During the propagation of electromagnetic wave in a particular...

${\left(\dfrac{U}{V}\right)}_{elec} = \dfrac{1}{2}{\in}_{0}{E}^{2}$ ....(i)

${\left(\dfrac{U}{V}\right)}_{mag} = \dfrac{1}{2}{B}^{2} / {\mu}_{0}$ ....(ii)

$\text{B} = \dfrac{\text{E}}{\text{C}}$ (for EMW) ....(iii)

and $C = \dfrac{1}{\sqrt{{\mu}_{0} {ε}_{0}}}$ .....(iv)

∴ devide eqⁿ (i) by (ii)

$\dfrac{{( U / V )}_{elec}}{{( U / V )}_{mag}} = \dfrac{\dfrac{1}{2}{ε}_{0}{E}^{2}}{\dfrac{1}{2}{B}^{2} / {\mu}_{0}}$

$= {( \dfrac{E}{B} )}^{2} {\mu}_{0} {ε}_{0}$

by using eq (iii) & (iv)

$= \dfrac{{C}^{2}}{{C}^{2}} = 1$

$⇒ {( U / V )}_{elec .} = {( \dfrac{U}{V} )}_{mag}$