• question_answer The photo sensitive surface is receiving light of wavelength $5000\text{ }\overset{\text{o}}{\mathop{\text{A}}}\,$ at the rate of ${{10}^{-8}}\text{ }J/s$. The number of photons received per second is: A) $2.5\times {{10}^{10}}$                     B) $2.5\times {{10}^{11}}$C) $2.5\,\,\times \,\,{{10}^{12}}$             D) $2.5\,\,\times \,\,{{10}^{9}}$

$\lambda \,\,=\,\,5000\,\overset{{}^\circ }{\mathop{A}}\,$ Energy received per second $=\,\,{{10}^{-}}^{\,8}J/sec$ $\therefore$ Energy of one photon is $E=hv=hc/\lambda$, $=\,\,\frac{6.63\times {{10}^{-34}}\times 3\times {{10}^{8}}}{5000\times {{10}^{-10}}}\,\,=\,\,3.99\times {{10}^{-19}}J$    $\therefore$ Number of photon received per second $=\frac{Energy\text{ }received\text{ }per\text{ }second}{Energy\text{ }of\text{ }one\text{ }photon}$ $=\,\,\frac{{{10}^{-8}}}{3.99\text{ }\times {{10}^{-}}^{19}}\,\,=\,\,2.5\times {{10}^{10}}$