Define the following terms in connection with spherical lenses:
(i) Centre of curvature (ii) Radius of curvature (iii) Principal axis (iv) Principal focus (v) Optical centre (vi) Focal length (vii) Aperture.
Definitions in connection with spherical lenses
(i) Centre of curvature (C): The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part. Because a lens has two surfaces, so it has two centres of curvature (C1, C2).
(ii) Radius of curvature (R): The radius of curvature of the surface of a lens is the radius of the sphere of which the surface forms a part.
(iii) Principal axis (C1C2): It is the line passing through the centres of curvature of the lens.
Fig. Characteristics of (a) convex lens (b) concave lens
(iv) Principal focus (F): A narrow beam of light parallel to the principal axis either converges to a point or appears to diverge from a point on the principal axis after refraction through the lens. This point is called principal focus. All lenses have two principal focii.
Fig. Principal focus of (a) convex lens (b) concave lens
(v) Optical centre (O): It is a point situated within the lens through which a ray of light passes undeviated.
(vi) Focal length (f): It is the distance between the principal focus and the optical centre of the lens.
(vii) Aperture: It is the diameter of the circular boundary of the lens.
