Question

Explain why photoelectric effect cannot be explained on the basis of wave nature of light.

Answer 1

Failures of classical wave theory:

From Maxwell’s theory, light is an electromagnetic wave consisting of coupled electric and magnetic oscillations that move with the speed of light and exhibit typical wave behaviour. Let us try to explain the experimental observations of the photoelectric effect using wave pictures of light.

1. When light is incident on the target, there is a continuous supply of energy to the electrons. According to wave theory, the light of greater intensity should impart greater kinetic energy to the liberated electrons (Here, Intensity of light is the energy delivered per unit area per unit time). But this does not happen. The experiments show that the maximum kinetic energy of the photoelectrons does not depend on the intensity of the incident light.
2. According to wave theory, if a sufficiently intense beam of light is incident on the surface, electrons will be liberated from the surface of the target, however low the frequency of the radiation is. From the experiments, we know that photoelectric emission is not possible below a certain minimum frequency. Therefore, the wave theory fails to explain the existence of threshold frequency.
3. Since the energy of light is spread across the wavefront, the electrons which receive energy from it are large in number. Each electron needs a considerable amount of time (a few hours) to get energy sufficient to overcome the work function and to get liberated from the surface. But experiments show that photoelectric emission is an almost instantaneous process (the time lag is less than 10⁻⁹ s after the surface is illuminated) which could not be explained by the wave theory.