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
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.