Returning to diagrams (a) and (b) for reference, we now state that the primary
focal point F is an axial point having the property that any ray coming from it or proceeding
toward it travels parallel to the axis after refraction. Referring to diagrams
(c) and (d), we make the similar statement that the secondary focal point F' is an
axial point having the property that any incident ray traveling parallel to the axis will,
after refraction, proceed toward, or appear to come from, F'.