GRAVITATIONAL LENSES

According to Einstein's general theory of relativity, a gravitational field bends electromagnetic waves in much the same way as low atmosphe ric air layers (subject to a vertical density gradient) curve the traj ectory of a propagating light ray. Large mass concentrations in the un iverse (stars, galaxies, clusters, etc) can thereby act as a type of l ens, a gravitational lens. Magnificent cosmic mirages may sometimes re sult and it is therefore not surprising that gravitational lensing per turbs our view of the distant universe and also affects our physical u nderstanding of various classes of extragalactic objects. After briefl y reviewing the history of gravitational lensing since the early thoug hts of Newton in 1704 until the serendipitous discovery of the first g ravitational lens system by Walsh, Carswell and Weymann in 1979, we re call the basic principles of atmospheric and gravitational lensing. We then describe a simple optical gravitational lens experiment which ha s the merit of accounting for all types of image configuration observe d among currently known gravitational lens systems. Various types of g ravitational lens models (point mass, singular isothermal sphere, unif orm disk, spiral galaxy, etc) are described in detail as well as the r esulting image properties of a distant source. An updated list as well as colour illustrations of the best known examples of multiply imaged quasars, radio rings and giant luminous arcs and arclets are presente d. Some of these observations are discussed in detail. Finally, we sho w how it is possible to use gravitational lensing as a cosmological an d astrophysical tool, the most interesting applications being the dete rmination of the Hubble parameter H-0, the mass of very distant lensin g galaxies as well as the distribution of luminous and dark matter in the universe. We also show how to determine the size and structure of distant quasars from observations of micro-lensing effects. Such obser vations may also be used to probe the nature of dark matter in the uni verse.