(a) According to wave theory :
(i) The maximum kinetic energy of the emitted electron should be directly proportional to the intensity of incident radiations, but it is not
observed experimentally. Also the maximum kinetic energy of the emitted electrons should not depend upon incident frequency according to wave theory, but it is not so.
(ii) Electron emission should take place at all frequencies of radiations, i.e. there should not exist the threshold frequency. This fact contradicts experimental observation.
(iii) There should be a time lag in photo electric emission but according to observation photo electric emission is instantaneous.
(b) According to photon picture :
(i) Each quantum of radiation has energy hv.
(ii) In photoelectric effect, the electrons in the metal absorb this quantum of energy (hv).
(iii) When this energy exceeds the minimum energy needed for the ejection of photo electron, the flow of photo current starts. [CBSE Marking Scheme, 2013]