Centra of curvature: It is defined as the centre of the spheres of which the lens is originally a part of. Because the spherical lens consists of two spherical surfaces, the lens has two centres of curvature.
Optical centra: The centre point of a lens is known as its optical centre. It always lies inside the lens. A light beam passing through the optical centre immerges without any deviation.
Principal axis: The line joining the centre of curvature and the optical centre is called the principal axis.
Principal focus: A light ray parallel to the principal axis of the lens meets at a point on the principal axis. This point is called the principal focus.
Focal length of concave lens, f = - 20 cm
Image distance, v= - 15 cm
(Negative sign is taken because of sign convention.)
Using the lens formula, we get: 1f = 1v.