With the help of a suitable diagram, explain the meaning of terms: (a) centres of curvature (b) optical centre (c) focus
(a) Centres of curvature: A lens is generally made up of two curved surfaces. These curved surfaces are parts of spheres. Centres of these spheres are called the centre of curvature. Here, C1 and C2 are the centres of curvature.
(b) Optical centre: The centre inside the lens is called optical centre. A ray of light passing through the optical centre emerges without deviation.
(c) Focus: When rays of light parallel to the principal axis, they converge at a point in case of convex lens. In case of a concave lens, they rays appear to diverge from a point. This point is called the focus of the lens.