Gravitational lenses

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Abstract
According to Einstein's general theory of relativity, a gravitational field bends electromagnetic waves in much the same way as low atmospheric air layers (subject to a vertical density gradient) curve the trajectory of a propagating light ray. Large mass concentrations in the universe (stars, galaxies, clusters, etc.) can thereby act as a type of lens, a gravitational lens. Magnificent cosmic mirages may sometimes result and it is therefore not surprising that gravitational lensing perturbs our view of the distant universe and also affects our physical understanding of various classes of extragalactic objects. 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 by Walsh, Carswell and Weymann in 1979, we recall the basic principles of atmospheric and gravitational lensing. We 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 (point mass, singular isothermal sphere, uniform disk, spiral galaxy, etc.) 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. Same of these observations are discussed in detail. Finally, we 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 Ho, the mass of very distant lensing 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 observations may also be used to probe the nature of dark matter in the universe.