When light moves from one medium into another (for example, from air to glass), the speed of light changes. The eﬀect is that the light ray passing into a new medium is refracted, or bent.
When it travels from an optically dense medium like water or glass to a less dense medium like air, it will be refracted away from the normal .
If it travels from a less dense medium to a denser one, it will be refracted towards the normal.
Find out more:
1. what happened to light the light ray if it luminous perpendicularly to the surface of two different medium ?
Learn more : http://dev.physicslab.org/Document.aspx?doctype=3&filename=GeometricOptics_RefractionPhenomena.xml