With reference to spherical lenses, state the meaning of the following terms or define the following terms and draw the diagrams to illustrate the same : (1) centre of curvature (C) (2) radii of curvature `(R_(1),R_(2))` (3) principal axis (4) optical centre (O) (5) principal focus (F) (6) focal length (f)

Explanation

Centre of curvature (C) : The centres of the spheres whose parts form the surfaces of a lens are called the centres of curvatures of the lens . A lens has two centres of curvatures `C_(1)` and `C_(2)` for its two spherical surfaces.
(2) Radii of curvature `(R_(1), R_(2))` : The radius of the spheres whose parts form surfaces of a lens are called the radii of curvatures of the lens.
(3) Principal axis : The imaginary striaght line passing through the two centres of curvatures of a lens is called the principal axis of the lens.
(4) Optical centre (O) : The point inside a lens on the principal axis, through which light rays pass without changing their path is called the optical centre (O) of the lens.
(5) Principal focus (F): When ligth rays parallel to the principal axis are incident on a convex lens they converage at a point on the principal axis. This point is called the principal focus (F) of the convex lens. Light rays travelling parallel to the principal axis of a concave lens diverage after refraction in such a way that they appear to be coming out of a point on the principal axis. This point is called the principal focus of the concave lens. A lens has two principal foci `F_1 and F_(2)` .
(6) Focal length (f) : The distance between the optical centre adn the principal focus of a lens is called the focal length f of the lens .

The cross sections of convex and concave lenses are shown in parts (a) and (b) Fig. The surfaces marked as 1 is part of sphere `S_(1)` while the surface marked as 2 is part of sphere `S_(2)`