Focal points and focal length কী?

Rays arriving parallel to the axis of a converging lens all cross the axis at one point behind it, the image focal point $F'$; symmetrically, rays leaving the object focal point $F$ (in front, with $OF = OF'$) exit parallel to the axis. The focal length $f' = \overline{OF'}$ is positive for a converging lens; through a diverging lens, parallel rays exit as if from a point $F'$ in front of the lens: $f' < 0$.