energy model resolved the photoelectric paradox by explaining why frequency, not intensity, determines electron ejection.

When a photon with energy greater than the work function strikes an electron, the excess energy beyond what is needed to overcome binding energy is transferred to the electron as kinetic energy. The ejected electron's kinetic energy equals the photon energy minus the work function of the metal.

Light intensity corresponds to the number of photons striking the metal surface per unit time. Greater intensity means more photons hit the metal, increasing the probability of collisions with electrons and thus ejecting more electrons. However, intensity does not affect the kinetic energy of individual ejected electrons.

The threshold frequency is the minimum frequency of light required to eject electrons from a metal. It is specific to each metal because different metals have different work functions—different binding energies holding electrons in place. A photon must have energy at least equal to the work function to free an electron.

The photoelectric effect shows that light behaves as discrete particles called photons, each carrying quantized energy dependent on frequency. Classical wave theory predicted energy should depend on intensity, not frequency, but observations matched the particle model. This phenomenon, along with interference and diffraction wave nature of light, reveals light's dual character.

Classical wave theory predicted that energy depends on wave amplitude (intensity), so increasing brightness should eject electrons regardless of frequency. Einstein's quantized photon model showed energy depends on frequency instead. This explained why only high-frequency light ejects electrons and why intensity affects only the number, not the energy, of ejected electrons.