The terms and conditions for the original code still apply. Check http://www.falstad.com/licensing.html before redistributing or modifying the code. You must always consult the original licensing information but, in case the link is unavailable, here is a copy of the original license (as of 2013-05-08):
You have permission to use these applets in a classroom setting or take
screenshots as long as the applets are unmodified. Modification or
redistribution for non-commercial purposes is allowed, as long as you
credit me (Paul Falstad) and provide a link to my page (the page you
found the applet(s) on, or http://www.falstad.com/mathphysics.html).
Contact me for any other uses. The source code for each applet is
generally available on that applet's web page, but some of the applets
use third-party source code that has restrictions.
THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
Rodrigo Hausen's changes are in the public domain (but it would be nice if you credited me alongside Paul if you use this version).