Why does a ray of light bend towards normal as it passes from air to glass ?

In going from a rarer to a denser medium, a ray of light bends towards normal. And in going from a denser to a rarer medium, a ray of light bends away from normal. Explain why.

Why does a ray of light bend when it travels from one medium to another?

Why does a ray of light bend when it travels from one medium to another.