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NCERT Class XII Chapter
Dual Nature of Radiation and Matter
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Question : 5 of 37
Marks: +1, -0
The energy flux of sunlight reaching the surface of the earth is 1.388 × 10310^{3} W m−2m^{-2}. 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.
Solution:  
Energy of each photon, E = hcλ\frac{hc}{\lambda} =
6.63×10−34×3×108550×10−9\frac{6.63 \times 10^{-34} \times 3 \times 10^{8}}{550 \times 10^{-9}}
= 3.62 × 10−1910^{-19} J
Number of photons incident on earth’s surface per second per square metre total energy per square metre
=
total energy per square metre per second reaching on earth surfaceenergy of each photon\frac{\text{total energy per square metre per second reaching on earth surface}}{\text{energy of each photon}}
= 1.388×1033.62×10−19\frac{1.388 \times 10^{3}}{3.62 \times 10^{-19}} = 3.8 × 102110^{21}
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