What is a black hole shadow, and how is it formed?

• A. The shadow is the dark region against a bright background caused by light bending around the black hole and photon capture by the horizon.
• B. The shadow is a bright ring around the black hole formed by emitting matter.
• C. The shadow is the region where light can escape most easily.
• D. The shadow is created by the accretion disk's inner edge.

Answer: (D)

The key idea is that a black hole casts a silhouette because gravity is so strong that many light paths are bent into the hole rather than reaching us. Photons that come too close to the black hole are swallowed by the event horizon, so directions from which those photons would have traveled appear dark to an observer. The boundary of this dark region is shaped by how light is bent around the hole and by the fact that some photons can skim very near the photon sphere and then escape after being deflected, forming a bright ring around the shadow. The glow you see around it mainly comes from the surrounding glowing matter (the accretion flow) being warped and lensed, while the inner edge of that disk can influence where the light is brightest, it does not create the shadow by itself. In short, the shadow is the dark patch where light cannot escape to us because photons are captured by the horizon, with a bright ring produced by strongly bent light from the surrounding gas.