Values of work function ( W _ 0 ) for a few metals are given below...

The minimum amount of energy necessary to start the emission of electrons from a metal's surface is known as its work function. The work function is the minimum amount of energy required to remove an electron from a metal's surface to infinity.

Energy of incident photon $= \dfrac{hc}{\lambda}$

$= \dfrac{6 . 6 \times {10}^{- 34} \times 3 \times {10}^{8}}{400 \times {10}^{- 9}}J$

$= \dfrac{6 . 6 \times {10}^{- 34} \times 3 \times {10}^{8}}{400 \times {10}^{- 9} \times 1 . 6 \times {10}^{- 9}}eV$

$= 3 . 1eV$

If radiation energy is greater than work function of metal, then metal show photo electric effect.

$∴ Li, Na$ and $K$ will show photoelectric effect.