Here's why:

* Refraction is the bending of light as it passes from one medium to another. This bending occurs because the speed of light changes as it moves from one medium to another.

* Speed of light: The speed of light is not constant in all mediums. It travels fastest in a vacuum (like space), and slows down when it enters materials like water, glass, or air.

Example: When light travels from air into water, it slows down, causing it to bend towards the normal (an imaginary line perpendicular to the surface). This is why a straw in a glass of water appears to be broken at the surface.