Surface of certain metal is first illuminated with light of wavelength $\lambda $₁ = 350 nm and then, by light of wavelength $\lambda $₂ = 540 nm. It is found that the maximum speed of the photo electrons in the two cases differ by a factor of 2. The work function of the metal (in eV) is close to :

(Energy of photon n = ${{1240} \over {\lambda (in\,mm)}}$eV)