Average radius, r = 1.49 × 1011 m. intensity of sunlight received by earth = I = 1400 W/m2 ,
λ = mean wavelength = 6000 Å.
Energy emitted per second by the sum= Power of the sun = W
but, Power, W= nhν, where n is number of photons emitted by sun per sec. .....(i)
Again, intensity at a distance ‘r’ from a point source of power W.
[From equation (i)]
n = 1.18 × 1045 photons per sec.