Question

Why was the photoelectric effect not explained by Classical Wave Theory? Explain.

Answer 1

Hint : To answer this question, we need to explain the classical wave theory of light. Then we have to use its results to explain the observations of the photoelectric effect. The contradicting results by the wave theory of light will be the required reason for its failure to explain the photoelectric effect.

Complete step by step answer:
According to the classical wave theory light is composed of the oscillating electric and magnetic fields. And the amplitude of the electric and the magnetic fields are indicated by its intensity. The intensity is proportional to the square of the amplitude. So greater the intensity, the greater will be the amplitudes of the electric and magnetic fields. So consequently the greater will be the energy carried by it, which must be absorbed by the electrons present on the surface of the metal. Therefore, according to this theory, a light beam having sufficient intensity must be enough to initiate the photoelectric effect.
But we know that the photoelectrons are not emitted until the light having minimum threshold frequency is incident on the metal.
Also, according to this theory, the electrons absorb the energy from the light continuously over the entire wavefront. We know that there are a large number of electrons present on the surface of the metal. So it must take hours for the photoelectric emission to get initiated. But we know that when a light having frequency equal to or more than the threshold frequency is incident on the metal surface, then the photoelectric emission is instantly initiated.
Because of the two reasons stated above, the classical wave theory failed to explain the photoelectric effect.

Note:
The photoelectric effect, not explained by the classical wave theory, has been well explained by the quantum theory of light, proposed by Einstein. Although the wave theory couldn’t explain this effect, it has explained other effects of lights such as interference, diffraction and polarisation.