The spherical region around a nonrotating black hole where light can orbit is called the photon sphere.
In the simplest black-hole model, the photon sphere lies at a radius of three-halves the Schwarzschild radius, or three times the black hole’s mass in geometrized units. At that location, gravity bends light into circular paths. These paths are unstable: a tiny disturbance can send a photon spiraling inward or escaping outward.
The photon sphere is different from the event horizon. The event horizon is the one-way boundary from which light cannot escape, while the photon sphere lies outside it and permits circular light paths only under ideal conditions. A nonrotating black hole’s photon sphere is located at 1.5 times its Schwarzschild radius.
The term is also connected to black-hole imaging. Light from material behind a black hole can be strongly bent around the photon region, helping create a bright ring-like appearance around a dark central shadow. For spinning black holes, frame dragging changes the photon paths, so the geometry is no longer a perfectly simple sphere.