Black Hole Gravitational Lensing Explained
What is gravitational lensing around a black hole?
In general relativity, gravity is described as curvature of spacetime. Light follows “straightest possible” paths in that curved geometry (technically: null geodesics). Near a black hole, spacetime curvature is strong enough that those light paths bend significantly.
The key point: the black hole is not acting like a glass lens. There’s no refraction. The bending happens because the geometry that light moves through is curved.
What are you actually seeing in a “black hole image”?
A rendered black hole image usually shows a bright region (light arriving from background stars, an accretion disk, or a textured sky map) that has been distorted by lensing. The central dark region is often called the “shadow” in astrophotography contexts.
In simplified educational renderers (including many ray-traced demos), the “shadow” is the set of directions on the observer’s sky that correspond to light rays which do not reach the observer because they fall into the black hole.
What is an Einstein ring?
An Einstein ring happens when a light source, the lensing mass, and the observer are aligned (or nearly aligned). The source’s light is bent into a ring-like appearance around the lens.
Around black holes, ring-like structures can appear for multiple reasons: background alignment, strong-field lensing, and light that orbits close to the hole before escaping. The exact appearance depends on the scene and the geometry.
What is the photon sphere?
The photon sphere is a region where gravity is strong enough that photons can orbit in (unstable) circular paths. “Unstable” matters: a tiny perturbation can send the photon either outward to infinity or inward into the black hole.
In visual terms, light rays that pass near this region can loop around the black hole, dramatically increasing the amount of bending.
Why do rings repeat?
If a ray loops around the black hole once and then escapes to the camera, it maps the background to one ring. If it loops twice, it maps the background again into a thinner, fainter ring, and so on.
In a physical scene, each additional loop typically corresponds to a much longer path and more extreme focusing, so later rings become hard to see. In simulations, visibility depends on the scene brightness and the rendering choices.
Try it in the simulator
- Black hole image simulation: upload an image and see how lensing distorts it.
- Chapter notes on gravitational lensing (more technical detail).
- Photon sphere and impact parameter.