Light of frequency $7.21 \times 10^{14} Hz$ is incident on a metal surface . Electron with a maximum speed of $6.0 \times 10^5\;ms^{-1}$ are ejected from the surface. What is the threshold frequency for photo emission of electron ? $ h =6.63 \times 10^{-34}Js, m_e=9.1 \times 10^{-31} \;kg$
