Chapter 7 - Quantam Theory of the Atom - Questions and Problems - Page 298: 7.119

Please see the work below.

We know that $\nu=\frac{c}{\lambda}$ We plug in the known values to obtain: $\nu=\frac{2.998\times 10^8}{0.125}=2.398\times 10^9 s^{-1}$ We also know that $E=h\nu$ We plug in the known values to obtain: $E=6.626\times 10^{-34}\times 2.398\times 10^9=1.589\times 10^{-24}J$ Now $n=8.368\times 10^4\times \frac{1 \space photon}{1.589\times 10^{-24}}=5.2656\times 10^{28} photons$