A 25 W source emits monochromatic light of wavelength 6600 $\text{Å}$. What is the photoelectric current (in A) caused by the emitted light assuming 3% efficiency for photoelectric effect? Given $\mathrm{h}=6.6\times {10}^{-34} \mathrm{Js}$.

Energy of each photon$=\frac{\mathrm{hc}}{\mathrm{\lambda }}=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{6600\times {10}^{-10}}=3\times {10}^{-19}\mathrm{J}$

Total energy emitted per second by 25 W source = 25 J 

Number of photons emitted per second$=\frac{25}{3\times {10}^{-19}}=8.33\times {10}^{19}$

Photoelectric current = 3% of photons emitted per second $\times$ charge of an electron

$=\left(\frac{3}{100}\right)\times \left(8.33\times {10}^{19}\right)\times \left(1.6\times {10}^{-19}\right)=0.4 \mathrm{A}$