What is Gravitational lensing?
[[File:Einstein_cross.jpg|thumb|right|250px|Einstein's Cross: four images from one [[quasar]]]] '''Gravitational lensing''' is caused by a [[mass]]ive body between a distant object and ourselves. It can create the appearance of two or more objects where there is really only one. The light from the object gets bent round the massive body in between. The massive body, such as a [[galaxy]] or [[black hole]], creates a very strong [[gravitational field]] in space. The exact nature of the effect depends on: *relative distance and position between observer, lens and lightsource *size of the lens *mass inside the lens == Einstein == {{General relativity}} [[Albert Einstein]] predicted the possibility of gravitational lensing. The discovery that our Sun bends the light of distant objects when their light passes close to it was evidence that the [[general theory of relativity]] was correct. ==Types of lensing== There are three types of lensing: *strong lensing *weak lensing *microlensing ===Strong lensing=== Strong lensing betrays itself by producing multiple images from the same object. A well-known example is ''[[Einstein's Cross]]'' (Q2237+0305) at a distance of 8 billion lightyears. Here, the gravitational lens produces four images from the same object (a [[quasar]]), although it is only one object. The light of the quasar does not travel in a straight line to [[earth]]. Instead, it is bent along the gravitational field of a galaxy in front of it. This galaxy is at a distance of 400 million lightyears. The first discovery of this kind (except the [[Sun]]) happened in 1979. Two quasars were close to each other. They both had the same [[spectrum]] and proved to be two images of the same quasar (Q0957+651). In 1980 scientists found out which group of galaxies functioned as the lens. ===Weak lensing=== [[Image:Shear-components.svg|thumb|right|125px|Distortions.]] Weak lensing does not produce multiple images of the same object. Instead it produces a highly deformed or stretched image from an object far beyond the