In an experiment on photoelectric emission from a metallic surface, the wavelength of incident light is $2\times {10}^{-7} \mathrm{m}$ and stopping potential is $2.5 \mathrm{V}$. The threshold frequency of the metal (in $\mathrm{Hz}$ ) approximately is (charge on electron $e=1.6\times {10}^{-19}$ C, Planck's constant $h=6.6\times {10}^{-34}$ $\mathrm{J} \mathrm{s}$)