Why does a wave change direction when it enters a different medium?

Different materials have different densities which affects the speed that a wave will be transmitted through it. In general, the denser the material, the more slowly a wave, such as light travels through it, and in a less dense material a wave will travel faster. When a wave is transmitted through a medium with a change in density at a boundary, this change in density causes refraction of the wave to occur, where the wave effectively changes direction due to the change in density causing a change in speed of the wave.