Past exam of the physics course of the year 4 of the natural sciences course of the University of Cambridge / 2014 / Paper 1 / A9

Approximate the beam as uniform over its circular cross section. Its mean intensity is $I=P/[\pi (D/2{)}^2]=1.27\times 10^6\mathrm{W}{\mathrm{m}}^{-2}$. For the root mean square magnetic field from beam power in a vacuum electromagnetic wave, $I=c{B}_{\mathrm{rms}}^2/{\mu }_0$, soThis is an rms field, not the peak sinusoidal amplitude. A Gaussian-beam peak would need a stated diameter convention.