The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is (where, c = speed of electromagnetic waves) 

Intensity in terms of electric field,  $(I{)}_{\text{electric field }}=\frac{1}{2}{\epsilon }_0{E}_0^2·c$

Intensity  in terms of magnetic fieId $(I{)}_{\text{magnetic filed }}=\frac{1}{2}\frac{B_0^2}{{\mu }_0}·c$

Now, taking the intensity in terms of electric field,

$(I{)}_{\text{electric field }}=\frac{1}{2}{\epsilon }_0{E}_0^2·c$

$\Rightarrow =\frac{1}{2}{\epsilon }_0{\left(c{B}_0\right)}^2·c$    $\left(\because E_0=c{B}_0\right)$

$=\frac{1}{2}{\epsilon }_0\times c^2{B}_0^2·c$

But,    $c=\frac{1}{\sqrt{{\mu }_0{\epsilon }_0}}$

$\therefore (I{)}_{\text{electric field }}=\frac{1}{2}{\epsilon }_0\times \frac{1}{{\mu }_0{\epsilon }_0}{B}_0^{2}c$

$=\frac{1}{2}\frac{B_0^2}{{\mu }_0}·c$

$=(I{)}_{\text{magnetic field }}$

Thus, the energy in electromagnetic wave is divided equally 'between electric field vector and magnetic field vector.
Therefore, the ratio of contributions by the electric field and magnetic field components to the intensity of an electromagnetic wave is 1 : 1.