Reason for inability of E.M. theory
Resolution through photon picture
The explanation based on e.m. theory does not agree with the experimental observations (instantaneous nature, max K.E. of emitted photoelectron is independent of intensity, existence of threshold frequency) on the photoelectric effect.
[Note : Do not deduct any mark if the student does not mention the relevant experimental observation or mentions any one or any two of these observations].
The photon picture resolves this problem by saying that light, in interaction with matter behaves as if it is made of quanta or packets of energy, each of energy hv. This picture enables us to get a correct explanation of all the observed experimental features of photoelectric effect.
[Note : Award the first mark if the student just writes "As per E.M. theory the free electrons at the surface of the metal absorb the radiant energy continuously, this leads us to conclusions which do not match with the experimental observations"].
Also award the second mark if the student just writes "The photon picture give us the Einstein photoelectric equation
Kmax(=eV0)=hv−ϕ0
which provides a correct explanation of the observed features of the photoelectric effect.