If a ball travels with a constant acceleration from a high horizontal plane to a low horizontal plane, during the time that it travels between the planes it will speed up and change direction. If it 'falls over' the edge at an angle to the edge, it will change direction toward the normal to the edge.

If Newton's corpuscle model for light was to explain refraction, or the bending of light as it passes from one medium to another, then the speed of the light corpuscle would have to speed up when it passed from the air into another material such as water.

REFRACTION of a light ray is the bending or redirection of the travel of the ray when it passes from one medium into another medium.