The page you are viewing is taken from an exhibit called MIRA: Exploring the Universe from the Central Coast.
The exhibit ran from 1 July through 24 September 2000 at the Pacific Grove Museum of Natural History.

How They Work
According to relativity theory, a beam of light passing a massive object will be bent toward the object, just as if it were a stream of particles. Thus if we observe a distant star near the edge of the sun, it appears to be a little further from the sun than it really is.

The gravitational lenses studied at MIRA work this way too, but light source is usually a distant quasar and the mass is in a closer foreground galaxy. If the brightness of the quasar changes with time, as it often does, we can calculate the difference in distance along the different light paths from the difference in arrival time. If one image changes without the other, then something is happening along the path, probably in the foreground galaxy.

The Einstein Cross
The Einstein Cross is a good example of a gravitational lens.

Light from the quasar passes close to the mass concentration in the foreground galaxy, where it is slightly deflected, so that the observer sees two images slightly separated from one another. Changes in brightness of the quasar reach the observer at different times, where the time delay is equal to the difference in path length divided by the speed of light.

The Einstein Cross imaged by the Hubble Telescope. The colors are used to show differences in brightness, not the real colors of the objects. The four bright spots are separate images of the same background quasar.)

The Einstein Cross image processed for maximum sharpness. This is done so as to separate the lensed
quasar from the foreground galaxy.

How We Can Use Them?
If the quasar is varying, we can measure the distance to it using the time delay. We can often find the ratio of lengths A and B from the geometry, and the difference in lengths from the time delay. If we know the difference between two numbers and their ratio, we can calculate their values, or the distance to the quasar.

These are the most distant objects we can see, and this is the most direct way of measuring their distances, which are needed for finding the size and age of the universe.

From the geometry of the images and the time delay, we can often say something about the mass distribution in the galaxy. If one image varies independently of the others, we are probably observing a star in the foreground galaxy passing near the light path. In this way we can estimate the star density in parts of the galaxy. We might even detect a planet. All of the images should have the same color. If they do not, it is because there is different light absorption due to dust along the different light paths through the foreground galaxy.

If we see the same light variation in all images, we can be sure that it is in the quasar itself, and we can try to understand the reason for the variation.

Change with time in the brightness of the four images in the Einstein Cross.

As the source brightness changes, the images reach peak brightness at different times, showing the difference in length along the four rays.

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