Answer:
Photo-electric effect is the phenomenon of emission of electrons from the surface of metals, when light radiations of suitable frequency fall on them. Einstein photo-electric equation is in accordance with the energy conservation law as applied to the photo-absorption by an electron in the metal. Thus Energy of incident photon \[=\]Maximum K.E. of photoelectron \[+\]Work function of metal or \[hv=\frac{1}{2}m\upsilon _{\max .}^{2}+{{W}_{0}}\] or \[{{K}_{\max }}=\frac{1}{2}m\upsilon _{\max .}^{2}=hv-{{W}_{0}}\] At threshold frequency \[{{v}_{0}}\] , no K.E. is given to the electron. So \[h{{v}_{0}}={{W}_{0}}\] Hence \[{{K}_{\max }}=hv-{{W}_{0}}=h(v-{{v}_{0}})\] This is Einstein's photoelectric equation. (i) Clearly, above the threshold frequency \[{{v}_{0}},{{K}_{\max }}\propto v\]i.e., the maximum K.E. of the emitted electrons depends linearly on the frequency of incident radiation. (ii) When \[v<{{v}_{0}},{{K}_{\max }}\]becomes negative. The kinetic energy becomes negative which has no physical meaning. Hence there is no photo-electric emission below the threshold frequency \[{{v}_{0}}\] . (iii) It is obvious from the photo-electric equation that the maximum K.E. of photo-electrons does not depend on the intensity of incident light.
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