Class 12 Physics
CBSE and UP Board
Dual Nature of Radiation and Matter
Chapter-11 Exercise 11.5
NCERT Solutions for Class 12 Physics Chapter 11 Question-5
The energy flux of sunlight reaching the surface of the earth is 1.388 × 10³ W/m² . 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
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Energy flux of sunlight reaching the surface of earth,φ= 1.388 x 103 W/m2
Hence, power of sunlight per square metre, P = 1.388 x 103 W
Speed of light, c = 3 x 108 m/s
Planck’s constant, h = 6.626 x 10⁻34 Js
Average wavelength of photons present in sunlight, λ = 550 nm = 550 x 10⁻9 m
Number of photons per square metre incident on earth per second = n
Hence, the equation for power can be written as:
P = nE
Therefore, n = P/E = P λ /hc = (1.388 x 103) x (550 x 10⁻9) /( 6.626 x 10⁻34 ) (3 x 108 )
= 3.84 x 1021 photons/m²/s
Therefore, every second, 3.84 x 1021 photons are incident per square metre on earth.