Recall from the wave tank the bending of waves as they entered a shallower water. The change of speed cause the direction of the wave to change as well. Light behaves the same way. As a light wave passes into a medium such as glass, it slows down, due to the greater optical density of the glass. 

          optical density:   property of a material; inverse measure of the speed of light through the material 

Because light is wavelike in nature, it behaves in the same manner as a water wave entering shallower water.

As one part of the wavefront enters the glass, it slows down, while the other part of the wavefront is still in the air, traveling fast. Because different parts of the wave enter the glass at different times, the wave changes direction. 

          refraction:  the bending of light as it passes obliquely into a new medium 

As the wave leaves the glass, it again refracts, since different parts of the wavefront speed up at different times.

We can talk about refraction in terms of  rays of propagation and normal lines: 

As light passes from air into glass, the ray of propagation is bent toward the normal line, and the angle of incidence, i, is greater than the angle of refraction, r. As light passes from glass into air, the ray of propagation is refracted away from the normal line, and the angle of incidence i is less than the angle of refraction r. In general, 

• When light enters a more optically dense medium, i > r; ray of propagation is refracted toward the normal line

• When light enters a less optically dense medium, i < r ray of propagation is refracted away from the normal lin