# The work function of a substance is $4\: eV$. 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) 310 nm
$hc/\lambda_{\circ} = \phi$
$\Rightarrow \lambda_{max} = hc/\phi = 6.6 \times 10^{-34} \times 3 \times 10^8 / (4 \times 1.6 \times 10^{-19} ) = 310 nm$

edited Mar 13, 2014