Answer
$2.985\times10^{18}$
Work Step by Step
If $\lambda$ be the wavelength of the laser, the energy of a photon emitted as laser is given by
$E=\frac{ch}{\lambda}$
$\text{Power of the laser}=\frac{\text{Total energy of the photon emitted in a given time} }{\text{time}}$
$=\frac{\text{Total number of the photon emitted in a given time}\times\text{energy of the photon}}{\text{time}}$
$=\text{Rate of photon emitted}\times\text{energy of the photon}$
Therefore,
$\text{Rate of photon emitted}=\frac{\text{Power of the laser}}{\text{energy of the photon}}=\frac{P\lambda}{ch}$
or, $\text{Rate of photon emitted}=\frac{2.80\times 10^{6}\times424\times10^{-9}}{3\times10^{8}\times6.63\times10^{-34}}\;s^{-1}$
or, $\text{Rate of photon emitted}=5.97\times10^{24}\;s^{-1}$
Therefore, the number of photon generated in time $0.500\mu s$ is
$=5.97\times10^{24}\times 0.5\times10^{-6}=2.985\times10^{18}$
As the atoms contributing to the pulse underwent stimulated emission only
once during the $0.500\;\mu$, the number atoms contributed in laser action is $2.985\times10^{18}$
