Matter consists mainly of vacuum. So even in matter we would expect electromagnetic waves to propagate with the vacuum speed of light c nearly everywhere. However, we know from Physics 101 that this is wrong. Electromagnetic waves in matter have a velocity c/n, where n is the refractive index (for simplicity let me only consider non-dispersive media). How does this come about? What is the microscopic mechanism behind generating an electromagnetic wave with velocity c/n? The answer to this rather obvious question is definitely not addressed in Physics 101, and as a matter of fact not widely known in the first place. It is given by the Ewald-Oseen extinction theorem dating from 1915! It demonstrates a remarkable and intricate interplay between electric dipoles, that get excited by the incident wave and then in turn produce fields that act back on the dipoles themselves. The solution to this self-consistency problem shows that the incident wave becomes extinct and is replaced by a wave with wave vector k=n 𝜔/c. It is a fun linear algebra programming exercise that gives real insight. For more details check out The Ewald–Oseen extinction theorem and extinction lengths and An extinction theorem for electromagnetic waves in a point dipole model.