According to the wave theory of light, electromagnetic waves carry the energy stored in oscillating electric and magnetic fields. When enough energy is absorbed by an electron in a substance, it should be liberated as a photoelectron. Frequency of light does not come into picture in this case. Hence, there should not be any threshold frequency for emission of electrons. But it is found that there exists threshold frequency and it depends on the metal.
Experimentally, the maximum kinetic energy of photoelectrons increases linearly with the frequency of light. This cannot be accounted by the wave theory of light.
If a source of light is weak or far away from a metal surface, emission of an electron will not be almost instantaneous. The electron may have to wait for several hours/days for absorption of enough energy from the incident light as by the wave theory of light, energy is spread over the wave front. But experimentally, for an appropriate frequency of incident light, photoelectric effect is almost instantaneous.
Only one observation, photoelectric current ∝ intensity of incident light can be accounted by the wave theory of light.
