Since power emitting by a source is given as $\begin{aligned}
& =\frac{\text { Total energy emitted }}{\text { time }} \
& =\frac{\left(E_1 \text { photon }\right) \times \text { Number of photons }(N)}{t} \
& P_1=\left(E_1\right) n
\end{aligned}\begin{aligned}
& \frac{\mathrm{P}_1}{\mathrm{P}_2}=\frac{\left(\mathrm{E}_1\right) \mathrm{n}_1}{\left(\mathrm{E}_2\right) \mathrm{n}_2}=\frac{\left(\frac{\mathrm{hC}}{\lambda_1}\right) \mathrm{n}_1}{\left(\frac{\mathrm{hC}}{\lambda_2}\right) \mathrm{n}_2} \
& \frac{\mathrm{P}_1}{\mathrm{P}_2}=\left(\frac{\lambda_2}{\lambda_1}\right) \frac{\mathrm{n}_1}{\mathrm{n}_2}
\end{aligned}Substitutingthegivenvalues\begin{aligned}
& 2=\left(\frac{300}{600}\right) \times \frac{2 \times 10^{15}}{\mathrm{n}_2} \
& \mathrm{n}_2=\frac{1}{2} \times 10^{15}=5 \times 10^{14} \text { Photon } / \mathrm{sec}
\end{aligned}$