Answer
$E_m=8.7\times 10^{-2}N/C$
Work Step by Step
The intensity of an electromagnetic wave is equal to the intensity of the Poynting vector, which is $$S=\frac{E_mB_m}{2\mu_o}$$ Since $E_m=cB_m$ and $B_m=\frac{E_m}{c}$, the equation becomes $$I=\frac{E_m^2}{2\mu_o c}$$ Solving for $E_m$ yields $$E_m=\sqrt{2\mu_o cI}$$ Substituting known values of $I=10 \times 10^{-6}W/m^2$ and the constants yield $$E_m=\sqrt{2(4\pi \times 10^{-7}N/A^2)(3.00\times 10^8m/s)(10\times 10^{-6}W/m^2)}$$ $$=8.7\times 10^{-2}N/C$$
