Transcript: In Einstein’s theory of relativity mass bends light, and this leads to the phenomenon of gravitational lensing. A single massive galaxy can deflect light by a very small angle, about one arcsecond. However, this can be resolved from the ground and especially with the Hubble Space Telescope, and so many situations of gravitational lensing have been discovered. Elliptical galaxies are massive and concentrated enough in their centers to cause multiple image formation of background active galaxies or quasars. Astronomers know nearly one hundred situations where a single quasar image has been turned into multiple images by an intervening galaxy. Typically two or four images are seen; however, there is an odd numbered image, the fifth or the third, which is demagnified and superimposed on the lensing galaxy itself. The situation of gravitational lensing is important in astronomy because the gravity of all matter, visible and dark matter, causes the bending of the light and so astronomers can model the entire mass of a galaxy in this way.