What happens when light passes through a medium?

Refraction is the bending of light that occurs because light travels at different speeds in different materials.
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