Answer
$1.03\times10^{16}$ photons.
Work Step by Step
Energy of one photon, E = hν = $\frac{hc}{λ}$ ( as ν = $\frac{c}{λ}$)
h = $6.626\times10^{-34} Js$
c = $2.997925\times10^{8} ms^{-1}$
λ = 532 nm = $532\times10^{-9}m$
E = $\frac{6.626\times10^{-34} Js\times2.997925\times10^{8} ms^{-1}}{532\times10^{-9}m}$ = $3.73\times10^{-19}J$
Total energy = 3.85 mJ = $3.85\times10^{-3}$
Number of photons = Total energy/Energy of one photon
= $\frac{3.85\times10^{-3}J}{3.73\times10^{-19}J}$ = $1.03\times10^{16}$
