The laws of photoelectric emission are as follows
(I) For a given material and a given frequency of incident radiation, the photoelectric current or number of photoelectrons ejected per second is directly proportional to the intensity of the incident light.
(II) For a given material and frequency of incident radiation, situation current is found to be proportional to the intensity of incident radiation whereas the stopping potential is independent of its intensity.
(III) For a given material, there exist a certain minimum frequency of the incident radiation below which no emission of photoelectrons take place. This frequency is called threshold frequency.
Above the threshold frequency, the maximum kinetic energy of the emitted photoelectrons or equivalent stopping potential is independent of the intensity of the incident light but depends upon only the the frequency(or wavelength) of the incident light.
(IV) The photoelectric emission is an instantaneous process. The time lag between the incidence of radiations and emission of photoelectrons is very small, less than even 10⁻⁹ s.