BBC
Bbc: Gcse Bitesize: Lenses
Lenses are precisely shaped pieces of glass that have been developed and used in corrective glasses, telescopes, microscopes, binoculars, and magnifying glasses. They can be convex or concave. Links are provided to a video an a test.
Physics Classroom
The Physics Classroom: Image Formation by Lenses
This lesson explains what lenses are, the different between a converging and diverging lens, the difference between a double convex and double concave lens, and gives definitions for terms related to lenses.
Other
Science Hobbyist: Making Telescopes
Site describes how to construct a telescope and what you can do with it. Also provides a simple explanation of how telescopes work.
Georgia State University
Georgia State University: Hyper Physics: Thin Lens Equation
This is an informative site from Georgia State University. It gives a discussion of the thin lens equation and an illustration of its use in determining the image distance based upon the object distance and the focal length.