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.

(i) Centre of curvature (C): The centre of the sphere, of which the spherical lens forms a part is called as the centre of curvature. It is represented by the alphabet C. Since a lens has two surfaces, so it has two centres of curvature (C₁, C₂).

(ii) Radius of curvature (R): The radius of the sphere of which the lens forms a part is known as the radius of curvature. Is is symbolised as R.

(iii) Principal axis (C₁C₂): The line passing through the centre of curvature of the lens is the principal axis. 


Fig. Characteristics of (a) convex lens (b) concave lens

(iv) Principal focus (F): A narrow beam of light which is incident 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): A point situated within the lens through which a ray of light passes undeviated.

(vi) Focal length (f): Distance between the principal focus and the optical centre of the lens. 

(vii) Aperture: The diameter of the circular boundary of the lens is called the aperture of the lens.