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:In order to arrive at the correct answer to this question, it is important to be extremely aware of the drawbacks and shortcomings of both the wave nature of light and the particle nature of light. When we study the drawbacks of both, we will be able to explain the phenomenon based on both.

Complete step by step answer:
(a) The wave nature of light fails to explain the photoelectric effect; this is because of the following reasons:
1) According to the Wave Theory/ picture of light, the energy carried/ possessed by a beam of light is measured in terms of the intensity of that beam of light as greater intensity implies greater amplitude and hence, the beam is said to be carrying greater energy. When a beam of light occurs on the metal, the energy possessed by the beam will be equally distributed among the electrons and these electrons on accepting energy will get ejected from the metal surface. When the waves of light of higher intensity fall on the metal surface, more energy will be imparted to the electrons. Hence, the kinetic energy of the emitted/ ejected electrons increases.
This was contradictory to the fact that: The maximum kinetic energy of the emitted electron is actually independent of the intensity of the incident beam of light.

2) According to the Wave Picture of Light, the energy possessed by the beam of light is smoothly and uniformly distributed across its advancing wavefronts. Thus, when the beam of light is incident on the metal surface, the energy carried by the beam is not received by just one electron. Instead, the energy is equally distributed among all the electrons that fall in the part of the surface of the metal which is illuminated. Thus, all the electrons receive this amount of energy distributed among them. Thus, each electron receives only a negligible amount of energy. So, the electron should take a finite amount of time to escape the surface of the metal.
This was contradictory to the fact that: The emission of photo - electrons takes place almost instantly or immediately after the light incidents on the metal surface.
(b) The phenomena of interference, diffraction and polarisation were explained by the wave picture of light. But the experimental study of the photoelectric effect cannot be explained with the help of the wave Picture of light. The wave picture was not able to explain the most basic features of photoelectric emission. So, a new theory was proposed to explain the phenomenon of photoelectric effect that is the Photon Picture of light. Photon is a packet or quanta of energy associated with electromagnetic radiation. The energy of a photon is given by E=hv , where,v is frequency associated with photon and h is Planck’s constant.

Note:The wave nature of light shows up in the phenomena of interference, diffraction and polarisation. On the other hand, in the photoelectric effect and the Compton Effect which involve energy and momentum transfer, radiation or light behaves as if it is made up of a bunch of particles – the photons.