gravitational lens

In Einstein’s general relativity, gravity necessarily acts not only on material bodies, but also on light – light passing a massive body is deflected. This deflection can be so strong that light of one and the same cosmic object reaches an observer along multiple paths – corresponding to the observer seeing multiple images of that object in the sky. Masses that, in this sense, act like very special optical lenses are called gravitational lenses.

More information can be found in the spotlight text A brief history of gravitational lenses.