Question

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

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

Answer 1

Answer: (A)

$4 \times 10^{21}$

Answer 2

: Here $I = 1.388 \times 10^{3}Wm^{- 2}$

$\lambda = 550 \times 10^{- 9}m,h = 6.63 \times 10^{- 34}js$

Number of photons incident on earth’s surface per second per square metre is

$n = \frac{1}{E} = \frac{1\lambda}{hc}\left( \because E = \frac{hc}{\lambda} \right)$

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

$= 4 \times 10^{21}$photons per $m^{2}$per second