$\lambda  = 6670\mathop {\,{\text{A}}}\limits^o $

$E$ of a photon $ = \frac{{12400\,\,{\text{eV}}\mathop {\text{A}}\limits^o }}{{6670\mathop {\,{\text{A}}}\limits^o }}$ $ = \frac{{12400}}{{6670}} \times 1.6 \times {10^{ – 19}}{\text{J}}$

Energy emitted per second, power $P=9 \times 10^{-3}\, \mathrm{J}$

$\therefore$ Number of photons incident $=\frac{\text { Power }}{\text { Energy }}=\frac{P}{E}$

$ = \frac{{9 \times {{10}^{ – 3}} \times 6670}}{{12400 \times 1.6 \times {{10}^{ – 19}}}}$ $ = 3 \times {10^{16}}$