Some definitions:

1. The Focal Point is the location at which rays parallel to the optical axis of an ideal lens/curved surfaces converge to a point. (Primary focal point F is an axial point having the property that any ray coming from it or proceeding towards it, travels parallel to the axis after refraction;secondary focal point F' is an axial point having the property that any incident ray traveling parallel to the axis, after refraction, proceed towards or appear to come from this point.) (Fig.8.1.3)(Fig.8.1.4)
2. The Focal Length is the distance between the focal point and the middle of the lens (vertex of curved surfaces). It is represented by f. The distance between the primary focal point and the vertex is called primary focal length (f) and the distance between the secondary focal point and vertex is called the secondary focal length (f ') . For a thin lens, the two focal lengths are equal.
3. The distance from center of the lens (curved surfaces) to the object (O) is referred as u (Fig.8.1.5)
4. The distance from the center of the lens (curved surfaces) to the image (I) is referred as v (Fig.8.1.5)
   

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