Active learning of phase and group velocities

I report on my observations during an active-learning activity on the topic of phase and group velocities with in-service physics teachers and a small group of high school students. The activity was designed to be as active as possible, and to guide the participants to the discovery (or reminder) that phase velocity is not the only velocity relevant to wave propagation. I observed that both groups, even after having been exposed to waves with dispersion, had difficulties using any velocity but phase velocity in their predictions and their reasoning. In the article, I describe the activity, and the observations. I discuss the possible reasons for the participants' difficulties and suggest approaches that might improve the active-learning outcome.