Why if a ray pass though different materials its direction changes?

[scientifico]

Hello, why if a ray pass though different materials its direction changes ? couldn't its propagation speed just increase or decrease without having to change the angle ?

Thank you

 

[barryj]

The way I was taught is that light has a wave front that is perpendicular to the direction of travel. When the wave front hits a different medium, one edge of the wave front enters the medium first and its velocity changes. This causes the wave front to rotate and change the direction of the ray.