It was observed that if the frequency of incident radiation is below a certain minimum value, no emission takes place however high the intensity of light maybe. Another important feature observed was that the kinetic energy of the electrons emitted was independent of the intensity of light. The kinetic energy of the electrons increases linearly with the frequency of incident light radiation. This was highly contrary to the laws of Physics at that time i.e. the energy of electrons should have been proportional to the intensity of the light, not to the frequency.
These features could not be properly explained on the basis of Maxwell's concept of light i.e. light as electromagnetic wave.
In 1905, Einstein applied Planck's quantum theory of light to account for the extraordinary features of the photoelectric effect. He introduced a new concept that light shows dual nature.