(a) (i) Optical center. It is a point within the lens that lies on the principal axis through which a ray of light passes undeflected.
(ii) Centre of curvature. The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part. A lens has two centres of curvature because it has two surfaces.
(iii) Principal axis. It is a line through the centres of curvature of the lens.
(iv) Aperture. The diameter of the circular boundary of the lens is called the aperture of the lens.
(v) Principal focus. A beam of light parallel to the principal axis wither converges to a point or appears to diverge from a point on the principal axis after refractin through the lens, is called the principal focus. All lenses have two principal focuses.
(vi) Focal length. The distance between the optical centre and the principal focus of the lens is called its focal length.
(b) A converging lens is a convex lens.
`f= +12 cm ` `u = ?` `v= +48 cm ( + ve` as it is formed on other side of the object )
According to lens formula
`(1)/(f) = (1)/(v) - (1)/(u) implies (1)/(12)=(1)/(48) - (1)/(u)`
`(1)/(u) = (1)/(48)-(1)/(12)=(1-4)/(48)=(-3)/(48)= (1)/(16)" " :. u = -16`
`:. ` The object should be placed 16 cm away from the lens.
