Refraction

 
All types of waves can display the phenomena of refraction. Refraction occurs when light travels from one medium to another which changes the speed at which the light travels.  This causes light to bend upon incidence with the interface of a new material.  Materials are described by indices of refraction which are ratios of the speed of light in vacuum to the speed of light in the specific materials.
This means that a small index of refraction allows light to travel at a faster speed while a larger index of refraction slows light.  If light travels from a medium with a small index to a medium with a higher index of refraction, light will bend toward the normal.  Likewise if light travels from a medium with a large index of refraction to a medium with a lower

Here a laser beam enters the water from the left, bounces off a snail with a mirror on its shell and exits the water to the right.  This is an example of light traveling from a lower index of refraction (higher speed) to a higher index (slower speed) and back to the lower index. Photo by Madelina Furis, University of Vermont Physics Department

index, light will bend away from the normal.  The normal is an imaginary line which falls perpendicular to the surface where the light beam strikes.
 

In the picture to the left water waves are projected onto a screen with the waves traveling from bottom to top.  In the center of the tank is a rectangular piece of Plexiglass that changes the depth of the water and also the speed of the water wave.  The wavefront lags in the shallower water where the index of refraction is higher.

A glass rod with is partially submerged in vegetable oil.  Since the index of refraction of the rod and the oil are almost exactly the same, the rod “disappears” in the oil.

Photo by David Hammond, University of Vermont Physics Department.