A lens has two surfaces. Those two surfaces of the lens are the part of two different spheres. The centre of the sphere, whose part is the lens surface, is called the centre of the curvature of that surface of the lens. Every lens has two surfaces that mean each lens has two centre of curvature.
It is a straight line which connects the two centre of curvature of a particular lens. In fig. C1C2 is the principal axis.
Optical point is a point on the principal axis of the lens such that a ray of light passing through this point emerges parallel to its direction of incidence.
Optical centre of a thin lens is the point on the principal axis of the lens through which a ray of light passes undeviated.
In Fig. PQ is the incidence ray; it passes through QR in the lens. After refraction the ray pass through RS. Here PQ and RS are parallel. The ray intersects the principal axis at the point O. Thus here O is the optical point of the lens.
A light ray can pass through a lens from either direction; therefore, a lens has two principal foci which are situated at equal distance from the optical centre, one on either side on the lens. These are known as the first focal point (or first focal) F1 and the second focal point (or second focus) F2.
The distance from the optical centre O of the lens to its first focal point F1 is called the first focal length f1 of the lens.OF1 = f1.
The distance from the optical centre O of the lens to its first focal point F2 is called the second focal length f2 of the lens. OF2 = f2.