In my physics class, we learned that light moves slower when traveling in different materials like water or air. However, it is the fastest in a vacuum. I was wondering why this happens. I read something about a refraction index in the different materials. For example, water has a refraction index of 1.3 or something. So, the speed of light in water is technically c/1.333....;the problem is, I still don't understand WHY this happens. Any help would be really nice. Thx!