Chapter 7 - Quantum Theory and the Electronic Structure of Atoms - Questions & Problems - Page 319: 7.114

$1.2\times10^{2}$ photons.

Energy of one photon $E=h\nu=h\frac{c}{\lambda}$ $=\frac{(6.63\times10^{-34}\,J\cdot s)(3.00\times10^{8}\,m/s)}{600\times10^{-9}m}=3.315\times10^{-19}\,J$ Number of photons=$\frac{4.0\times10^{-17}\,J}{3.315\times10^{-19}\,J}=1.2\times10^{2}$