Answer
The pulse energy is 0.017 J, and there are $5.5\times10^{16}$ photons per pulse.
Work Step by Step
The energy per pulse is the average power multiplied by the duration.
$$E=P\Delta t=(0.68W)(25\times10^{-3}s)=0.017J$$
The number of photons contained in a pulse is the energy of a pulse, divided by the energy of a single photon of that frequency. Use equation 27–4.
$$N=\frac{E}{hf}=\frac{E\lambda}{hc}=\frac{(0.017J)(640\times10^{-9}m)}{(6.626\times10^{-34}J \cdot s)(3.00\times10^{8}m/s)}$$
$$= 5.5\times10^{16}photons $$
The pulse energy is 0.017 J, and there are $5.5\times10^{16}$ photons per pulse.
