Question

At noon on a clear day, sunlight reaches the earth\'s surface at Madison, Wisconsin, with an average power of approximately 2.00 kJ·s–1·m–2. If the sunlight consists of photons with an average wavelength of 510.0 nm, how many photons strike a 2.70 cm2 area per second?

Answer 1

Answer is: 1,39·10¹⁸ photons.
E = h·c/λ.
E = 6,62·10⁻³⁴ J·s · 3·10⁸ m/s ÷ 5,1·10⁻⁷ m.
E = 19,86·10⁻²⁶ J·m ÷ 5,1·10⁻⁷ m.
E = 3,89·10⁻¹⁹ J.
E - energy of one photon.
average power = 2,00 kJ/s·m² = 2000 J/s·m² = 0,2 J/s·cm².
E₁ = 0,2 J/s·cm².· 2,70 cm²·s = 0,54 J.
number of photons = E₁/E = 0,54 J ÷ 3,89·10⁻¹⁹ J.
number of photons = 1,39·10¹⁸.