Question 11.5: The energy flux of sunlight reaching the surface of the earth is 1.388 × 103 W/m2. How many photons are incident on the Earth per second/square meter? Assume an average wavelength of 550 nm.

Hint: P = nE
Step 1: Find the power.
Power of sunlight per square meter, $$P=1.388\times10^3~W$$
Step 2: Find the number of photons.
$$\text{The power, P = n}E\\ \Rightarrow n=\frac PE=\frac{Ph}{hc}\\ =\frac{1.388\times10^3\times550\times10^{-9}}{6.626\times10^{-34}\times3\times10^8}\\ =3.84\times10^{21}~~~\text{photons}/m^2/s.$$