Georgia State University
Georgia State University: Hyper Physics: Lens Maker's Formula
A discussion of the lens-maker's formula and its use in determining the effect of lens shape and index of refraction upon the magnifying power of a lens. Includes an interactive problem-solving section.
University of Chicago
Telescopes at Yerkes Observatory / Virtual Tour
This site describes the difference between reflective and refractive telescopes and includes a virtual tour of Yerkes Observatory which houses the largest refracting telescope in the world.
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.
Exploratorium
Exploratorium: Make Your Own Telescope
At this site students can make their own telescope and see how it works with this experiment. Out of a light, a pair of glasses, a magnifying glass and wax paper.
Other
Rice University/ats: Virtual Museum of Telescope History
This virtual museum has information on early telescopes which are mostly refractors. It contains research information on telescopes and early observatories as well as navigational equipment. Some parts still under construction.
Georgia State University
Georgia State University: Hyper Physics: Refractive Telescopes
This site from Georgia State University Physics Department gives a discussion of the manner in which light is refracted by refractive telescopes. Discusses the Galielean telescope and the astronomical telescope.