Gravitational lensing is a phenomenon that occurs when the gravitational field of a massive object, such as a galaxy or a cluster of galaxies, bends the path of light coming from a more distant object, like another galaxy or a quasar. This effect is a prediction of Einstein's theory of general relativity, which describes how mass curves spacetime.
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Gravitational lensing is the deflection and focusing of light by spacetime curvature along the line of sight.
A useful first model is a point-mass gravitational lens: one displaced source gives two images of a point-mass lens, while exact alignment gives an Einstein ring. The Einstein angular radius sets the angular scale, and image parity in gravitational lensing explains why the two images have different handedness. The spectroscopic test of the Twin Quasar lens shows how optical spectra can test whether two apparent quasars represent one source.