Schematic of the reflection process in the black hole Cygnus X-1. The light emitted from the jet is shown in blue, which we say originates from a point source some distance above the black hole. The X-rays following path a) are observed directly by our telescopes. The light that follows paths (labelled b & c), reflect off the disc and follow the green trajectories to reach us. Light travelling along a) follows the shortest path to reach us, whereas the light travelling along path c) takes the longest path, and therefore takes the longest time to reach us.
Patrick O’Neill. Background image: NASA/JPL-Caltech