Faraday discovered that a varying magnetic field can produce varying electric field. Maxwell showed that varying electric field can produce varying magnetic field.

"Such moving electric and magnetic fields can propagate out in the surrounding space are know as electro-magnetic waves".

In electromagnetic waves both electric and magnetic fields are perpendicular to each other and their direction reverse together with each cycle. As electric and magnetic fields can exists in vacuum, therefore no medium is required to transmit energy by electromagnetic waves.

The speed of the electro magnetic waves depends upon the permeability and permitivity of the medium through which is propagates.