Dear ElectroMagneticians! Several students conveyed to me interest in and confusion about phase velocity and group velocity. I found a set of lecture notes on phase & group velocity that I thought provided more useful details than the discussion in Griffiths 9.4 (these notes were originally available at http://www.colorado.edu/physics/EducationIssues/baily/, but that link is now broken). The notes link to a nice series of visualizations that begin here, which I will show in class.