The image M'Q' in Fig. 3D is a real image in the sense that if a flat screen is located
there, a sharply defined image of the object MQ will be formed on the screen. Not
all images, however, can be formed on a screen, as is illustrated in Fig. 3E. Light
rays from an object point Q are shown refracted by a concave spherical surface
separating the two media of index n = 1.0 and n' = 1.50, respectively. The focal
lengths have the ratio I: 1.50.