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.

When light passes through the cluster from an object lying behind it, the light is bent and focused to produce an image or images of the source.

The image may be magnified, distorted, or multiplied by the lens, depending upon the position of the source with respect to the lensing mass.

The characteristics of the gravitationally lensed image depend upon the alignment of the observer, the lens and the background object.

If the alignment is perfect, the resulting image is an Einstein Ring. 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.