Gravitational lensing - the phenomenon in which diverging light rays from a source are refocused due to the warping of space by a massive object - has in recent years evolved from a beautiful test of General Relativity to a precision tool with which to measure quantities of astrophysical and cosmological importance. A particular example of this is the use of weak lensing of far away background galaxies by an intervening galactic cluster to allow the reconstruction of the two-dimensional mass profile of that cluster, and a measure of its total mass. The rough idea is represented in the figure below, which I took from NASA's web page on lensing.
(A good place to find out more about the subject is on Joanne Cohn's lensing page, and links therein.) When this technique is applied to actual astrophysical images, one is often led to remarkable results, as, for example, in the ...
