Question

The energy flux of sunlight reaching the surface of the earth is $1.52\times 10^{3}W{m}^{-2}$. The photons in the sunlight have an average wavelength of $520nm$. How many photons per square metre are incident on the earth per second?

• A. $4\times 10^{21}$
• B. $4\times 10^{34}$
• C. $4\times 10^{31}$
• D. $4\times 10^{28}$

Answer 1

Answer: (A)

Here, $I=1.52\times 10^{3}W{m}^{-2}$
$\lambda =520\times 10^{-9}m$
$\Rightarrow h=6.63\times 10^{-34}Js$
Number of photons incident on earth's surface per second per square metre is
$n=\frac{I}{E}=\frac{I}{hc}\lambda \left[\because E=\frac{hc}{\lambda }\right]$
$n=\frac{1.52\times 10^3\times 520\times 10^{-9}}{6.63\times 10^{-34}\times 3\times 10^8}$
$=4\times 10^{21}$ Photons per $m^2$ per second