Gravitational Lensing

An Einstein ring forms only when observer, lens, and source are perfectly collinear. Hubble captured the first optical Einstein ring in 1998; JWST now resolves them by the hundreds and uses them to weigh dark-matter halos around foreground galaxies with precision no other method can match. Gravitational lensing reveals mass regardless of whether it emits light — making lensing surveys among the strongest evidence that roughly 27% of the universe's energy density is cold dark matter.

Any concentration of mass curves the space around it, and light passing nearby follows that curve — so a foreground galaxy bends the light of a more distant object behind it. Instead of a single image you see two, an arc, or a complete ring, and each one is brightened. The heavier the lens and the closer the alignment, the larger the Einstein radius θ_E this tool computes. Astronomers exploit this as a free zoom lens to study galaxies too faint to see directly.

A gravitational lens is a spectacularly bad optical instrument. A glass converging lens bends rays farther from its center the most, funnelling parallel light to a single focus; gravity does the opposite — the deflection α = 4GM/(c²b) grows as light passes closer to the mass. So parallel rays never meet at one point but pile up along a line stretching to infinity. That very 'flaw' is why lensed sources smear into rings and arcs instead of tidy dots.