 # Explain the following terms related to spherical lenses : 1. Centres of curvature

Explain the following terms related to spherical lenses :

1. Centres of curvature
2. Principal axis
3. Optical centre
4. Principal focus

At what distance from a concave lens of focal length 20 cm should a 6 cm tall object be placed so that it forms an image at 15 cm from the lens? Also determine the size of the image formed.

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.