There is an interdependance between the electric and magnetric fields when microwaves propagate through space. In these waves the time-changing magnetic field(H) generates a time varying electric field(E), which in turn generates a magnetic field, and as the process repeats, energy is propagated through empty space at the velocity of light. The directions of E & H are everywhere perpendicular and are both transverse to the direction of wave propagation. Waves of this type are called Transverse Electromagnetic(TEM).

Figure 1 - Diagramatic of a plane wave
(Oregan University,Phys)

The wave impedance can be found from the ratio of E/H, in free space the ratio is known as the "intrinsic impedance" it has a value of 377 Ohms.

At any instant and any point, the energy stored in the electric field per unit volume is equal to the energy stored in the magnetic field. However if the wave is a standing wave then the electric energy density is a maximum when the magnetic is zero, and vice versa, these points of maximum are 1/4 wavelength apart.

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