On May 20, 2009, Prof Patricia Burchat, the Chair of the Department of Physics at Stanford University, presented the last lecture in the 10th season of the Silicon Valley Astronomy Lectures. She gave what I have found to be the best demonstration of gravitational lensing.

Of course, there are many examples of gravitational lensing in Hubble photos of galaxies. These are sometimes called Einstein rings and are the direct result of the bending of light by gravitational fields.

By scanning the lensed images, it’s possible to back out what the mass distribution might be that causes the lensing effects. This is the chief way of mapping the distribution of dark matter- matter invisible to electromagnetic fields, but interacts only gravitationally.

Prof Burchat takes a wine glass and breaks off the stem, and polishes the end flat. The density of the glass is roughly in the shape of the gravitational field of a point mass. Its optical qualities match the gravitational field of a lensing galaxy.

Now, place this wine stem on top of a point on a piece of paper and the point image of the source is refracted into a ring, if centered, and an arc, if the object is offset from the optical axis of the wine stem.

This illustrates the exact shapes of imaged background galaxies viewed in gravitational lensing events happening on cosmic scales.

How cool is that!