Drag the alignment slider to see how closely a background quasar or galaxy lines up with a foreground lens changes the number and shape of the images you see - from a stretched arc down to a single point.
Drag to orbit and scroll or pinch to zoom on the scene above. For the special symmetric case where alignment is exact and the images merge into one full circle, see the Einstein Ring 3D Explorer instead.
Strong Gravitational Lensing 3D Explorer
Pick the Einstein Cross case or the Abell 2218 case and drag the alignment slider in this browser explorer to see why a foreground galaxy or group of galaxies splits a single background quasar or galaxy into multiple images or long arcs, using the real figures from both cases.
The Einstein Cross (formally Q2237+0305) is a quasar at redshift 1.695 whose light is bent by a foreground galaxy at redshift 0.0394 into 4 separate point images arranged around the lens - discovered by Huchra et al. in 1985. Abell 2218 is a massive group of galaxies at redshift 0.171 whose combined gravity stretches dozens of background galaxies, some out to redshift 2.5, into long thin arcs; it produced one of the most famous strong-lensing images captured by the Hubble Space Telescope. Both cases follow the same rule: the closer the background source lines up with the lens along the line of sight, the more images appear and the more they stretch into arcs; move the source away from that alignment and the extra images fade, leaving one dominant, only mildly distorted image.
- Case buttons switch between the Einstein Cross (4-point-image) illustration and the Abell 2218 (multi-arc) illustration, each with that case's real published figures
- Alignment slider (0-100) drives how closely the background source lines up with the lens - low values show more images or longer arcs, high values collapse toward a single image
- A faint ring marks the lens's critical curve, the boundary near which the strongest arcs and extra images form
- Drag to orbit, scroll or pinch to zoom
- Runs fully in the browser with the vendored three.js engine - no account, no upload
| Case | Redshift (lens / source) | Image count | Discovery |
|---|---|---|---|
| Einstein Cross (Q2237+0305) | 0.0394 / 1.695 | 4 point images | Huchra et al., 1985 |
| Abell 2218 galaxy group | 0.171 / up to about 2.5 | Dozens of arcs | Hubble Space Telescope imaging |
For the special symmetric case where the alignment is exact and the images merge into a single unbroken circle, see the Einstein Ring 3D Explorer instead - this page covers the general, more common case where alignment is imperfect and produces separate images or arcs rather than one full ring.
Everything renders on your device with WebGL. The 3D engine loads once (about 0.7 MB) and is cached.
This is an educational approximation, not a numerically-solved lens equation for either object's exact measured mass distribution - the image count, spacing, and arc stretch are stylized to show how alignment changes the lensing pattern, not a ray-traced reconstruction of Q2237+0305 or Abell 2218.
Frequently Asked Questions
What is strong gravitational lensing?
Strong gravitational lensing happens when a foreground galaxy or group of galaxies is massive enough to bend light from a background object into multiple separate images or long stretched arcs, rather than the barely-noticeable weak lensing that most galaxies produce.
What is the Einstein Cross?
The Einstein Cross (Q2237+0305) is a quasar at redshift 1.695 whose light a foreground galaxy at redshift 0.0394 bends into 4 separate point images arranged around the lens galaxy - discovered by Huchra et al. in 1985.
What is the Abell 2218 arc field?
Abell 2218 is a massive group of galaxies at redshift 0.171. Its combined gravity stretches dozens of background galaxies, some out to redshift about 2.5, into long thin arcs - one of the most famous strong-lensing images captured by the Hubble Space Telescope.
Why do some cases show 4 images and others show arcs?
A single point-like lens galaxy tends to produce a small number of point images, like the Einstein Cross's 4. An extended lens made of many galaxies, like Abell 2218, has a more complex combined mass distribution that stretches background galaxies into long arcs instead of simple points.
What does the alignment slider control?
It controls how closely the background source lines up with the lens along the line of sight. Near-perfect alignment produces more images or longer arcs; moving away from alignment fades the extra images until only one dominant, mildly distorted image remains.
How is this different from the Einstein Ring explorer?
The Einstein Ring page shows the special symmetric case where alignment is exact and the images merge into one unbroken circle. This page shows the more general, more common case where alignment is imperfect, producing separate images or arcs instead of one full ring.
Is the on-screen image count or arc shape a physics simulation?
No. This is an educational approximation - the image count, spacing, and arc stretch are stylized to show how alignment changes the lensing pattern, not a numerically-solved lens equation for either object's exact measured mass distribution.
Where do these figures come from?
Huchra et al. 1985 for the Einstein Cross discovery and its redshifts, and NASA/ESA Hubble Space Telescope imaging plus published redshift data for the Abell 2218 arc field.