The threshold frequency is the minimum frequency required for emission of electrons from the outermost shell. The light with frequencies lower than threshold would not eject photoelectrons from the shell of an atom. As for the emission kinetic energy of an electron should be greater than or equal to zero. Hence, the photoelectric emission will take place only when the energy of a photon is greater than the energy possessed by the electron. Atoms are ionised resulting in transfer of energy when photons of sufficient energy strike the metal surface.