EM average (rms)

[Dx]

Hello!
A 15 mW laser puts out a narrow beam 2mm in diameter. what is the average(rms) value of the magnetic field?
I have the formula E^2 = E^2_0/2 but I need some additional help figuring out how to set up the problem to solve for it. Can you help, plz?
Thanks!
Dx [;)]

[Tom Mattson]

You know the power and you know the cross sectional area of the beam. That is enough for you to calculate the magnitude of the Poynting vector. You still have two unknowns then: Em and Bm. To eliminate Em, there is a simple relationship between it and Bm, and you should be able to find it in your book without any trouble.