\(E\) 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 \(=\frac{\text { Power }}{\text { Energy }}=\frac{P}{E}\)

\( = \frac{{9 \times {{10}^{ - 3}} \times 6670}}{{12400 \times 1.6 \times {{10}^{ - 19}}}}\) \( = 3 \times {10^{16}}\)