There are two main types of aberrations or optical defects in images produced by a lens.
- Spherical aberration 2. Chromatic aberration
1. Spherical aberration
It is observed that the rays of light which pass through thick lenses or lenses of large aperture do not focus at a single point. Thus the images formed are not sharp and well defined. This defect is called “Spherical Aberration”.
If parallel rays of monochromatic light are incident on a lens then the rays near the rim of the lens (called marginal rays)after refraction come to focus at a point Fm nearer to the lens but the parallel rays near the axis (i.e., paraxial Rays) are focused at point Fp (fig. 14 .15). The second point Fp is farther from the lens than the point Fm. On account of this defect images formed are not sharp or well defined.
This defect is minimized by covering the lens with a disc of size equal to the lens and having a small hole at the Centre of this disc. This disc allows only the central or paraxial rays to pass through the central part of the lens but cut off the marginal rays (fig 14. 16). This method is used in cheap optical instruments. In expensive instruments this defect is removed by using a complicated lens made by combining lenses of different shapes.
2. Chromatic Aberration
You might have noted that an object illuminated by white light and viewed through a cheap convex lens shows coloured tinge in the image. This makes the image blurred.
A lens acts like two prisms placed end to end. Thus white light while passing through a lens is refracted as well as dispersed. As a result violet rays are focused nearest the lens, while red rays are focused at the farthest away (fig. 14. 17). This defect is called chromatic aberration. It can be minimized by combining a convex lens of crown glass and a concave lens of flint glass in such a way that dispersion of light produced by the convex lens is neutralized by the concave lens. Such a combination of lenses is called an “Achromatic Lens” (colour corrected lens) (fig. 14. 18). In high class cameras and optical instruments a complicated combination of lenses is used.
