The energy density associated with electric field $\vec{E}$ and magnetic field $\vec{B}$ of an electromagnetic wave in free space is given by $\left(\epsilon_{0}-\right.$ permittivity of free space, $\mu_{0}-$ permeability of free space)
$U_{E}=\frac{\epsilon_{0} E^{2}}{2}, U_{B}=\frac{B^{2}}{2 \mu_{0}}$
$U_{E}=\frac{E^{2}}{2 \epsilon_{0}}, U_{B}=\frac{\mu_{0} B^{2}}{2}$
$U_{E}=\frac{\epsilon_{0} E^{2}}{2}, U_{B}=\frac{\mu_{0} B^{2}}{2}$
$U_{E}=\frac{E^{2}}{2 \epsilon_{0}}, U_{B}=\frac{B^{2}}{2 \mu_{0}}$
