Answer
$1.1\times10^{21}$ photons
Work Step by Step
Total energy= $32.8\,J$
Energy of one photon= $\frac{hc}{\lambda}$
$=\frac{(6.626\times10^{-34}\,J\cdot s)(3.00\times10^{8}\,m/s)}{6.5\times10^{-6}\,m}$
$=3.058\times10^{-20}\,J$
Number of photons= $\frac{\text{Total energy}}{\text{Energy of one photon}}$
$=\frac{32.8\,J}{3.058\times10^{-20}\,J}$
$=1.1\times10^{21}$
