(a) Why photoelectric effect cannot be explained on the basis of wave nature of light ? Give reasons.
(b) Write the basis features of photon picture of electromagnetic radiation on which einstein's photoelectric equation is based.

Explanation

(a) Wave nature of light fails to explain photoelectric effect due to the following reason:
(i) In wave nature, the energy carried by a light beam is measured by its intensity. Thus, by increasing intensity of light (irrespective of wavelength or frequency) we can increase the energy of light beam and accordingly energy of photoelectrons should increase. However, experimentally, K.E. of photoelectrons is found to be independent of the intensity of incident light.
(ii) Again as per wave nature photo-emission should take place from a metal surface for all frequencies but in reality it is not to be so.
(iii) According to wave nature photoelectric emission cannot be an instantaneous process.
(b) The basis features of photon picture of electromagnetic radiation are as follows:
(i) Each photon travels in vacuum (or air) with a constant speed c, the speed of light, having a value `c=3xx10^(8)ms^(-1)`. this speed does not depend on frequency or energy/momentum of photon or on relative motion of source and observer etc.
(ii) Each photon having a frequency v has energy `E=hv and` momentum `p=(hv)/(c)=(h)/(lamda)`,
where `lamda` is the wavelength. thus, all photons of light of a particular frequency have the same energy and momentum, irrespective of the intensity of light. the intensity of light indicates the number of photons striking per unit area of given surface per second.
(iii) When a photon of appropriate energy interacts with an electron in a metal, it may transfer its energy to eelectron and electron may be ejected from the metal surface giving rise to phenomenon of photoelectric emission.