\(\frac{{{\text{dn}}}}{{{\text{dt}}}} = \frac{{2 \times {{10}^{ - 3}}\lambda }}{{{\text{hc}}}}\)
\( = \frac{{2 \times {{10}^{ - 3}} \times 500 \times {{10}^{ - 9}}}}{{6.6 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}\)
\( = \frac{{1000}}{{6.6 \times 3}} \times {10^{14}}\) \( = 5 \times {10^{15}}\)