Gravitational lensing. False-colour optical image of a large luminous arc" associated with Galaxy Cluster 2242-02. Several theories have been put forward to explain such arcs. Most popular of these is that the arc is part of an Einstein ring: the smeared-out image of a distant galaxy or quasar whose light has been "lensed" by the gravity of the cluster of galaxies. Einstein proposed the effect of gravitational lensing in 1936. This picture was taken by a CCD (charge- coupled device), a kind of silicon chip used for imaging, on one of the optical telescopes of the US National Optical Astronomy Observatories (NOAO)."