Gravitational lensing is the focusing of electromagnetic radiation by a massive body. EMR such as visible light passing near the massive body (such as a galaxy or galaxy cluster) is turned from its straight path by the force of gravity. Such a configuration is termed a gravitation lens (or in context, just a lens). A galaxy has the mass to lens a more distant galaxy or quasar. A strong gravitational lens is such a lens producing decided effects such as multiple images of the same object, or turning a contiguous object into a ring (called an Einstein ring, a ring-shaped image formed as the lensing object redirects light passing it from the lensed object toward the observer), and a weak gravitational lens is one in which only less decided effects occur: distortion and displacement of the lensed object(s). The latter can be detected through apparent distortion of objects surrounding the lens, or to detect lensing by unidentified objects, through differences between the statistics of the viewed object shapes versus those of the expected shapes, this statistical difference termed cosmic shear.
Gravitational microlensing is the gravitational effect on electromagnetic radiation by a smaller body such as a star, and is a means of detecting the presence of bodies otherwise unseen but able to lens the passing light enough to create a transient. The smaller body can be a transiting planet, dim binary star companion, brown dwarf, or "free" planet. Searches for such lensing transients have been carried out to search for MACHOs.
Gravitational lensing of a jet can be useful in observing jet and active galactic nucleus detail otherwise not visible. A jet which should be straight and appears bent offers one means of detecting such lensing.
Gravitational lens models that are used for analysis of observation include the Schwarzschild lens, which models the lensing mass as a point, and the embedded lens which models the lens is a mass concentration surrounding by a low-density region so on a larger scale, the density is uniform. Both are approximations.