Another nice example of a self-illustrating phenomenon is photoelasticity. Plastic is a birefringent material, which means that the speed of light is different in different directions. Since the index of refraction is the ratio of the speed of light in a vacuum to the speed in the material, the index of refraction is a also a function of direction. When placed under stress, the index of refraction changes as a function of the stress tensor. If you take a picture of plastic illuminated with polarized light, you can see the underlying stress patterns. In this image, isochromatic lines represent lines of constant principal-stress difference.
Notice how clearly this image shows the concentration of stress at the bottom of the crane hook.
 http://www.doitpoms.ac.uk/tlplib/photoelasticity/index.php for a nice introduction.
NOTE: I still don?t *really* understand this!