Wave theory predicts that the photoelectric effect should occur at any frequency, provided the light intensity is high enough. But as observed in the photoelectric experiments, the light must have a sufficiently high frequency for the effect to occur irrespective of the intensity of the incident light.
Wave theory predicts that all the electrons along the wavefront absorb energy continuously. Each electron takes time to pick up sufficient energy to overcome the work function and get ejected out of the metal. But as observed in the photoelectric experiments, the photoelectric emission is an instantaneous phenomenon.
