Animation 5. How reradiation from free electrons causes an increase in phase speed. In the top panel, three electrons respond to a distant EM radiation source, as indicated by the blue wave and and parallel phase lines. The initial radiation is in the form of a wave packet, the leading edge of the wave packet is indicated by a yellow line. The electrons absorb some of the energy and accelerate. This causes new EM fields (red waves and curved phase lines). Unlike all the previous figures on this and the preceding page, the phase lines of the reradiation (in red) correspond to the wave troughs, not peaks. Therefore, when the original and reradiated phase lines (as drawn) overlap, there is a cancellation of the resulting signal. The bottom panel shows a wave diagram of the original wave packet (blue), the effect of all the reradiation (red) and the net effect resultant wave (green), determined by adding the original and reradiated EM field.

In the figure, look at the region where the reradiation waves intersect. Notice that that the trough phase lines from the reradiation are slightly behind (to the left) the peaks of the the original phase lines. This causes a cancellation of the back part of the original wave peaks and effectively advances the phase of the resultant signal.