At the cutoff frequency, the photon energy equals the metal’s work function. Below this cutoff, individual photons have insufficient energy to eject any electrons, no matter how many photons are present.
Work Step by Step
The wave theory says that light of any frequency can eject electrons from metal, if the light is intense enough (i.e., if the electric field amplitude is high enough). On the other hand, the particle theory says that each photon carries a fixed amount of energy that depends on the light frequency, E = hf. Increasing the intensity of the light only increases the number of photons, not their individual energy.