# Calculate the electric field amplitude

1. Sep 9, 2012

### dmriser

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

Calculate the average irradiance for the given laser beam.

λ=645nm, P=3mW, d=1mm.

2. Relevant equations

S=[E X H]
S_avg = (1/2) * c * n * ε0 * E0^2

3. The attempt at a solution

I understand the Poynting vector and I can derive the average irradiance formula. I cannot however find the field amplitude E_0 with the information I have.

I know that the field amplitude is proportional to photon flux, but I can't find anywhere what the constant of proportionality is.

First, I calculated the energy per photon.. 3 * 10^-19 J, then I find the photon flux by finding intensity (P/A). From this information I arrive at the photon flux.. From here I don't know what to do.

To be clear, I have something like this for photon flux: 1x1019 photons/m2*s