Question

Ultraviolet light of wavelength 200 nm is incident on polished surface of Fe (iron). Work function of the surface is 4.71 eV. What will be its stopping potential?
$(h=6.626\times 10^{-34}J-s;1eV=1.6\times 10^{-19}J$,
c = $3\times 10^{8}m{s}^{-1})$

• A. 0.5 V
• B. 2.5 V
• C. 1.5 V
• D. None of these

Answer 1

Answer: (C)

Given that, the wavelength of incident light
$(\lambda )=200nm$
= $200\times 10^{-9}m$
work function ( $\varphi$) = 4.71 eV
From Einstein's photoelectric equation
$K{E}_{max}=hv-\varphi$ ...(i)
But $K{E}_{max}=e{V}_0$ ...(ii)
So, $e{V}_0=hv-\varphi$ $(V_0=$ Stopping potential)
or $e{V}_0=\frac{hc}{\lambda }-\varphi$
$\therefore 1.6\times 10^{-19}{V}_0=\frac{6.6\times 10^{-34}\times 3\times 10^8-4.71\times 1.6\times 10^{-19}}{200\times 10^{-9}}$
$\Rightarrow V_0=\frac{6.6\times 10^{-34}\times 3\times 10^8}{2\times 10^{-7}\times 1.6\times 10^{-19}}-\frac{4.71\times 1.6\times 10^{-19}}{1.6\times 10^{-19}}$
$\Rightarrow V_0=(6.19-4.71)$
$\Rightarrow V_0=1.48$
$\Rightarrow V_0\approx 1.50$ V