JEE Main · 2021 · 16 Mar 2021 · Shift 1 · EM Waves

In a plane electromagnetic wave in free space the fields are tied at every instant by $E=cB$ with $c=\frac{1}{\sqrt{{\mu }_0{\epsilon }_0}}$. The two energy densities are $u_e=\frac{1}{2}{\epsilon }_0{E}^2$ and $u_m=\frac{B^2}{2{\mu }_0}$. Substituting $B=\frac{E}{c}$,

Since the two densities are equal at every instant, their averages over a cycle are equal, $U_e=U_m$.