in the introduction to jackson's classical electrodynamics, he states that the number of photons incident to a 1 square centimeter area in 1 s for a 100W bulb (1 m away) is on the order of 10^15. also he gives a similar quantity for an FM antenna operating at 100W at a frequency of 10^8 Hz. what's the derivation for these numbers? I suspect that you need to find the intensity of the beams, then multiply by the 1 cm square to get the total energy - and then divide by the energy of 1 photon ( given by E = hf ). what I don't know is how to get the intensity.