1.NounOf a black hole, a spherical limit around a black hole at such distance that any photon meeting it tangentially is constrained to travel in a circular orbit; the region between this limit and the event horizon.
"The radius of the photon sphere is also the lower bound for any stable orbit.; For a Schwarzschild (non-spinning) black hole, the radius of the photon sphere is 1.5 times the Schwarzschild radius."