1. Refraction and Snell's Law
You may recall that waves change speed when they pass from one medium into another - their frequency stays the same but their wavelength changes. This in turn cause a change in their direction as one side of the wavefront slows down or speeds up before the other.
2. EM Wavespeed and Refractive Index
A little bit about little c and how this is related to the refractive index of a material.
3. Refraction Extra Points
A little bit extra on how as the wave speed changes this affects the wavelength and frequency of the wave.