The kinetic energy of most energetic electrons emitted from a metallic surface is doubled when the wavelength λ of the incident radiation is changed from 400 nm to 310 nm. The work function of the metal is -

2.2 eV

$K=\frac{12400}{4000}-\phi=3.1-\phi$

$2K=\frac{12400}{4000}-\phi=4-\phi$

$⇒6.2-2\phi=4-\phi$

$or\, 2.2 eV = \phi$

$∴\phi=2.2eV$