Answer
See explanation.
Work Step by Step
a. The energy is $Pt=(0.600W)(0.0200)=0.0120J=7.50\times10^{16}eV$.
b. Find the energy of one photon.
$$ E=hf=h\frac{c}{\lambda}$$
$$E=\frac{1.24\times10^{-6}eV\cdot m}{652\times10^{-9}m}=1.90 eV=3.04\times10^{-19}J$$
c. The number of photons in a pulse is $\frac{0.0120J }{3.04\times10^{-19}J }=3.94\times10^{16}$.
