Why do light rays bend when they enter from one optical medium to another?

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

What happens when a light ray enters obliquely from a rarer medium to a denser medium?

What happens when a light ray enters obliquely from a denser medium to a rarer medium?

Why are the car lights dimmed when the starter is operated?

A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

A cyclist has to bend a little inwards from his vertical position while turning. Why?

Why does velocity increase when water flowing in broader pipe enters a narrow pipe?