Answer:
(i) Cut-off voltage or stopping potential. The minimum negative potential given to the anode of a photocell for which the photoelectric current becomes zero is called cut-off voltage or stopping potential. (ii) Threshold frequency. The minimum value of the frequency of incident radiation below which the photo- electric emission stops is called threshold frequency. According to Einstein's photoelectric equation, the maximum K.E. of a photoelectron is given by \[{{K}_{\max }}=hv-{{W}_{0}}\] If \[{{V}_{0}}\] is the stopping potential, then \[{{K}_{\max }}=e{{V}_{0}}\] \[\therefore \] \[e{{V}_{0}}=hv-{{W}_{0}}\] or \[{{V}_{0}}=\left( \frac{h}{e} \right)v-\frac{{{W}_{0}}}{e}\] ?(1) So the graph of \[{{V}_{0}}\] versus \[v\] is a straight line as shown in Fig. We can read the value of threshold frequency,\[{{v}_{0}}\] from the graph. From equation (1), we can find stopping potential \[{{V}_{0}}\] for any frequency \[\text{v}\].
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