Gravitational lenses produce different shaped images depending on the shape of the lensing body. If the lens is spherical then the image appears as an Einstein ring (in other words as a ring of light) (top); if the lens is elongated then the image is an Einstein cross (it appears split into four distinct images) (middle), and if the lens is a galaxy cluster, like Abell 2218, then arcs and arclets (banana-shaped images) of light are formed (bottom). Image credit: ESA.
A gravitational lens is the gravitational equivalent of a magnifying glass: the focusing or distorting effect produced when a concentrated mass lies along the line of sight from an observer to a more distance light source. Typically, the distant source is a quasar and the lensing mass is a galaxy lying between the quasar and Earth. Gravitational lenses arise from the warping of spacetime around a massive object, as described by Einstein's general theory of relativity and first discussed by Einstein in 1936. Light rays, in following the shortest path through the curved region of spacetime, are deflected. But the effect is not as simple as that produced by an ordinary glass lens; instead it is comparable to the distortion produced by looking through the base of a wineglass. Depending on the geometry of the gravitational lens, the resulting image of the lensed object may be an arc, a complete circle (known as an Einstein ring), or a series of multiple images. More than 20 examples of cosmic-scale gravitational lenses have been discovered since the first in 1979, including the remarkable Einstein Cross.
Gravitational lensing also occurs on a smaller scale, known as microlensing, when a dark object in our Galaxy passes directly in front of a more distant star (for example, one of the stars in the Magellanic Clouds) and makes its image brighten briefly. Microlensing can be used as a sensitive method of searching for extrasolar planets and MACHOs. If the lensing object happens to be a star and its planetary system, details about the orbiting planets can be worked out from observations of how the lens affects the light of a background star during alignment.