Albert Einstein showed that gravity will cause light to bend. The effect is normally extremely small, but when light passes close to a very massive object such as a massive galaxy, a galaxy cluster, or a supermassive black hole, the bending of the light rays becomes more easily noticeable.
When light from a very distant object passes a galaxy much closer to us, it can detour around the foreground object. Typically, the light bends around the object in one of two, or four different routes, thus magnifying the light from the more distant galaxy directly behind it. This natural telescope, called a gravitational lens, provides a larger and brighter – though also distorted – view of the distant galaxy. These distortions, which stretch beyond the limits of the Hubble Space Telescope, can be effectively handled by a new space telescope on the drawing boards – the James Webb Space Telescope
As many as 20 percent of the most distant galaxies currently detected appear brighter than they actually are, because of an effect called “strong gravitational lensing,” astronomers have discovered. This graphic illustrates how, when astronomers view distant galaxies in a telescope, some of those galaxies line up with our view of nearby galaxies. The gravity of the nearby galaxies bends and magnifies the light coming from some of those distant galaxies, so that they appear brighter than they actually are. Thus galaxies that would normally be too faint to detect become visible in telescope images.
A very massive object – or collection of objects – distorts the view of faint objects beyond it so much that the distant images are smeared into multiple arc-shaped images around the foreground object. According to Rogier Windhorst, this effect is analogous to looking through a glass coke bottle at a light on a balcony and noticing how it is distorted as it passes through the bottle. Cosmologists such as Windhorst believe that gravitational lensing likely distorted the measurements of the flux and number density of the most distant galaxies seen in the recent deep near-IR surveys with the Hubble Space Telescope Wide Field Camera 3.
Read more here Cosmic magnifying lenses distort view of distant galaxies.