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. A galaxy has the mass to lens a more distant galaxy or quasar.
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.