<P> The photon sphere is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole . For non-rotating black holes , the photon sphere has a radius 1.5 times the Schwarzschild radius . Their orbits would be dynamically unstable , hence any small perturbation , such as a particle of infalling matter , would cause an instability that would grow over time , either setting the photon on an outward trajectory causing it to escape the black hole , or on an inward spiral where it would eventually cross the event horizon . </P>
compared to the event horizon the photon sphere of a black hole is
a radius 1.5 times the Schwarzschild radius