Light refracts or bends when it leaves one medium and enters another one (ex. when it goes from air to water; or when it goes from glass to air). This is because the speed of light is different in different materials (light travels in a vacuum at a speed of 3 x 10^8 m/s and is slower in all other mediums). When light slows down (ex. moves from air into water), the light bends towards the normal; when light speeds up (ex. moves from water into air), the light bends away from the normal.