I've recently been stumped after hearing about refraction and its effects on light. I have little knowledge of physics and after doing my own research I thought I could best get an answer here. Any help would be appreciated, or even corrections on what I thought I knew. Essentially, I know that the speed of light is about 299,792,458 m/s in a vacuum. However, I've been told that when light travels through a medium that has mass, it moves slower. My previous knowledge would have suggested that light travels at a constant speed (c), but given what I've heard about refraction this does not seem to be true. So for starters, is this correct? The theory that I have led myself to believe is that light itself does travel at c, but there is somewhat of an illusion that it does not. What I had thought was that for light to travel from point a to b, should the medium have mass, it would take a longer amount of time than if the medium was a vacuum. Light would however travel at c, but would simply take a longer route by bouncing off of the mass on its way through (or something along the lines of that). I suppose that my question could be narrowed down to, does light/a photon always travel at c, regardless of whether or not it is in a vacuum. If clarifcation of what I've said is required then feel free to let me know as I truly am confused on the topic. Thanks.