Light of wavelength 4000$\stackrel{0}{A}$ falls on a photosensitive metal and a negative 2 V potential stops the emitted electrons. The work function of the material (in eV) is approximately $\left(\mathrm{h}=6.6\times {10}^{-34}\mathrm{Js}, \mathrm{e}=1.6\times {10}^{-19}\mathrm{C}, \mathrm{c}=3\times {10}^8 {\mathrm{ms}}^{-1}\right)$