The photoelectron emission is possible if the energy of the incident photon is more than the workfunction of the metal, i.e., the energy required to just liberate an electron from the surface of metal. If the energy of the incident photon is less than the work function of a metal, no emission of photoelectrons will take place from a metal surface. The energy of a photon depends upon the frequency of the incident light. Thus the radiations of all frequenycies (i.e, greater than threshold frequency) whose photons are of energy more than work function of a metal, will cause photoelectric emission but the radiations of frequency less than threshold frequency will not cause photoelectric emission.
