How does gravitational lensing help us observe black holes?

• A. Lensing creates multiple images and time delays that obscure mass measurements around black holes.
• B. Light from background sources is bent around the black hole, producing distorted images or a shadow, enabling mass measurement and shadow studies.
• C. Lensing only occurs for cosmological distances and not relevant to black holes.
• D. Gravitational lensing eliminates the black hole shadow entirely.

Answer: (B)

Gravitational lensing around a black hole comes from the way its strong gravity curves spacetime and deflects light from background sources. This bending can produce distorted images, rings, or a dark shadow where light cannot escape, and the details of these effects depend on the black hole’s mass and the geometry of the system. By analyzing how light is bent and the size and shape of the shadow, astronomers can infer the black hole’s mass and study the shadow itself. Observations like the black hole image produced by the Event Horizon Telescope illustrate how the shadow and surrounding light patterns provide direct clues to the black hole’s properties. The idea that lensing obscures mass measurements isn’t correct, and lensing isn’t limited to cosmological distances—it also helps reveal and measure black holes nearby. It certainly doesn’t erase the shadow; it helps reveal and characterize it.