Gravitational lenses

According to Einstein's general theory of relativity, a gravitational field bends electromagnetic waves in much the same way as low atmospheric air layers curve the trajectory of a propagating light ray. Large mass concentrations in the universe can thereby act as a type of lens, a gravitational lens. After briefly reviewing the history of gravitational lensing since the early thoughts of Newton in 1704 until the serendipitous discovery of the first gravitational lens system in 1979, the authors recall the basic principles of atmospheric and gravitational lensing. They then describe a simple optical gravitational lens experiment which has the merit of accounting for all types of image configuration observed among currently known gravitational lens systems. Various types of gravitational lens models are described in detail as well as the resulting 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 presented. Some of these observations are discussed in detail. Finally, the authors show how it is possible to use gravitational lensing as a cosmological and astrophysical tool, the most interesting applications being the determination of the Hubble parameter H[SUB]0[/SUB], the mass of very distant lensing galaxies as well as the distribution of luminous and dark matter in the universe. They also show how to determine the size and structure of distant quasars from observations of micro-lensing effects.