A subtle but important point is that E and B are both vectors. This vector character to EM radiation becomes important when we consider the concept of polarization. Familiar to most as an aspect of expensive sunglasses, polarization shows up in both optical observations and radar. A brief illustration of how EM waves propagate becomes necessary at this point.
Figure 2.2 shows how the electric and magnetic fields oscillate with respect to one another in an EM wave (in a vacuum). The electric and magnetic fields are perpendicular to one another and to the direction of propagation k. These waves are transverse as opposed to longitudinal (or compressional) waves. The orientation of the electric field vector in the plane perpendicular to k defines the polarization direction. Here, linear polarization is illustrated—other forms of polarization are possible, but harder to illustrate.
