Refraction analogy. Illustration showing a fast car travelling at speed on a road (grey) hitting an area of thick mud (brown) at an angle. The friction of the mud slows the wheels on one side of the car, and the path of the car is bent. The more it is slowed, the more its path is bent. This is an analogy for the refraction (bending) of waves as they slow down when passing from one medium into another of greater optical density.