The energy flux of sunlight reaching the surface of the earth is 1.388 × 103 W/m2. How many photons (nearly) per square metre are incident on the Earth per second? Assume that the photons in the sunlight have an average wavelength of 550 nm.
Energy flux of sunlight reaching the surface of earth, Φ = 1.388 × 103 W/m2
Hence, power of sunlight per square metre, P = 1.388 × 103 W
Speed of light, c = 3 × 108 m/s
Planck’s constant, h = 6.626 × 10−34 Js
Average wavelength of photons present in sunlight, $\lambda=550 \mathrm{~nm}$
$=550 \times 10^{-9} \mathrm{~m}$
Number of photons per square metre incident on earth per second = n
Hence, the equation for power can be written as:
$P=n E$
$\therefore n=\frac{P}{E}=\frac{P \lambda}{h c}$
$=\frac{1.388 \times 10^{3} \times 550 \times 10^{-9}}{6.626 \times 10^{-34} \times 3 \times 10^{8}}=3.84 \times 10^{21}$ photons $/ \mathrm{m}^{2} / \mathrm{s}$
Therefore, every second, $3.84 \times 10^{21}$ photons are incident per square metre on earth.