The work function of a metal is 4.2 eV. If radiations of $2000\stackrel{0}{\mathrm{A}}$ fall on the metal, then the kinetic energy of the fastest photoelectron is:

KE=(Energy of radiation – Work function)           $=(\mathrm{h}\times \frac{\mathrm{c}}{\mathrm{\lambda }}-4.2)$
$=\left(\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{2000\times {10}^{-10}}\right)-(4.2\times 1.602\times {10}^{-19})=(9.9\times {10}^{-19})-(6.7\times {10}^{-19})=3.2\times {10}^{-19}\mathrm{J}$