ing antenna (rods and loops) extract from the (6.8) wave, voltages related to the electric and magnetic fields. Further- more many other features of the wave can be accurately formulated from the theory (some of which are given here, and others may be Elimination of H from this set of simultaneous equations produces found in text books with mathematical derivation). Thus, there is a a wave equation, as in Equation 6.9. tendency to attach more significance to the theory than is justified. It does not prove that the wave is composed of distinct electric and (6.9) magnetic vectors. It only shows that it is very convenient for us to make the distinction. 6/4 Electromagnetic waves 6.1.3 Boundary conditions For n, <n2 Snell's law givesa.<a,sofor(a)+q): If there are discontinuities in the electrical properties of the medium, 1. Greater than 2: rv is positive, i.e. no phase change. determination of the wave behaviour requires a knowledge of what 2. Less than 2: rv is negative, i.e. a It phase change. are called 'the boundary conditions'. The most relevant of these is 3. Equal /2: rv is zero and a equals the Brewster angle. that components of both E and H tangential to a surface must be continuous. Thus, it is possible to determine how a wave behaves if it is incident on a plane surface where there is a change of imped- 6.1.3.2 Case 2 ance from n1 to n2 and of refractive index from nl to n2. If the incident wave-normal makes an angle ajwith the plane-normal, as Electrical vector perpendicular to the plane of incidence (equivalent shown in Figure 6.1, Snell's law gives the angle a2 for the wave to a horizontally polarised transmitter), as shown in Figure 6.2, passing into medium 2. gives Equations 6.19 and 6.20. Medium 1 n 1, N, Medium 1 Medium : n2.22 Medium 2 Figure 6.1 Reflection and refraction at a boundary with the electric vector in the plane of incidence Figure 6.2 Reflection and refraction at a boundary with the magnetic vector in the plane of incidence (6.15) To determine the reflection r and transmission t coeffi