In a photoelectric experiment ,if the work function of the target material is $1.24eV$ and the wavelength of incident light is $4000\stackrel{\circ }{A}$. What retarding potential is necessary to stop the emission of the electrons? Take $hc=1240eVnm$

Energy of incident photons in $eV$on metal surface is

$E=\frac{12400}{4000}eV=3.1eV$

According to Einstein's photo electric equation, we have

$E={\varphi }_0+K_{\max }$

$\Rightarrow K_{\max }=E-{\varphi }_0$

$\Rightarrow K_{\max }=(3.1-1.24)eV=1.86eV$

The stopping potential for these ejected electrons is given by

$V_s=\frac{K_{\max }}{e}=\frac{1.86eV}{e}$

$\Rightarrow V_0=1.86\text{ volt }$