Which phenomenon refers to the emission of electrons from a metal when it absorbs light?

The emission of electrons from a metal when it absorbs light is known as the photoelectric effect. This phenomenon occurs when light, particularly ultraviolet or visible light, strikes the surface of a metal and provides sufficient energy to overcome the work function, which is the minimum energy required to release an electron from the metal's surface.

As a result, the absorbed light energy is converted into kinetic energy of the emitted electrons. The photoelectric effect demonstrates that light can behave as both a wave and a particle, corroborating the particle-like nature of light called photons. This effect was crucial for the development of quantum mechanics and supported the idea that energy is quantized.

In contrast, other phenomena such as Compton scattering involve the change in wavelength of X-rays or gamma rays as they collide with electrons, rather than the direct emission of electrons due to light absorption. Electron diffraction pertains to the wave-like behavior of electrons when they pass through a lattice, while thermionic emission refers to the release of electrons from a heated material and is not related to light absorption. Therefore, the photoelectric effect is specifically associated with the absorption of light leading to the liberation of electrons, making it the correct choice.