The velocity of wave changes when wave move from one media into another media.
For light: The index of refraction is defined as $n=c/v$, where $c$ is the speed of light in vacuum, and $v$ is the speed of light in media.

The following animation use many dogs running from one region into another region.
Due to the condition on the ground , dogy's speed is different when running from one region into another region.
It obeys Snell's Law $n_1 \sin\theta_1 =n_2 \sin\theta_2$.

You can use $n_1,n_2$ slider to change speed of running dogs and c1 to change it moving direction.

/htdocs/ntnujava/ejsuser/2/users/ntnu/fkh/dogyrunning_refraction_pkg/dogyrunning_refraction.propertiesFull screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list