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According to Einstein’s the discrete unit in radiation is called quanta of energy of radiation. Each particle have an energy
.
is work function which is constant for a given substance.
From this, it is clear that light is made up of these type of discrete units or packets of energy or quantum energy. it is called photon and it carries energy. When a photon incident on metal the energy of a photon is utilised for two purposes. One is to overcome the attraction of the metallic surface and another part is for gaining kinetic energy.
so kinetic energy of electrons = Energy obtained from the photon - energy used to escape from the metallic surface.
That is
Kinetic energy
where
This above equation is Einstein’s photoelectric equation.
The features of photoelectric effect which cannot be explained by the wave theory of light are
i) Kinetic energy of emitted electrons are found to be independent of incident light
ii) The electrons are emitted only at a particular frequency called threshold frequency. Below that frequency. no photoemission takes place
iii) Number of photoelectrons emitted from the metal per second is directly proportional to the intensity of radiation. As intensity increases the number of energy quanta per unit area per second increases