What's your best illustration for......?

To demonstrate refraction, I would suggest filling a clear container halfway with water. Then place a straight object like a pencil or ruler into the container. Look at it from the side. At the surface of the water, the object's appearance will change; it's width and position will both appear to be discontinuous at this junction. This is because water has a different refractive index than air.

Try this site, it has several diagrams; see if one or more can be of any help:

http://www.extinctionshift.com/topic_06.htm