Refraction: How light rays get kinks

Have you ever observed what happens when you stick a pencil in a glass of water, and then look at it from different directions? This picture shows you that the pencil doesn't seem to be straight any more, it looks like it has a kink! What does this mean?  

Shining a laser beam, for instance from a laser pointer, onto a bowl of water can give us a clue: the light beam does not continue straight at the border between air and water, it has a kink! The phenomenon is called refraction: a light ray bends when it goes from air into water, or vice versa, or from glass into air, or vice versa – generally, when it goes from one medium into another. When we use optical lenses, we are taking advantage of the refraction of light.

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