question_answer

Write two characteristic features observed in photoelectric effect which support the  photon picture of electromagnetic radiation. Draw a graph between the frequency of incident radiation \[\left( \mathbf{v} \right)\] and the maximum kinetic energy of the electrons emitted from the surface of a photosensitive material. State clearly how this graph can be used to determine (i) Planck's constant and (ii) work function of the material.                 

Answer:

                The following features observed in photoelectric effect support the photon picture of e. m. radiation: (i) The maximum kinetic energy of emitted electrons is independent of the intensity of incident radiation. (ii) For each photoemitter, there exists a threshold frequency (of incident light) below which no emission takes place. (iii) Photoelectric emission is an instantaneous process. According to Einstein's photoelectric equation \[{{K}_{\max }}=hv-{{W}_{0}}\] So the graph between \[{{K}_{\max }}\]and \[\text{v}\] is a straight line as shown in Fig. (i)  Slope of \[{{K}_{\max }}-v\] graph \[=\frac{\Delta {{K}_{\max }}}{\Delta v}=h\] \[\therefore \] Slope of \[{{K}_{\max }}-v\] graph gives the value of Planck's constant. (ii) Intercept on the negative \[{{K}_{\max }}\]axis \[={{W}_{0}}\]  \[\therefore \]Intercept on the negative \[{{K}_{\max }}\] axis gives the value of work function.