The surface of certain metal is first illuminated with light of wavelength ${\lambda }_1=350 \mathrm{n}\mathrm{m}$ and then, by a light of wavelength ${\lambda }_2=540 \mathrm{n}\mathrm{m}.$ It is found that the maximum speed of the photoelectrons in the two cases differ by a factor of $2$. The work function of the metal (in $\mathrm{e}\mathrm{V}$) is close to
(Energy of photon $=\frac{1240}{\lambda \left(\mathrm{i}\mathrm{n}\mathrm{ }\mathrm{n}\mathrm{m}\right)} \mathrm{e}\mathrm{V}$ )