The average magnetic energy density of an electromagnetic wave of wavelength λ travelling in free space is given by:

$\frac{B^2}{2 \lambda}$ $\frac{B^2}{2 \mu_0}$ $\frac{B^2}{\mu_0 \lambda}$ $\frac{B}{\mu_0 \lambda}$

$\frac{B^2}{2 \mu_0}$

The average magnetic energy density of an electromagnetic wave travelling in free space is $u_{B}=\frac{B^2}{2 \mu_0}$