When a clean surface of an alkali metal (ex potassium, rubidium, etc) is exposed to a beam of light, ejection of electron from the metal surface takes place. Only if the frequency of incident light is greater than a certain minimum value, characteristic of the metal.
This phenomenon is known as photoelectric effect.It is observed that violet light is able to eject electrons from potassium, but red light which has lower frequency has no effect. The explanation for the frequency dependence of the photoelectric effect is given by Albert Einstein. He could explain the photoelectric effect using Plank's Quantum theory of electromagnetic radiation as the starting point.
Photoelectric Effect : Plank's Quantam theoryThe energy emitted or absorbed is in the form of small packets of energy called photon. And this energy is directly proportional to the frequency of radiation.E = hv. h = plank's constantWhen a photon of sufficient energy strikes an electron in the atom of the metal, it transfer its energy instantaneously to the electron during the collision and the electron is ejected with out any time delay.Greater the energy possessed by the photon, Greater will be the transfer of energy to the electron and greater the kinetic energy of the ejected electron.
Photoelectric Effect : EquationsWhen a photon strikes the surface of the metal, the energy (hv) of the photon is absorbed by the electron in the metal and a part of its energy is used to set free the electron from the attractive forces in the metal. The remaining energy of the photon appears in the form of kinetic energy of the released electron. Thus we can write,hv = W + K.E , where hv = energy of the photonW = energy required to overcome the attractive forces on the electron in the metal also called workfunctionK.E = kinetic energy of the emitted electron.Therefore hv = hv0 + 1/2 me v2 ; me = mass of the electronv = velocity of th ejected electronvo = threshold frequencyThe frequency of the incident light which should be greater than a certain minimum value, characteristic of the metal where the photoelectric effect takes place is called threshold frequency.