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

Answer 1

The correct option is A $4\times {10}^{21}$

Here,$I=1.388\times {10}^{3}W{m}^{-2}$
$\lambda =550\times {10}^{-9}m,h=6.63\times {10}^{-34}Js$

Number of photos incident on earth's surface per second per square metre is
$n=\frac{I}{E}=\frac{I\lambda }{hc}$ $[\therefore E=\frac{hc}{\lambda }]$

$=\frac{1.388\times {10}^3\times 550\times {10}^{-9}}{6.63\times {10}^{-34}\times 3\times {10}^{-8}}=4\times {10}^{21}$