Explanation:
\({V_1} = \frac{{hc}}{{e{\lambda _1}}} - \frac{\phi }{e}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( 1 \right)\)
\({V_2} = \frac{{hc}}{{e{\lambda _2}}} - \frac{\phi }{e}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left( 2 \right)\)
From Eqs. (1) and (2),
\({V_{1}-V_{2}=\dfrac{h c}{e}\left[\dfrac{1}{\lambda_{1}}-\dfrac{1}{\lambda_{2}}\right]}\)