Some photons can orbit a black hole at a specific radius called the photon sphere, creating loops of light before either escaping or falling in. Observing this helps model black hole gravity and lensing effects.
It matters because photon spheres allow testing of light behavior in extreme gravity.
It also impacts imaging of black holes like M87.
Light can circle a black hole multiple times in a photon sphere before escaping.
[ESA, esa.int]