Caustica is a free, interactive gravitational lensing simulator. Move a lens mass to warp background sources into Einstein rings, giant arcs, and caustics, and watch the images change in real time. It is a browser-based tool for exploring how gravity bends light, one of the cleanest ways to map dark matter.
This interactive tool runs in your browser. If it has not loaded, make sure JavaScript and WebGL2 (hardware acceleration) are enabled in your browser settings.