Electromagnetic radiation is a transverse electric field travelling at the speed of light. (There is always
a changing magnetic field at right angles to it, but it is simplest to consider only the electic lines of force). It cannot be a longitudinal wave (like sound) essentially because, due to the Lorentz contraction, at the speed of light, a length diminishes to zero in the direction of motion. Thus a transverse wave is similar to the displacements along a taut string that is has had one end accelerated: If accelerated only long a straight line (Y-axis in figure) the resulting wave is plane polarised.