Create more realistic transparent materials in Cycles with this videotutorial by BlenderDiplom.
Gottfried Hofmann writes:
Have you ever wondered how those crazy node-groups for faking absorption with the new ray-length-node are working? This tutorial shows you how the trick is done from the very basic of how Cycles is treating light and transparency to the fully-fledged node-group for re-use in your own projects. You will also learn how to translate a physical formula into Cycles nodes.
This is probably my most didactic tutorial ever.