Answer
13 V/m, $4.4\times10^{-8}T$.
Work Step by Step
The intensity is the power per unit area. The light’s power is distributed uniformly over the surface area of a 2.50-radius sphere.
The energy per unit area per unit time is the magnitude of the average intensity, equation 22–8. Find the strength of the electric field.
$$\overline{I}=\frac{P}{A}=\frac{P}{4\pi r^2}=\frac{1}{2}\epsilon_ocE_o^2$$
$$E_o=\sqrt{\frac{P}{2\pi r^2\epsilon_oc }}$$
$$E_o=\sqrt{\frac{18W }{(2\pi (2.50m)^2) (8.85\times10^{-12} C^2/(N\cdot m^2))(3.00\times10^8m/s) }}$$
$$\approx 13V/m$$
Now find the strength of the magnetic field.
$$B_o=\frac{E_o}{c}=\frac{13.14V/m}{3.00\times10^8m/s }=4.4\times10^{-8}T$$