The surface of certain metal is first illuminated with light of wavelength $\lambda _ { 1 } = 350 \mathrm {~nm}$ and then, by a light of wavelength $\lambda _ { 2 } = 540 \mathrm {~nm}$. 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 eV) is close to (Energy of photon $= \frac { 1240 } { \lambda \mathrm{ in nm} } \mathrm { eV }$)\\
(1) 2.5\\
(2) 1.8\\
(3) 5.6\\
(4) 1.4