Primary and Secondary Focal Points

Because a thin lens may separate two different media on its two sides, it has two distinct focal points. The primary focus $F_1$ is the object position that produces parallel emergent rays after passing through the lens; the distance $PF_1$ is the primary focal length $f_1$. For a converging lens this is a real object position; for a diverging lens it is a virtual object position. The secondary focus $F_2$ is the point where parallel incident rays converge (or appear to diverge from) after refraction through the lens; the distance $PF_2$ is the secondary focal length $f_2$ -- a real image point for a converging lens, virtual for a diverging lens. If the media on both sides of the lens are identical, $f_1=f_2$; this chapter …