"<a href=""https://www.youtube.com/watch?v=-jA2ABHBc6Y&amp;t=1h08m14s"">1:08:14</a> Wait. What? How is this used for magnification? What's a gravitational lens?<br><br>Edit: Ah", it bends more rays towards you.,(reply>), Lots about gravitational lensing on Wikipedia. The basic theory is simple trigonometry using a result from Einstein's General Theory of Relativity. The bendng of light according to Einstein is twice that expected from Newton's gravity, and its observation by Eddington at the end of WWI  during an eclipse of the Sun showed that General Relativity is a better description of gravity than Newton's theory. It has not failed any tests so far.