Two identical photo-cathodes receive light of frequencies ${f_1}$ and ${f_2}$. If the velocities of the photo electrons (of mass $m$ ) coming out are respectively ${v_1}$ and ${v_2},$ then
$v_1^2 - v_2^2 = {{2h} \over m}\left( {{f_1} - {f_2}} \right)$
${v_1} + {v_2} = {\left[ {{{2h} \over m}\left( {{f_1} + {f_2}} \right)} \right]^{1/2}}$
$v_1^2 + v_2^2 = {{2h} \over m}\left( {{f_1} + {f_2}} \right)$
${v_1} - {v_2} = {\left[ {{{2h} \over m}\left( {{f_1} - {f_2}} \right)} \right]^{1/2}}$
