Final answer:
The direction of wave propagation in the given equation is in the positive y-axis direction, determined by the standard wave equation form and the negative sign before the y-term in the cosine function.
Step-by-step explanation:
The direction of wave propagation can be deduced from the argument of the cosine function in the magnetic field wave equation. For the given magnetic field H = 30cos(10^8t - 0.5y) mA/m, the wave is moving in the positive y-axis direction since the argument of the cosine includes a negative sign in front of the y-term. This is a standard form for representing wave equations where the term -ky indicates the direction of the wave.
In an electromagnetic wave, the electric field (E) and the magnetic field (B) are perpendicular to each other and to the direction of propagation, as described in Figure 2.5.1. Using this right-hand rule, if the magnetic field is given in the problem, and we know the direction of the wave propagation (the y-direction), we can deduce that the electric field would be oriented in the z-direction if the magnetic field is in the x-direction, or vice versa.