Refraction of light — Describe

What happens to light as it passes through the boundary between two transparent media?

When light travels from a less dense medium (e.g. air) into a denser medium (e.g. glass) at an angle, it slows down and bends towards the normal. When it passes back from the denser medium into the less dense medium, it speeds up and bends away from the normal. Light travelling exactly along the normal changes speed at the boundary but does not change direction.