Gravitational lens optics

Recently, interest in gravitational lenses has expanded to include the study of the images of extended sources such as galaxies as well as of those of unresolved point sources like quasars. The giant luminous arcs associated with the rich clusters Abell 370, Cl2244-02, and Abell 963, together with the ring radio source MG1131+0456, are now widely believed to be gravitationally imaged. Some simple geometrical ways to understand the formation of extended images are described in this article. It is explained how the image shape is controlled by the location of the source relative to the caustic surfaces formed by the lens. Multiply-imaged extended sources generally create larger magnifications and furnish more information about the gravitational potential of the lens than do multiply-imaged point sources. A simple iterative method for deriving the source structure is described and some preliminary results for MG1131+0456 are exhibited. Future observational prospects are briefly discussed