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.