Refraction is a change in direction caused by a change in wave speed

Refraction & Total Internal Reflection — AQA A-Level Physics

Key Definition

Refraction — The change in direction of a wave when it passes through a boundary between media of different optical density.

The direction change is caused by different parts of the wavefront changing speed at different times as they cross the boundary.
When a wave enters a more optically dense medium, it slows down and bends towards the normal.
When a wave enters a less optically dense medium, it speeds up and bends away from the normal.
When a ray hits the boundary at 90 degrees (along the normal), it passes straight through with no change in direction.
During refractionThe change in direction of a wave as it passes from one medium to another, caused by a change in wave speed., speed and wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). change but frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). stays the same.