When light of suitable frequency falls on a metal surface, electrons are emitted. These photo (light) generated electrons are called photoelectrons.Metallic surfaces when illuminated by ultraviolet radiation then electrons are emitted from them. This effect is known as the photoelectric effect. By absorbing energy from the incident electromagnetic radiation, the electrons in the metal escape the attraction of ions in the metal.
Hertz and Lenards observation
In 1887 by Heinrich Hertz observed that when light falls on a metal surface, some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions in the material of the surface. After gaining sufficient energy from the incident light, the electrons escape from the surface of the metal into the surrounding space.
In 1902, Lenard observed that the kinetic energy of electrons emitted increased with the frequency of radiation used. This could not be explained as Maxwell’s electromagnetic theory (which Hertz proved correct) predicted that the kinetic energy should be only dependent on light intensity (not frequency).