What is the meaning of aberration?
The defect in the image formed by a lens or mirror,revealed as a blurring and possibly a false coloration,these principal aberrations are spherical aberrations,coma,and astigmatism.
Aberrations occur when light rays do not pass close to optic axis of the system but make a considerable angle to it such that it is no longer accurate to replace the sine of the angle involved by the angle itself.Out of six aberrations only spherical and chromatic aberrations are found in the images of axial points.The other four occur only when off axis points are involved.
What is a spherical aberration and a chromatic aberration?
there are two basic types of aberrations that are given below in detail.
An aberration that can occur in optical systems when rays are traced after reflection or refraction at large aperture surfaces that do not unite accurately at a focus.
When the outer zones focus within the focal point for paraxial rays,the spherical aberration is said to be positive.The image of a point appears as a circular disc.
The term spherical aberration is also used to embrace all the aberrations due to the sphericity of surfaces.
How can we remove spherical aberration?
This defect can be removed by using only the central portion of a lens.This defect can also be removed by using a lens made by combining lenses of different shapes.In mirrors it is corrected by grinding them to the shape of a paraboloid.
When a lens shows image of different colours at different positions of white object then lens is defected.This defect is called chromatic aberration.
In thin lenses small portion of a lens behaves as a prism.When white light passes through such portion of the lens ,it disperse into component colours.As a result,the images of various colours are formed overlapping each other.
How can we remove chromatic aberration?
To remove this defect a combination of convex and concave lens is used.
The concave lens is made of glass of suitable focal length.The convex lens reduces deviation.This combination is known as “achromatic aberration lenses.”