Short answers, pulled from the story.
A gravitational lens is any mass, such as a galaxy or a cluster of galaxies, that bends light from a distant source as that light travels toward an observer. The bending happens because, according to Albert Einstein's general theory of relativity, light follows the curvature of spacetime around massive objects. The effect can produce multiple images, arcs, or complete rings of the background source.
The first gravitational lens was confirmed in 1979, when Dennis Walsh, Bob Carswell, and Ray Weymann discovered the Twin QSO using the 2.1 meter telescope at Kitt Peak National Observatory. The object, officially named SBS 0957+561, appeared as two identical quasars but was actually a single quasar whose light was split by a foreground galaxy.
An Einstein ring appears when a light source, a massive lensing object, and an observer align in a perfectly straight line, causing the source to appear as a complete circular halo around the lens. The phenomenon was first noted in print by Orest Khvolson in 1924 and quantified by Albert Einstein in 1936. It is named after Einstein because Khvolson did not calculate the ring's flux or radius.
The three classes are strong lensing, which produces visible Einstein rings, arcs, and multiple images; weak lensing, which causes subtle statistical stretching in the shapes of many background galaxies and can reveal dark matter distributions; and microlensing, where no shape distortion is seen but a background object temporarily brightens as a foreground object passes in front of it.
Arthur Eddington and Frank Watson Dyson observed the total solar eclipse on the 29th of May 1919 from two locations simultaneously: Sobral in Ceará, Brazil, and the island of São Tomé and Príncipe off the west coast of Africa. By photographing stars near the Sun during the eclipse, they showed that starlight passing close to the Sun was slightly deflected, matching the prediction of Einstein's general theory of relativity.
The Euclid Space Telescope, launched in 2023, expects to find around 100,000 strong-lens candidates over its six-year survey. The Vera C. Rubin Observatory, which began collecting data in 2025 in Chile, projects 62,000 to 120,000 galaxy-scale lenses over ten years. The Nancy Grace Roman Space Telescope, planned for launch in 2026 or 2027, is expected to discover around 160,000 additional strong gravitational lenses.