This is the change in direction of light hen it moves from one transparent medium to another due to a change in speed of light. The figure below shows the part of a ray of light moving from air to gas;

As light enters into glass, is bent or refracted towards the normal and this makes the angle of incidence (i) to be greater than the angle of refraction (r).

Similar results are obtained when light passes from air into water or paraffin.

Transparent materials like glass, air and water have different optical densities and the greater the optical density, the greater the refracting affects it have on light.

In terms of optical density; Glass > water > air

Generally, light moving into a denser medium bends towards the normal; light moving into a less dense medium bends away from the normal.

Both cases are seen in the figures below;

N.B; from dense to less dense medium, light bends away from the normal

        From less dense to denser medium, light bends towards the normal.

When the ray of light meets the boundary at a right angle (90^o), no deviation occurs as seen in the figure below;

The optical density of air is approximately equal to that of a vacuum, so light does not refract appreciably when it moves from air to a vacuum or vice versa.

 The speed of light reduces when it enters a denser medium and increases when it enters a less dense medium.