(a) Explain the following terms related to spherical lens: (i) Optical centre

(a) Explain the following terms related to spherical lens:

  1. Optical centre
  2. Centres of curvature
  3. Principal axis
  4. Aperture
  5. Principal focus
  6. Focal length

(b) A converging lens has a focal length of 12 cm. Calculate at what distance should the object be placed from the lens so that it forms an image at 48 cm on the other side of the lens.


  1. Optical centra: 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 immerges without any deviation.

  2. Centra 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.

  3. Principal axis: The line joining the centre of curvature and the optical centre is called the principal axis.

  4. Aperture: This is the length or breadth of the lens through which refraction takes place.

  5. 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.

  6. Focal length: The distance of the point from the center of lens or mirror at which a parallel ray of beam converge (or diverge) is called focal length and the point is called focus.

(b) Focal length of the converging lens, f = -12 cm.
Image distance, v = - 48 cm (Negative sign is taken because of sign convention.) Using the lens formula, we get: