For more information about thin spherical lens, define :
(a) First principal focus `(F_1)`
(b) Second principal focus `(F_2)`
(c) Optical centre (C) (d) Focal length (f)

Explanation

If a point object is placed on the principal axis of a convex lens such that the rays refracted are parallel to the axis figure (a), then the position of the point object is called the first principal focus `(F_1)` of the lens,
If the object is situated at infinite (figures (b) and (c)), refracted rays meet (or appear to meet) for convex (or concave) lens to a point `(F_2)`, then the position of this point is known as second principal focus `(F_2)`.
The geometrical centre of the medium of the lens is called its optical centre (C).
Distance of principal focus from the optical centre (C) is known as focal length (f) of the lens.
As per the sign convention, f is positive for convex lens and negative for concave lens.