Answer
$3.6\times10^{21}\frac{photons}{s\cdot m^2}$
Work Step by Step
The intensity is the energy output per second reaching one square meter in the atmosphere. Divide this intensity by the energy of one photon to find the number of photons per second per square meter.
Equation 27–4 gives us the energy of one photon.
$$E_{photon}=hf=\frac{hc}{\lambda}$$
Number per second per unit area = $\frac{I}{E_{photon}}=\frac{I \lambda}{hc}$
$$=\frac{(1300W/m^2)(550\times10^{-9}m)}{(6.63\times10^{-34}J \cdot s)(3.00\times 10^8 m/s)}$$
$$=3.6\times10^{21}\frac{photons}{s\cdot m^2}$$
