Light changes direction when it goes from air into another medium. This effect is called refraction. 
The angle B (inside the medium) is greater (closer to being a right angle) than the angle A (outside), but their numerical relationship depends on the material.  Different materials cause different angles.  For instance light passing from air into water deflects a certain amount, while light passing from air into glass bends a different amount.  Regardless of how much the light is actually deflected, this idea is already enough to explain why a clear lake is deeper than it looks.  In the picture, the light that reaches our eye from the fish is deflected when it leaves the water and enters the air.  But our brain imagines that it came to us in a straight line, without the bend at the surface.  So it appears that the fish is higher in the water than it actually is.    
It also explains why light changes direction when it goes through a prism.

The angles A and A' generally are not the same, but the are always smaller than the angles B and B'.

Different materials affect the degree to which light is deflected, and the color of the light also plays a role.  This leads to the color effects that can be seen with a prism.  Different wavelengths of light are deflected different amounts.  To get a really good color effect, you need a prism made of a special material for which this wavelength dependence is unusually large (like lead glass or diamond, for example).  The effect is larger if you turn the prism so that the light is travelling almost parallel to the surface when it emerges from the prism.
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Make a microscope!