Question

why does light bend when it goes from one medium to another

Answer 1

It's because of a phenomena called refraction. When a light beam travels from one medium to another, then it's velocity decreases or increases depending upon the optical density of the other medium. Notice the word beam here. A beam is a collection of many light rays. All light rays in a beam move individually. In reality we only have a narrow beam of light, a ray of light is idealization. When a light beam enters another medium at an slanting angle the ray that reaches the medium first changes it's velocity before any other ray, the change in velocity causes the beam to bend.