Strong Lensing

Effect due to gravitational bending of light by mass concentrations which results in (1) the strong distortion of background galaxy images (arcs) behind some massive foreground galaxy clusters, and (2) multiple images of the same background quasar behind a foreground galaxy. Over 50 lensed quasi-stellar objects are known.


Image source: http://www.centauri-dreams.org/wp-content/uploads/2006/05/lensed_quasar_2.jpg.