An aligned source behind an axisymmetric gravitational lens produces a circular image at an Einstein radius. Its angular radius satisfies . Exact alignment gives a ring; displaced sources generally give arcs or distinct images rather than a complete ring.
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An Einstein ring is a fascinating astronomical phenomenon that occurs due to gravitational lensing, a prediction of Albert Einstein's General Theory of Relativity. This effect takes place when a massive object, like a galaxy or a cluster of galaxies, lies directly between an observer (such as Earth) and a more distant source of light (like a galaxy or a quasar). When the gravitational field of the foreground object distorts the spacetime around it, it bends the light from the background object.