Question

The energy flux of sunlight reaching the surface of the earth is $1.388\times 10^{3}\, W\, m^{-2}$. The photons in the sunlight have an average wavelength of $550\, nm$. How many photons per square metre are incident on the earth per second ?

• A. $4\times10^{21}$
• B. $4\times10^{34}$
• C. $4\times10^{31}$
• D. $4\times10^{28}$

Answer 1

Answer: (A)

Here, $I=1.388\times10^{3}\,W\,m^{-2}$
$\lambda=550\times10^{-9}\,m, h=6.63\times10^{-34}J\,s$
Number of photons incident on earth’s surface per second per square metre is
$n=\frac{I}{E}=\frac{I\lambda}{hc} \left(\because E=\frac{hc}{\lambda}\right)$
$=\frac{1.388\times10^{3}\times550\times10^{-9}}{6.63\times10^{-34}\times3\times10^{8}}=4\times10^{21}$