We know what lenses are. We have them in glasses, cameras, telescopes, and our eyes. These optical lenses are made of transparent material, and they focus light. However, astronomers now also use gravitational lenses to detect distant galaxies, dark matter and extrasolar planets. What is a gravitational lens and how does it work?

Matter warps space and bends light
The story starts with Albert Einstein. His General Theory of Relativity says that matter warps the space around it. Therefore you could observe that light bends as it passes through the space around a massive body such as a star. But in practice how would that work?

The theory predicts that light from stars behind the Sun bends a specific amount as it comes towards us. Unfortunately, the Sun's glare makes this impossible to observe. Except during a total solar eclipse.

In 1919 British astronomer Arthur Eddington (1882-1944) and his collaborators tested the theory during an eclipse. They photographed the stars in the night sky six months before the eclipse. These were the same stars they would see during the darkness of the eclipse, so they could then determine whether the star positions had appeared to change – and if so, by how much. The results confirmed Einstein's predictions, and both Einstein and Eddington became international celebrities.

Gravitational lensing
Einstein realized that another theoretically observable effect of massive objects bending light was a gravitational lens. The more massive the object, the stronger its gravitational field, and therefore the more the light rays would be bent.

At its simplest, it works like this. Imagine a faraway object such as the distant galaxy in this diagram. Between us and the galaxy is a massive cluster of galaxies whose gravity acts like a lens to bend the light coming around it. The white lines show light which we won't see from Earth. The orange lines show the paths of light which are bent by the lens, and which we will see. There are two separate paths, so we would see two distinct images of the distant galaxy.

The first known gravitational lens
Scientists other than Einstein also wrote about the theory of gravitational lenses, but everyone agreed that we wouldn't be able to see them. And with the equipment available in the first part of the twentieth century, that would have been true.

The first gravitational lens wasn't discovered until 1979. It was the quasar Q0957+561. Quasars are massive objects that give out vast amounts of energy and appear somewhat starlike through a telescope.

In the case of Q0957+561, what they found was a pair very similar quasars. After some study, it was obvious that they were not twins, but two images of the same object. The galaxy YKOW G1, in the line of sight, had bent the quasar's light along two different paths, producing the double image visible right in the center of this picture. The Twin Quasar, as it was nicknamed, is just under nine billion light years from Earth, and the lensing galaxy is about four billion light years. We couldn't see the quasar without the lensing effect.

Different types of images
A gravitational lens isn't as simple as an optical lens. It doesn't have a single point of focus. In addition, the lensing object may be something like a group of galaxies which is geometrically rather messy. The image will also be related to the way the object, the lens and the observer are lined up. If everything is perfectly aligned, the distant object would look like a ring around the lensing object. This is called an Einstein ring. No one has yet seen a perfect one – but LRG 3-757 comes close. The gravity of a luminous red galaxy (LRG) has distorted the light of the more distant blue galaxy.

It's more usual to get multiple distorted images, either as arcs or interestingly, as an Einstein cross. Here's an Einstein cross formed by galaxy G2237+0305 lensing a quasar eight billion light years away.

Sometimes the effect is just to make the background object brighter, but more common are the rings, arcs and multiple images. The header image is an example of multiple images. There is a string of five separate images that looks like a serpent. That's the result of cluster Abell 370 warping the the light from a galaxy.

Using gravitational lensing
The discovery of gravitational lenses has supported Einstein's theory, but astronomers don't continue to study them for that reason or because they make nice pictures. They have become important observational tools with a number of uses. Here are some examples.

1. Gravitational lenses allow us to see deeper into the Universe than is otherwise possible. The most distant – and therefore youngest – galaxies we know of were discovered in this way. Since we can only see objects when their light reaches us, as we look farther away we're also looking back in time.

2. Dark matter is matter which doesn't interact with light or any kind of electromagnetic radiation. However it can be detected by its gravitational effects. Its contribution to lensing effects help astronomers to map dark matter.

3. One type of observation is known as microlensing and it's used to detected extrasolar planets. The upside is that the method can find low-mass planets around distant stars. The downside is that it occurs as an event resulting from a specific alignment of objects. This means that it's difficult – or impossible – to follow up later on.

NOTE: The images in this article are from the Hubble Space Telescope.