## University Physics with Modern Physics (14th Edition)

$4.00\times10^{17}$ photons per second.
Find the photon energy. $$E=hf=h\frac{c}{\lambda}$$ $$E=\frac{1.24\times10^{-6}eV\cdot m}{10.6\times10^{-6}m }= 0.117eV$$ We know the power of the laser beam is 7.50mW, so 7.50mJ per second is delivered. This is $4.69\times10^{16}$ eV. The number of photons delivered per second can then be calculated. $$N=\frac{4.69\times10^{16}eV/s}{0.117eV /photon}=4.00\times10^{17} photons/s$$