Jean-Pierre Luminet, Black Holes, Cambridge University Press, 1987

THE ILLUMINATION OF FOUR TYPES OF BODIES

Every natural body absorbs and reflects electromagnetic radiation in some way. The experiment illustrated in Figure 30 uses a parallel beam of light to illuminate several 'ideal' bodies, and to observe the reflected light in a direction perpendicular to the incident direction. The type of image received depends on the nature of the body and how it reacts to electromagnetic waves.