Jeff Hecht in New Scientist:
Astronomers have spotted the most distant Einstein ring ever seen. It offers valuable insight both on the galaxy which acts as a gravitational lens and on the more distant galaxy whose light it magnifies.
Gravitational lensing occurs because massive objects – ranging from stars to entire galaxies – distort the fabric of space-time, bending the path of light passing near them. A strong gravitational lens can form multiple images of the distant object, or spread its light into an arc.
If the lens lies directly between Earth and the distant background object, the light can be distorted into a ring, named after Einstein because the effect is described by general relativity.