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 discussion of dark matter in the bullet cluster, which seems to force us to take particulate dark matter even more seriously. If one wants to get a more hands-on feeling or how lensing works - strong versus weak; aligned source and lens versus nonaligned; etc. - then there do exist a few options. One is to visit Pete Kernan's Lens an Astrophysicist page, which allows one to submit the url of an image and then adjust the mass and position of a black hole, returning the (strongly) lensed image. I often use screen captures from this site in my colloquia, and here's one of a recognizable character.
Last week, I spent a delightful few days visiting U.Penn, and interacting with their cosmology and particle physics groups. My host was Bhuvnesh Jain, who is an expert on lensing, and who told me about a second way = an innovative teaching tool that he and collaborators have developed to illustrate lensing. Bhuv and friends have set up a precision optics system, and had a couple of special optical lenses machined so that their effect is that of a black hole or a galactic halo. Their site presents many of these applications - for example, here's a four image (ignoring the one at the center, that one typically doesn't see) one, and an actual astrophysical version or comparison
It's a wonderful way to allow students to play with the lenses and explore the circumstances under which different types of distortions occur. If you feel like building one yourself, they even tell you how to do it.
