# Question about a derivation of jackson

1. Jan 8, 2006

### ptabor

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.

2. Jan 8, 2006

### Tide

The intensity is the power (given) divided by the surface area of a sphere 1 m in radius.