Gravitational Lensing - software tool for astrophysics labs
Einstein's General Theory of Relativity demonstrates that a large mass can deform spacetime and bend the path of light. So, a very massive object, such as a cluster of galaxies can act as a gravitational lens. With this program, one can draw images to see the effect of gravitational lensing. You can easily test the effects of several parameters by changing them and redrawing the image.