What is photoelectric effect? State t he result of photoelectric effect experiment that could not be explained on the basis of laws of classical physics. Explain this effect on the b asis of quantum theory of electromagnetic radiations.

Explanation

Photoelectric effect when radiation with certain minimum frequency `(V_(0))` strke the surface of a metal. The elec trons are ejected from the surface of the metal. This phenomenon is called photoelectric effect. The electrons emitted are called photoelectrons.

Equipment for studying the photoelectric effect Light of a particular frequency strikes a clean metal surface inside a vacuum chamber.
Electrons are ejected from the metal and are counted by a detector that measures their kinetic energy
The result observed in this experiment were
(i) The electrons are ejected from the metal surface as soon as the beam of light strikes the surface. i.e. there is no tike lag between the striking of light beam and the ejection of electrons from the metal surface.
(ii) The number of electorons ejected is proportional to the intensity or brightness of light.
(iii) For each metal, there is a characteristic minimum frequency, `v_(0)` (also known as threshold frequency) below which photoelectric effect is not observed. At a f requency `vgtv_(0)` the ejected electrons come out with certain kinetic energy. the kinetic energies of these ejections increases withthe increase of frequency of the light used
The above observation cannot be explained by the electromagnetic wave theory. According to this theory. since radiations were continous. Therefore, it should be possible to accumulate energy on the surface of the metal. irrespective of it frequency and thus, radiations of al frequencies should be able to eject electrons.
Similarly according to this theory the energy of the electrons ejected should depend upon the intensity of the incident radiation.
Particle Nature of Electromagnetic Radiaton
To explain the phenomena of black body radiation and photoelectric effect. Max planck in 1900, put forward a theory known after his naem as Planck's quantum theory. This theory was further extended by Einstein in 1905.
(i) The radiant energy is emitted or absorbed not continously but discontinously in the form of small discrete packets of energy Each such packet of energy is called a quantum in case of light qunatum of energy is called a photon.
(ii) The energy of each quantum is directly proportional to the frequency of the radiation
i.e. `Epropv` or `E=hv`
where h is a proportionality constnat called planck's constnat its value to approximately equal to `6.626xx10^(-27)` erg s or `6.626xx10^(-34)J` s.
(iii) The toal amount of energy emitted or absorbed by a body will be some whole number quanta.
Hence,E=nhv (where,nis any integer)