\(\frac{\mathrm{hc}}{\lambda_{2}}=\phi+\mathrm{eV}_{2}\)   ...... \((ii)\)

\((i)\)  \(-\)  \((ii)\)

\(\operatorname{hc}\left(\frac{1}{\lambda_{1}}-\frac{1}{\lambda_{2}}\right)=e\left(V_{1}-V_{2}\right)\)

\( \Rightarrow {V_1} - {V_2} = \frac{{hc}}{e}\left( {\frac{{{\lambda _2} - {\lambda _1}}}{{{\lambda _1} - {\lambda _2}}}} \right)\)

\( = (1240\,\,{\text{nm}}\,\,V)\,\,\frac{{100\,{\text{nm}}}}{{300\,{\text{nm}} \times 400\,{\text{nm}}}}\)

\( = \frac{{12.4}}{{12}} \approx 1{\text{V}}\)