(i) Optical center: 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 emerges without any deviation.
(ii) Centre 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.
(iii) Aperture: This is the length or breadth of the lens through which refraction takes place.
(iv) 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.
(v) Focal length: The distance of the point from the centre of lens or mirror at which a parallel ray of beam converge (or diverge) is called focal-length and the point is called focus.
