# Homework Help: Number of photons/volume surrounding radio transmitter

1. Feb 6, 2009

### Mika3

1. The problem statement, all variables and given/known data

Seeking a method of calculating the number of photons instantaneously present within a given volume surrounding a radio transmitter at given power and frequency.

2. Relevant equations

$$E=hv$$, and other equations involving power and wavelength.

3. The attempt at a solution

I have no problem calculating the photon energy, and number of photons/second at a given frequency and transmitter power. But how to calculate the number of photons within a given spherical volume, with the transmitter in the center (assuming spherically-symmetrical pattern)?

For example: number of photons within the near-field at x watts and y frequency (I know things can get complicated in the near-field). This is for my own curiosity and is not a textbook question (at least not one that I am aware of). I do not know where to start.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Feb 6, 2009

### HallsofIvy

If, as you say, you have no problem with finding the number of photons of a given wavelength produced in, say, a second, then just determine how many of those photons will still be within the given radius of the source. Assume that the photons are produced at a constant rate during that second and that they move away from the source with speed, of course, c.

3. Feb 7, 2009

### Mika3

Thank you HallsofIvy. I was getting confused about the near-field, but real emitted photons are the only thing we have to work with you are right.