This is an effect observed by Sir J.J Thompson while performing some of his famous experiments. He observed that electrons are ejected by a metal surface when light of a certain frequency strikes the surface of a metal. This phenomenon observed is known as photoelectric effect. With some metals, this effect is observed under the action of visible light where as some show it under ultraviolet rays. Lower the ionization energy of a metal, more easy is to eject electron from its surface and hence more easily is the photoelectric effect observed.
Careful studies were made on this phenomenon and following observations were made: –
a) Every metal eject electron at a certain minimum frequency, below which it does not eject any electron. This frequency is called threshold frequency. It varies from metal to metal. Lower is the ionization energy, lower is the threshold frequency. The frequency must be at least equal to threshold frequency to observe the photoelectric effect otherwise it won’t be observed, doesn’t matter for how long the radiations fall or how dense the radiations are.
b) The kinetic energy of the ejected electron depends upon the frequency and not on the density.
c) The number of electrons ejected from the metal surface depends upon the density of the radiations and not on the frequency of the radiations.
