A photon sphere is a theoretical area in the vicinity of a black hole or another massive object where gravity is strong enough that photons (light particles) can orbit the object in unstable circular paths. This occurs at a specific radius, known as the photon sphere radius, which is typically located at 1.5 times the Schwarzschild radius of a non-rotating black hole.
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A photon sphere is a spherical set of circular null geodesics. In Schwarzschild spacetime it occurs at and is unstable.