Gravitanional lenses
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Summary. Gravitational lensing is one of the hot topics of interest in the so called BIG science. The phenomena was predicted by Einsteinís theory of curved spacetime. Deep space exploration started in 1920ís as Edwin Hubble discovered what galaxies really are. The first experimental attempt to prove Einsteinís prediction  was undertaken by Arthur Eddington in 1919. He measured deflection of light rays while observing stars during a solar eclipse.                                                                                                                   Radio-astronomy made possible the discovery of quasars in the 1960ís.  The first twin quasar was discovered  by Walsh and collaborators in 1979. Spectra of twin quasars were almost the same. This fact remained unexplained until, later that year, Alan Stockton at Mauna Kea Observatory found a galaxy between the pair of quasars.  

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The system is an example of gravitaional lensing: the observer is actually seeing two images of the same quasar. In gravitaional lensing there is always an odd number of images. The galaxy  that causes the effect is hardly ever aligned with quasars, and usually closer to one of the quasars.The problem becomes more complex when we take into consideration that the galaxy between the quasars is just a member of a cluster of galaxies.

Links.                                                                                                                                   http://www.physics.usyd.edu.au/refta/anrep93/node24.html                                                   http://dept.physics.upenn.edu/-myers/ASTE001/L37.html                                                      http://csep10.phys/utk.edu/guidry/violence/gravlens.html                                                        http://zebu.uoregon.edu/-soper/Mass/gravlens.html

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