There\'s a tutorial here on the COW about the caustics effect that covers what you\'re looking for.
http://library.creativecow.net/articles/tierney_jim/caustics.php
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.