PHYSICS TERM PAPERTOPIC:-LENS ABERRATIONS

LENS ABERRATIONSIn optical system all

rays from a point in the object plane would converge to the same point in the image plane.

The influences which cause the different rays to converge to different points are called aberrations.

TYPES OF ABERRATIONSMONOCHROMATIC ABERRATIONS CHROMATIC ABERRATIONS

These are those aberrations which include only one type of wavelength.

It includes following aberrations:-

Spherical Aberrations Coma Field Curvature Distortion Astigmatism

These are those aberrations which occur due to different bending of different wavelengths for same refractive index.

It includes following aberrations:-

Longitudinal Chromatic Aberrations

Transverse Chromatic Aberrations

SPHERICAL ABERRATIONS It occurs due to the

improper refraction of the light rays when they strike a lens near its edge, in comparison with those striking near to the centre.

This aberration is termed as positive spherical aberration when rays are bending too much.

And negative aberration when bending of rays is not enough.

APLANATIC POINT

1) A spherical lens has an aplanatic point(i.e. no spherical aberration) only at a radius equals the radius of the sphere divided by the refractive index of the lens material.2) For a normal glass this value is 43% i.e. 67% of diameter of a spherical lens is useful.

IMAGE FORMED IN SPHERICAL ABERRATION• Upper row is of negative

aberration• Middle one is zero

aberration• Lower one is the positive

aberration

COMA

Coma is an aberration that afflicts off-axis light bundles in a manner quite similar to the way in which spherical aberrations affects the axial bundles.

This result in an overall spot that is comatic in shape, having bright central core with a flare extending towards the optical axis of the lens.

IMAGE IN COMA

FIELD CURVATURE Most optical tend to form the

image on a curved surface. The nominal curvature of this

surface is reciprocal of the radius of the lens and this is referred to as the Petzval or field curvature of the lens.

This error arise due power error for rays at a larger angle.

These rays strike the lens as having an effectively smaller diameter and an effective high power, forming the image of the off axis points closer to the lens.

ASTIGMATISMIn this the rays that

propagates in two perpendicular planes have different foci.

If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be sharp but at different distances.

TYPES OF ASTIGMATISM

First one is third order aberration, occurs for objects away from optical axis.

This form of aberration occurs even when the optical system is symmetric.

The second form occurs when the optical system is not symmetric about the optical axis.

Occurs due to manufacturing error or misalignment of components.

IMAGE COMPARISION BETWEEN COMA AND ASTIGMATISM

IMAGE DISTORTION It is the deviation from rectilinear

projections. Means projection of straight line is not straight line.

Barrel Distortion:- Image magnification decreases with distance from the optical axis.

Pincushion Distortion:-Image magnification increases with the distance from the optical axis.

Moustache Distortion:-Combination of above types. It starts with barrel distortion at the centre and gradually turns to pincushion distortion at the periphery. The resultant image looks like a handlebar moustache.

Moustache Distortion

CHROMATIC ABERRATIONS It is an effect resulting from

dispersion in which there is failure of lens to focus all colours to some convergence point.

It has two main types: Longitudinal:-occurs when

different wavelengths focus at different distances from lens.

Transverse:-occurs when different wavelengths are focused at different positions in the focal plane because magnification of the lens also varies with wavelength.

LONGITUDINAL TRANSVERSE

IMAGE IN CHROMATIC ABERRATIONS

CORRECTIONS AND APPLICATIONS Spherical aberrations,

field curvature can be corrected by the use of combination of different lenses or by use of aspherical lenses or by use of stops.

Chromatic aberrations can be removed by the use of the achromatic lens. The other option is the use of two mated lens-concave and convex.

Astigmatism can be removed by the use of pantoscopic tilt glasses.

Coma can be minimized by using a complex lens system or by pair of lenses arranged symmetrically as in a relay lens system.

WHERE WE ARE USING THIS KNOWLEDGE(APPLICATION)• To make telescope, microscope the appropriate

lenses need to be selected so their knowledge is necessary.

• The elimination techniques of coma are used in designing of many lenses for instruments like borescopes and submarine periscopes.

• In making of camera lenses, high resolution optical devices, making of correctors and the most important the human eye which also contains a convex lens. So, this knowledge is also important for the correction of are eyes.

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