The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is
We know that the average energy of an electromagnetic wave is related to the electric field as
Where E0 and 
are the amplitude of the electric field and permittivity of vacuum respectively.
And to the magnetic field as,
Where B0 and 
are the amplitude of the magnetic field and permeability of vacuum respectively.
Now, we know that E0 and B0 are related as
Where c is the speed of the wave, therefore,
But
Therefore, electric and magnetic fields contribute equally, to the electromagnetic wave, thus the ratio of their contributions is 1:1
1