Question

(A) Why photoelectric effect cannot be explained on the basis of the wave nature of light? Give reasons. (B) Write the basic features of the photon picture of electromagnetic radiation on which Einstein’s photoelectric equation is based.

Answer 1

Hint: Photocurrent is the current produced when light falls on a photo diode or a metal with suitable work function. Work function is defined as the minimum energy required to eject an electron from the surface of the material. This phenomenon of ejection of an electron is known as photoelectric effect.

Complete answer:
Light is believed to have dual behavior: wave and particle. Some properties of light could be explained by using wave nature and some are explained by using the particle nature of the light. For instance, the photoelectric phenomenon is explained by using the particle nature of light.

Photoelectric effect cannot be explained on the basis of wave nature because experimentally we know that we need a particular energy called the work function of the metal surface. Unless this energy is provided, the electron won’t be ejected, irrespective of the time for which light is incident. But if light were a wave, which is a continuous source of energy, we must get electrons if energy is less that work function, if light is incident for a larger time! Which is not the case!

Photon- Photon is an imaginary particle which is believed to have rest mass zero (mass at rest). But it has some mass when in motion. The mass could be calculated by using Einstein’s special theory of relativity.
The speed of a photon is equal to the speed of light i.e. ‘c’. Thus using $E = mc^2$, we have:

$m = \dfrac{E}{c^2}$. Also momentum is defined as mass times velocity of a particle. So momentum (p) = mc.

Note:
Photons possess mass only when in motion. Hence it has definite kinetic energy and momentum as well only when it is in motion. Also, photoelectric current depends upon, Intensity of incident light, the potential difference applied between two electrons and the nature of incident material.