Question

Silver has a work function of $4.7 \,eV$. When UV light of wavelength $100 \,mm$ is incident upon it. A potential of $7.7 \,V$ is required to stop the photo electrons from reaching the collector plate. How much potential is required to stop photo electrons when light of wavelength $200 \,mm$ is incident upon silver?

• A. 1.5 V
• B. 2.35 V
• C. 3.85 V
• D. 15.4 V

Answer 1

Answer: (A)

$\frac{h c}{\lambda e}=\frac{W}{e}+V_{s}$ (Einstein's equations)
$\frac{h c}{100 \times 10^{-3} e}=4.7+7.7=12.4$ ......(1)
$\frac{h c}{200 \times 10^{-3} e}=4.7+V_{s}^{\prime} $ .....(2)
from $(1) $ and $(2) $
$\Rightarrow \frac{12.4 \times 100 \times 10^{-3}}{200 \times 10^{-3}}=4.7+V_{S}^{\prime} $
$V_{S}^{\prime}=6.2-4.7$
$V_{S}^{\prime}=1.5\, eV$
$\therefore V_{ S }^{\prime}=1.5 \,V $