How to find the polarization (phase) of the virtual source if image theory is used
Polarization and phase are not the same thing. 

Polarization is the plane that the electric field oscillates in (the magnetic field is at right angles to it). In a vertical antenna the electric field is vertically oriented, so the EM wave will also have vertical polarization. Polarization angle is not changed by reflection, so a vertically polarized wave will still be vertically polarized when reflected (thus "the polarization of the reflected waves must be as indicated").



Phase is the relative angle in time between waves. If two waves are going up and down at exactly the same time then they are 'in phase', and when combined their amplitudes will add ('constructive' interference). If they are 'out of phase' then the combined signal will not be as strong ('destructive' interference), the weakest point being when the phase difference is 180&deg;.

     

Reflection may cause a phase change depending on the angle of reflection, polarization of the incident wave (vertical E field is close to in phase at angles over about 30&deg;, horizontal E field is always 180&deg; out of phase) reflecting material and frequency. But phase also changes with distance, so two waves that start out in (or out) of phase may not be so after traveling over paths with different lengths. As the reception point is moved the the phase difference will continually change from constructive to destructive and back, so the signal will fade in and out.