Why does light change its path as the medium changes during the transit?

When light travels from one medium to another, its speed changes. So it causes a slight change in direction. This change of direction is called refraction. 

The amount of bending depends on:

If a substance causes the light to speed up or slow down more, it will refract (bend) more.

If the light is entering a new medium at a greater angle, the amount of refraction will also higher. But, if the light is entering the new substance with normal incidence (at 90° to the surface), the light will slow down, but it won’t change direction.