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
$I=n^{\prime }\times e=\frac{0.01\times {10}^{-3}\times 5000}{12420\times 1.6\times {10}^{-19}}\times 1.6\times {10}^{-19}$
$\Rightarrow 0.16\times {10}^{-16}=\frac{0.01\times 5\times x}{12420}$
$\Rightarrow x=0.0398\cong 0.04$