Question

(a) Describe briefly three experimentally observed features in the phenomenon of photoelectric effect.
(b) Discuss briefly how wave theory of light cannot explain these features.

Answer 1

(a) (i) The photoelectric effect will not occur when the frequency of the incident light is less than the threshold frequency. Different materials have different threshold frequencies and most elements have threshold frequencies in the ultraviolet region of the electromagnetic spectrum.
(ii) The maximum KE of a stream of photoelectrons increases linearly with the frequency of the incident light above the threshold frequency.
(iii) The rate at which photoelectrons are emitted from a photosensitive surface is directly proportional to the incident light when the frequency is constant.

(b) Classical wave theory cannot explain :
(i) The existence of threshold frequency because it predicts that electrons would absorb enough energy to escape and there would not be any threshold frequency.
(ii) It can not explain the almost immediate emission of photoelectrons as, according to this theory, electrons require a period of time before sufficient energy is absorbed by it to escape from the metal; however, such a thing does not happen practically.
(iii)It also can't explain why the maximum KE of photoelectrons is dependent on the frequency and independent of intensity.