Q UANTUM OF R ADIATION In , Albert Einstein ( - ) proposed a radically new picture of electromagnetic radiation to explain photoelectric effect. In this picture, photoelectric emission does not take place by continuous absorption of energy from radiation. Radiation energy is built up of discrete units – the so called quanta of energy of radiation . Each quantum of radiant energy has energy h + , where h is Planck’s constant and + the frequency of light.
In photoelectric effect, an electron absorbs a quantum of energy ( h + ) of radiation. If this quantum of energy absorbed exceeds the minimum energy needed for the electron to escape from the metal surface (work function - ), the electron is emitted with maximum kinetic energy K max = h + – - ( . ) More tightly bound electrons will emerge with kinetic energies less than the maximum value. Note that the intensity of light of a given frequency is determined by the number of photons incident per second.
Increasing the intensity will increase the number of emitted electrons per second. However, the maximum kinetic energy of the emitted photoelectrons is determined by the energy of each photon. Equation ( . ) is known as Einstein’s photoelectric equation .
We now see how this equation accounts in a simple and elegant manner all the observations on photoelectric effect given at the end of sub-section . . . According to Eq.
( . ), K max depends linearly on + , and is independent of intensity of radiation, in agreement with observation. This has happened because in Einstein’s picture, photoelectric effect arises from the absorption of a single quantum of radiation by a single electron. The intensity of radiation (that is proportional to the number of energy quanta per unit area per unit time) is irrelevant to this basic process.
Since K max must be non-negative, Eq. ( . ) implies that photoelectric emission is possible only if h + > - or + > + , where + = h ( . ) Equation ( .
) shows that the greater the work function - , the higher the minimum or threshold frequency + needed to emit photoelectrons. Thus, there exists a threshold frequency + (= - / h) for the metal surface, below which no photoelectric emission is possible, no matter how intense the incident radiation may be or how long it falls on the surface. In this picture, intensity of radiation as noted above, is proportional to the number of energy quanta per unit area per unit time. The greater the number of energy quanta available, the greater is the number of electrons absorbing the energy quanta and greater, therefore, is the number of electrons coming out of the metal (for + > + ).
This explains why, for + > + , photoelectric current is proportional to intensity.