
The gravitational deflection of light near a black hole used for probing spacetime and matter.Black hole light bending refers to the gravitational deflection of light rays as they pass near a black hole. This effect is a direct consequence of general relativity, where massive objects curve spacetime, and light follows the curved paths. Near a black hole, the bending can be so extreme that light can orbit the black hole multiple times before escaping, creating what are known as photon rings. This phenomenon is crucial for testing general relativity and for imaging black holes, as done by the Event Horizon Telescope.
The bending of light by gravity was first calculated by Einstein in 1915 and confirmed by Eddington's 1919 solar eclipse expedition. The specific case of light bending around black holes was studied in the 1930s and later refined with the concept of photon spheres by Bardeen, Press, and Teukolsky in 1972.
The bending angle depends on how close the light passes to the black hole and on the black hole's mass and spin. For a non-rotating black hole, the critical impact parameter for photon capture is at 3 times the Schwarzschild radius, where light can orbit in an unstable circle. Light passing closer than this is captured, while light passing slightly farther is deflected by large angles. The shadow of a black hole — the dark region seen in images — is the projection of this photon capture region, and its size and shape encode the black hole's properties.
It is used to measure black hole masses and spins from their shadows, to test general relativity in strong-field regimes, and to study the distribution of matter around black holes via gravitational lensing effects.
Light can orbit a black hole multiple times before escaping, and these higher-order photon rings are predicted to appear as a series of increasingly faint concentric rings in high-resolution images.




