# Aberration in Optics and Types of Aberration

## Aberration in Optics

In the simple theory of image formation by lenses, it is assumed that an object is situated very close to the principal axis and each point object produces a corresponding point image.

In other words, a lens produces a perfect or an ideal image of an object. But in reality, a lens forms a image of an object which departs from the ideal image. There are certain inherent characteristics of the lens which lead to the formation of defective images.

The deviations of an actual image from an ideal image are called aberration in Optics.

## Types of Aberration

In fact, aberrations in optics are classified into two types i.e., Chromatic Aberration and Monochromatic Aberrations.

### Chromatic Aberrations

The inability of a lens to bring all the rays of different colours to focus in one plane after refraction is called chromatic aberration.

### Monochromatic Aberrations

The aberrations which exist even if monochromatic light were used in image formation are called monochromatic aberrations.

Monochromatic aberrations are classified into five types

• Spherical aberration
• Coma
• Astigmatism
• Curvature of field
• Distortion

### Spherical Aberration

The inability of lens to bring to a focus a point object to a single point image is called spherical aberration.

### Coma

The aberration of defect Coma occurs when the image of a point object situated just off the axis is comet – like shaped. This type of aberration arises due to the fact that the different zones of a lens produce unequal lateral magnification of the image.

### Astigmatism

A lens or an optical system corrected for spherical aberration and come still shows another defect known as Astigmatism. A lens suffering from astigmatism will be unable to form a point image of a point object situated far away from the axis.

Instead of a point image is a pair of short lines normal to each other and at slightly different distances from the lens.

### Curvature of field

Curvature of field is present when the sharpest image is formed not on a flat plane but on a curved surface.

### Distortion

Distortion, is refers to the deformation of an image. There are two kinds of deformation, namely barrel distortion and pincushion distortion.