According to wave theory when light falls on a metal surface, energy is continuously distributed over the surface. All the free electrons receive light energy and when the energy received exceeds that of work function, an electron may escape the surface. If we user a low intensity source, it may take hours before an electron to come out.
But the experimental observation is not so. However small is the intensity, photoelectrons are ejected out and that too instantaneously. We explain with a particle theory that is photon theory. In photon theory, the intensity of the light is related to the number of photons and the energy of every photons and the energy of every photon is related to the frequency of light. So, when low intensity light falls on the metal, less number of photons fall on the metal and hence less number of electrons get a chance to absorb energy. The energy that gets the photon receiving all the energy ejects out immediately.
Hence, photoelectric effect can be explained with the particle nature of light.
