The photon ring forms at 1.5 Schwarzschild radii, where gravity bends light into a closed orbit. Rays that barely escape carry a compressed image of the entire disc.
Schwarzschild black hole shader adapted from E. BrunetonThe photon ring forms at 1.5 Schwarzschild radii, where gravity bends light into a closed orbit. Rays that barely escape carry a compressed image of the entire disc.
Schwarzschild black hole shader adapted from E. Bruneton