Gravitational microlensing

When a distant background star passes behind a black hole, the gravitational field of the black hole acts like a lens, distorting and magnifying the star. In microlensing the distorted arcs are too small to be seen on the sky, appearing as a single point, but we are able to detect the increase in brightness and the slight shift in the apparent position of the star.

Credit: Matthew Freeman (UC Berkeley/Moving Universe Lab), images courtesy NSO, NASA.