I've always associated the question of changes in direction in terms of a boundary, not in terms of the actualy media, at least for "corpuscles".
For example, consider a golf ball rolling (without friction) at a steady speed on a level surface. Now suppose the ball obliquely hits a ramp to a higher, but also level surface. Once it reaches the top surface, it will be a) travelling at a slower, steady speed, and b) heading farther away from the normal. The higher the ramp and the slower the ball is going at the end, the more it bends. If the ramp is high enough, you observe "total external reflection".