Curved Mirrors and Lenses - Interactive Applet from Surendrananth
Select what kind of optical device you want (convex lens, concave mirror, for example). Drag the red object around and see how the light rays are affected. The yellow dots are place at the focal length, and twice the focal length (special places!) Drag them to change the focal length.
Click http://surendranath.tripod.com/Applets/Optics/RRCS/RRCS.html link to open resource.