Oh boy, it seems you're one of today's lucky 10000 (https://xkcd.com/1053/ :-). The concept of EM waves slowing down in a medium is a curious one: it sounds obvious (you can hike a shorter distance in harder train even though exerting same effort) but generates interesting questions, e.g. what is the mechanism that "slows them down", how do photons speed up after they exit a the medium (https://physics.stackexchange.com/questions/330994/how-does-...). I think the problem here is similar to the concept of "relativistic mass", a simple way to think about things and work problems but at the same time creates another set of problems and confusions.
red75prime's short answer in this thread is correct, for a longer one see this Physics SE questions and answers (https://physics.stackexchange.com/questions/11820/what-reall...). In ELI5 terms: photons always move at speed c, but in a medium now and then (depending on properties of the medium, density, etc.) they interact with its atoms. In this case the atom absorbs the photon and after a short time generates another photon. So the speed of each photon is always c but the "overall light wave" becomes less than c.
red75prime's short answer in this thread is correct, for a longer one see this Physics SE questions and answers (https://physics.stackexchange.com/questions/11820/what-reall...). In ELI5 terms: photons always move at speed c, but in a medium now and then (depending on properties of the medium, density, etc.) they interact with its atoms. In this case the atom absorbs the photon and after a short time generates another photon. So the speed of each photon is always c but the "overall light wave" becomes less than c.
If you're a non physicist like me who would like to delve further into this, I suggest Feynman's excellent and very readable book QED: The Strange Theory of Light and Matter (https://www.amazon.com/QED-Strange-Theory-Light-Matter/dp/06...)