Defects In Image Formation By Lenses And Their Correction

Images formed by lenses suffer from the following defects:
1. Spherical aberration
2. Chromatic aberration

1. Spherical aberration : It is the inability of a lens to bring all the rays to meet to one point after suffering refraction.

                              
Spherical aberration is due to the greater deviation of the rays from outer edges of the lens as compared to that of the rays incident on the central part.

Removal Of Spherical Aberration : Spherical aberration can be removed or minimized by the following methods :
1. By using a stop or making the diameter of the lens small.
2. By using Lenses of large Focal lengths.
3. By selecting suitable curvature to the two surfaces of the lens.
4. By Minimising the deviation.
5. By suitable combination of lenses.

2. Chromatic aberration  or colour defect :  The inability of the lens to focus all the colours at one point is called chromatic aberration.

Achromatism : The removal  of chromatic aberration or color defect is called achromatism. 
It is possible to remove chromatic aberration by having a combination of a suitable convex lens with a concave lens. Such a combination is known as achromatic combination.


Some Multiple Choice Questions for practice :

1. Which lens is used to correct the defect of long sightedness :
(a)   Concave                                        (b) convex
(c)   double concave                              (d) plano concave

2. Aberration in a lens is due to:
(a) nature of light                       
(b) defect in lens material
(c) deviation from simple lens equation   
(d) defect  in experiment arrangement

3. chromatic aberration in alens is due to the;
(a) inability of the lens to focus axial and marginal rays at the same point 
(b) inability of the lens to focus longer and shorter wavelengths of the light at the same point
(c) total internal reflection
(d) the fact that two faces of the lens may have different curvatures

4. Where is the image of the distant object formed by a short sighted eye ?
inability of the lens to focal axial and marginal rays at the same point 
(a)  At yellow spot                          (b) At blind spot
(c)  In front of retina                      (d) Behind the retina

5. Chromatic aberration of a lens results, because :
(a) the marginal and paraxial rays do not meet at the same point                
(b) of the different radii of curvature of the two surfaces
(c) of the prismatic action of the lens                                              
(d) of the defect in manufacturing

6. Ability of the eye to see objects at all distances is called
(a)  Binocular vision                       (b) myopia
(c)  hypermetropia                         (d) accomodation

7.Defect of hypermetropia can be remedied by using a

(a) convex lens                            (b) concave lens
(c) cylinderical lens                      (d) bi-focal length lens

8. Defect of color blindness can be cured by using a 
(a) contact lens                           (b) cylinderical lens
(c) bi-focal length lens                 (d) None of these

9. Spherical aberration can be reduced by
(a) using stops
(b) using lenses of large focal lengths
(c) using plano-convex lenses
(d) All the above methods

10. A plano-convex lens is used as objective lens in a telescope, then to minimise spherical aberration its

(a) convex surface should face the parallel rays  
(b) plane surface should face the parallel rays
(c) centre is covered with black paper
(d) edges are covered with black paper

11. chromatic observation in the formation of images by a lens arises because

(a) of non paraxial rays                                              
(b) the focal length varies with wavelength 
(c) of the defect in grinding                                               
(d)  radii of curvature of the two sides are not same

12. The chromatic aberration in a lens is because of 
(a) the large size of object                                                
(b) the object being very close to object
(c) manufacturing defect               
(d) dispersion of light in the material of lens

13.In order to reduce the spherical aberration in optical instruments , one should use
(a) plano convex lenses               (b) convex lenses
(c) plane mirrors                         (d) concave lenses

14. How should people wearing their spectacles work with a microscope ?
(a) They should take off the spectacles                                          
(b) They should keep on wearing the spectacles
(c) It hardly matters whether they take off or put on their spectacles 
(d) Special lenses of more power are used in the spectacles

15. The defects in the formation of image arising due to marginal and paraxial rays undergoing different deviation in a lens is known as
(a) Chromatic aberration              (b) Coma
(c)  Distortion                              (d) Spherical aberration

Answers :

1. b
2. c
3. b
4. c
5. a
6. d
7. a
8. d
9. d
10. a
11. b
12. d
13. a
14. a
15. d


Courtesy:commons.wikimedia.org