# The work function of a substance is $4.0\: eV$. The longest wavelength of light that can cause photoelectron emission from this substance is approximately

$\begin {array} {1 1} (a)\;540\: nm & \quad (b)\;400\: nm \\ (c)\;310\: nm & \quad (d)\;220\: nm \end {array}$

Ans : (c)
work function,$\phi_{\circ} = hv_{\circ} – \large\frac{hc}{\lambda_{max}}$
$\Rightarrow \lambda_{max} = \large\frac{hc}{\phi_{\circ}} = \large\frac{1240}{4}$$= 310\: nm$

edited Mar 13, 2014