Jean Surdej{1}{*}, Sjur Refsdal{2} and Anna Pospieszalska-Surdej{1}
{1}Institut d'Astrophysique, Université de Liège, Avenue de Cointe 5, B-4000 Liège, Belgium
{2}Hamburg Observatory, Gojenbergsweg 112, D-21029, Hamburg-Bergedorf, Germany
{*}also Research Director, Belgian Fund for Scientific Research (FNRS)


  Because of refractive index variations in the air layers just above the ground, atmospheric lensing may significantly deform our view of distant Earth-objects. Similarly, gravitational lensing perturbs our view of the distant Universe and affects our physical understanding of various classes of extragalactic objects. After recalling the basic principles underlying the formation of atmospheric and gravitationally lensed mirages, we describe a simple optical lens experiment which accounts for all types of image configurations observed among presently known gravitational lens systems. Direct images of most of these systems as well as further information on gravitational lensing (bibliography, the present didactical experiments, etc.) may be found on the Gravitational Lensing World Wide Web page at the URL address:


The didactical experiments proposed in this contribution and most of the figures are adapted from Surdej (1990), Refsdal and Surdej (1992, 1994) and Surdej, Refsdal and Pospieszalska-Surdej (1993, 1995; see the exact references in the Gravitational Lensing Bibliography).