Power and Aberration of Lenses

The reciprocal of focal length of lens is called Power of lens i.e.


The defect (Blurring and distortion) in the image formed by lenses is due to two common reasons i.e. spherical aberration and chromatic aberration. These are known as Lens Aberrations or defects of lens.  

a

      The reciprocal of focal length of a lens is its power. The greater the focal length, the lesser will be the power of lens and smaller the focal length, the greater will be the power of lens.

In case of combination of two lenses,

       This shows that the power of combination of two lenses in contact is equal to the sum of their individual powers.

Unit

        The unit of power of lens is "Diopter" and it is defined as the reciprocal of focal length in meters.

i.e.

Defects of Lenses or Lens Aberrations

1-Spherical Aberration

       When rays of light parallel to the principal axis of a lens pass through zones near its edges, these are brought to focus nearer the lens than those rays which pass through region nearer to its center. Therefore, the focus is not a sharp point but is contained in a region called "focal region". This produces blurred images. This problem is known as spherical aberration as shown in figure:

  Fig   Spherical Aberration

Minimizing

• By using only central portion of the lens. This can be achieved by using a stop on the lens which makes effective aperture of the lens small.

• By making opposite surface of the lens of different curvatures.

• By combining a strongly convergent lens of producing little spherical aberration in the opposite direction.

2-Chromatic Aberration

      The fact that different wavelengths of light refracted by lens focus at different points give rise to chromatic aberration as shown in figure:

  Fig   Chromatic Aberration

       Actually a thick lens may be regarded as made up of two prisms placed one above the other and due to dispersion of the refracting medium of the lens different wavelength of light focus at different points. Hence image formed by lens consists of small linear spectrum. Chromatic aberration for a diverging lens is opposite to that for a converging lens.

Minimizing

• This defect can be reduced to a large extent by combining a converging lens of crown glass with a diverging lens of flint glass. These lenses are so chosen that the dispersion of one is equal and opposite to that the other. Lenses which are free from dispersion are called "Achromatic lenses".