An electromagnetic radiation has both an electric and a magnetic field oscillating at the same frequency. These two fields, perpendicular to each other, propagate in an orthogonal direction in the area (figure below).
The propagation of these waves is carried out at a speed that varies depending on the environment they are found in. In a vacuum situation, the propagation velocity is equal to 3.108 m.s-1.
Source : Bonn et Rochon, 1992
Bonn, F., Rochon, G.
Précis de télédétection. Volume 1 : Principes et méthodes. AUPELF-UREF, 1992. coll. Presses Universitaires du Québec, Montréal.