Refraction

When any wave strikes a boundary, some of the energy is reflected and some is transmitted or absorbed. When a two- or three-dimensional wave traveling in one medium crosses a boundary into a medium where its speed is different, the transmitted wave may move in a different direction than the incident wave, as shown in Fig. 11–42. This phenomenon is known as refraction.

Heavily related to Snell's Law, comparing the indexes of refraction ( $n = \frac{c}{v}$ )

https://www.youtube.com/watch?v=twURUE075y8