Final answer:
Refraction occurs when light crosses the border between two media at an angle, due to a change in speed associated with different indices of refraction. Light bends towards the normal when it enters a denser medium and away when it enters a less dense medium, as described by Snell's law and Huygens's principle.
Step-by-step explanation:
Light experiences refraction when it hits the border between two media at an angle due to a change in its speed. This change in speed is related to the indices of refraction of the two media. When light travels from a medium with a lower index of refraction to one with a higher index, it slows down and bends towards the normal. Conversely, when it moves from a medium with a higher index of refraction to one with a lower index, the light speeds up and bends away from the normal. This concept is explained by Snell's law, which is derived from the wavefront geometry described by Huygens's principle. This phenomenon is analogous to the motion change observed when, for example, pushing a lawnmower from a footpath to grass: the mower slows down and turns towards the area where it moves slower, akin to how light bends towards the normal in a denser medium.