Gravitational Lens, astronomical phenomenon predicted by the relativity theory of German American physicist Albert Einstein. According to that theory, objects in space that are sufficiently massive could act as a lens for light coming from more distant objects in the same line of sight with respect to observers on Earth. If the configuration of these objects was right, the lens effect could produce two or more identical images of the more distant object.

The first actual gravitational lens was discovered in 1979 by a British astronomer, Dennis Walsh. The lens is a giant elliptical galaxy, and it produces a double image of a more distant quasar. A few other such phenomena have since been observed, including the 1998 discovery of an “Einstein ring” produced by a perfect alignment of nearer and farther objects. Astronomers used a network of radio telescopes in Britain and the Hubble Space Telescope (HST) to discover the Einstein ring. The radio telescope array, called MERLIN, found the ring and the HST produced an image of it. The ring occurs because two distant galaxies happen to line up perfectly with Earth. The closer galaxy’s gravitational lens spreads out the light of the more distant galaxy into a complete circle. Vast luminous arcs, or imperfect rings, have also been observed.

Astronomers can use gravitational lenses to determine the amount of matter in a galaxy by measuring how much the galaxy’s gravitational force distorts the light of a more distant galaxy behind it. By comparing the gravitational effect of the nearer galaxy with the amount of visible matter in the same galaxy, astronomers can estimate the amount of invisible matter, or dark matter, that the galaxy must hold.