The intensity of an electromagnetic wave is given by:
I=Uavc
here, Uav is the average energy and c is the speed of light.
The average energy in terms of the electric field is written as:
Uav=12εoE2o
Average energy in terms of magnetic field:
Uav=12B2oμo
We know that, Eo=cBo and c=1√μoεo
∴Uav (electric field)=12εo×1μoεoB2o
⇒12B2oμo
Since the energy due to electric and magnetic field is same. so, both will have the same contribution to the intensity.