Question

10^-3 W of 5000 $\text{Å}$ Light is directed on a photoelectric cell. If the current in the cell is 0.16 μA, the percentage of incident photons which produce photoelectrons, is

• A. 0.4%
• B. 0.04%
• C. 20%
• D. 10%

Answer 1

The correct option is B

0.04%

The number of photons incident on photocell per unit of time is,
$n=\frac{{10}^{-3}\times \lambda }{hc}=\frac{{10}^{-3}\times 5000}{12420\times 1.6\times {10}^{-19}}$

Let $x\%$ of incident photons produce photoelectrons so the number of electron ejected per unit time is
$n^{\prime }=0.01\times x\times n$

Photocurrent $i=n^{\prime }\times e=\frac{0.01\times {10}^{-3}\times 5000}{12420\times 1.6\times {10}^{-19}}\times 1.6\times {10}^{-19}$

$\Rightarrow 0.16\times {10}^{-16}=\frac{0.01\times 5\times x}{12420}$

$\Rightarrow x=0.0398\cong 0.04$