Gravitational lensing is the effect that a galaxy, or large massive object such as a star, has on light effectively making it a lens to view objects behind it. The principles of gravitational lensing are based in Einstein's Theory of General Relativity. In this theory Einstein shows that all bodies in the universe that have mass create a dimple in space time. The effect is that every massive object literally bends the fabric of space that surrounds it and is a function of both mass and gravity. Einstein theorized that a star could be used as a galactic magnifying glass to view stars behind another star or stars. Yet, Einstein dismissed this function of general relativity as too small a magnification to be significant.
Gravitational lensing not only magnifies distant stars and objects but distorts them into separate images for viewing. Each "side" of the star or object being used in gravitational lensing produces an image of the distant object that can then be combined to create a full, more defined image for study.
Einstein failed to see the huge improvements to telescope technology that this presented -- and is now being implemented in almost every aspect of astronomy. This technology has led to the discovery of distant galaxies that are seen at a time shortly after the 'Big Bang' allowing scientists and researchers the pleasure and opportunity to see the early universe.
One of the first confirmations of Einstein's gravitational lensing was discovered in 1919 through the use of a solar eclipse and deflection. The light of distant stars were deflected around the sun and seen during an eclipse as the sun was blocked out due to the Moon. The mass of the sun causes the deflection due to the bending of space-time which is discussed in the Theory of General Relativity.
In addition, since galaxies are most often used as gravitational lenses, due to their extremely large masses, elliptical mass distributions affect the magnification of the lens, or galaxy. Mass distributions with elliptical densities provide far better realistic lenses, but need a higher degree of numeric calculations. Elliptical masses and deflection are used in tandem to determine the best possible lensing model available to a particular application or area of the universe intended to be viewed.
The use of gravitational lensing will occasionally show the image of a distant object as a ring or set of rings, called Einstein rings, as the mass and gravity of the "lens" bends the light of the object completely around itself when being viewed through a telescope. During this process, another affect of the gravitational lensing is red shift, or gravitational red shift. This makes the wavelengths of light seem longer than they truly are. Gravity and the mass of the lens being used can distort the wavelengths of light according to a function of the mass and gravity of the lens when compared and measured against the known distance to the star being viewed through the gravitational lens. Red shift is also a property of the doppler effect and is used in every aspect of telescope technology in astronomy.
The science and technology behind gravitational lensing is just starting in the field of astronomy and astrophysics but is proving to be an invaluable tool for research and science. The gravitational lensing can be applied to virtually any telescope as it relies upon the universe to see the universe.