\(h{f_2} = {W_0} + \frac{1}{2}mv_2^2\)….\(.(ii)\)
\( \Rightarrow h({f_1} - {f_2}) = \frac{1}{2}m(v_1^2 - v_2^2)\)
\( \Rightarrow (v_1^2 - v_2^2) = \frac{{2h}}{m}({f_1} - {f_2})\)
$\left(\mathrm{hc}=1240\; \mathrm{eV} \mathrm{nm}, 1\; \mathrm{eV}=1.6 \times 10^{-19} \;\mathrm{J}\right)$
$(\left| e \right| = 1.6 \times {10^{ - 19}}\,\,C,\,\,{m_e} = 9.1 \times {10^{ - 31}}\,kg,\,\,h = 6.6 \times {10^{ - 34}}\,\,Js)$