Dear ElectroMagneticians! Several students conveyed to me interest in and confusion about phase velocity and group velocity. I found a set of lecture notes that I thought provided more useful details than the discussion in Griffiths 9.4 at this page. If you scroll down to Course Materials, you’ll find a link to .pdf on phase & group velocity. The .pdf links to a nice series of visualizations that begin here. There are also notes on dispersion and EM waves in conductors (basically several supplements to Griffiths 9.4), also at Course Materials. These are optional supplements, but if you have time, some of you will find them illuminating.