1. Terminology

An electromagnetic wave is characterized by :

• the description at any instant of the amplitude and orientation of the electric E and magnetic H fields constituting it. The ratio of the components transverse to the direction of propagation E _T /H _T is the transverse wave impedance Z _T . The orientation of the electric field defines polarization;

• its frequency f (in Hz or multiples thereof) ;

• its propagation speed v. In a vacuum, this is equal to that of light, i.e. c = 2.997.10 ⁸ m.s ^-1 ; in any medium v = c/n where n is the index of the medium, generally...