# The work function of the following metals are : $Na = 2.75\: eV; K = 2.3\: eV ; Mo = 4.17\: eV ; Ni = 5.15 \: eV$. Which of these metals will not give photoelectric emission for a radiation of wavelength $3300\: A^{\circ}$ the photocell? from a $He – Cd$ laser placed $1m$ away from

$\begin {array} {1 1} (a)\;Mo,Ni & \quad (b)\;Na,Ni \\ (c)\;Ni,K & \quad (d)\;Na,K \end {array}$

Ans : (d) Na,K
$\lambda = 3300\: A^{\circ}$
$E= hc/ \lambda$
$= 6.6 \times 10^{-34} \times 3 \times 10^8 / (3300 \times 10^{-10} )$
$= 6 \times10^{-19} / 1.6 \times 10^{-19} = 3.158\: eV$
Since this is greater than work function of $Na$ and $K$ only,
so they will not give photoelectric emission.