A plane electromagnetic wave with electric field of amplitude $4\mathrm{V} {\mathrm{m}}^{-1}$ varies with time propagating along $\mathrm{z}-\mathrm{axis}.$ The average energy density of the magnetic field (in $\mathrm{J} {\mathrm{m}}^{-3}$) is:

Amplitude of electric field and magnetic field are related by the relation:

$\frac{{\mathrm{E}}_0}{{\mathrm{B}}_0}=\mathrm{c}$

Average energy density of the magnetic field is

$$\begin{gathered} {\mathrm{u}}_{\mathrm{B}}=\frac{1}{4}\frac{{{\mathrm{B}}_0}^2}{{\mathrm{\mu }}_0}=\frac{1}{4}\frac{{{\mathrm{E}}_0}^2}{{\mathrm{\mu }}_0{\mathrm{c}}^2} \left(\because {\mathrm{B}}_0=\frac{{\mathrm{E}}_0}{\mathrm{c}}\right) \\ =\frac{1}{4}{\mathrm{\epsilon }}_0{{\mathrm{E}}_0}^2 \left(\because \mathrm{c}=\frac{1}{\sqrt{{\mathrm{\mu }}_0{\mathrm{\epsilon }}_0}}\right) \\ =\frac{1}{4}\times 8.854\times {10}^{-12}\times {\left(4\right)}^2 \\ \approx 35.4\times {10}^{-12} \mathrm{J} {\mathrm{m}}^{-3} \end{gathered}$$