1. The problem statement, all variables and given/known data What is the photon flux (photons/ m2 s) at a distance of 1 km from a light source emitting 50W of radiation in the visible domain, with wavelength 6000 Angstroms. 2. Relevant equations 3. The attempt at a solution Energy of one photon = hc/λ = 3.32 x 10-19J. So, number of photons per second = 50/3.32 x 10-19 = 1.51 x 10-20. So, photon flux = 1.51 x 10-20/4π(1000)2 = 1.20 x 1013. Is this correct?