Aberrations in objective lens system.pdf

Aberrations in objective lens system Prepared by Vidula Bindu Aberrations: Deviation produced from the actual size, shape and position of an image are called the aberrations produced by a lens. The aberrations produced by the variation of...

Aberrations in objective lens system Prepared by Vidula Bindu Aberrations: Deviation produced from the actual size, shape and position of an image are called the aberrations produced by a lens. The aberrations produced by the variation of refractive index with wavelength of light are called chromatic aberrations. Aberrations are caused even if monochromatic light is used. It is known as monochromatic aberrations. Certain defects in the image are caused due to the spherical surface of the lens. These are called as spherical aberrations. Various types of defects are chromatic or spherical aberrations, distortion, curvature of field, astigmatism, coma and lateral color. 1. Chromatic aberrations White light is composed of different colors. Dispersive power of the lens :- A simple or compound lens composed of only one material will exhibit different focal lengths for the various constituents of light. Every wavelength is differently refracted. The shortest wave the most and the longest wave , the least. There will be series of color foci along the axis ,as a result image will be surrounded by colored zones or halos and called as chromatic aberration.  Figure – Chromatic aberration with white light Chromatic aberrations are corrected by using: i. Compound lenses ii. Apochromatic lenses (made up of fluorite in combination with lenses of optical glass ) are also used for correcting defects when light system of three different wavelengths are used. iii. Semi apochromats can also be used. 2. Spherical aberration Spherical aberrations caused due to greater power in the outer portion of the spherical surface of lens than in the inner portion. Paraxial rays of light form image at a longer distance than the marginal rays. This is called as spherical aberration and arises due to the fact that different annular zones have different focal lengths. The image is not sharp at any point of the axis. Figure: Spherical aberration. The distance Im to Ip measures the longitudinal spherical aberration and distance AB measures lateral spherical aberration.  Spherical aberrations are corrected by :- i. Using stop, which allows either axial rays or marginal rays of light. But the image is less bright as amount of light passing through the object is less. ii. Using crossed lens, distance between Im and Ip is reduced by bending of the lens. iii. Using planoconvex lenses, where deviation of marginal rays of light is made minimum. The focus of fm for a parallel beam will shift towards fp due to which spherical aberration can be minimized or removed. Using two planoconvex lenses, where the distance between the two lenses is equal to the difference in their focal lengths. iv. Using combination of convex (positive) and concave (negative)lenses, i.e. convergent and divergent lens systems. This defect leads to loss in contrast in the normal microscopic images.  3. Curvature of field Produces curved image of a flat object. Caused by a spherical lens surface. Marginal portion of the image come to a focus at different distance from those of a central portion of the image. The central portion of the image near the axis is in focus while the outer regions of the image away from the axis is blurred. Defect is caused due to the fact that the paraxial focal length is greater than the marginal focal length. 4. Distortion Square object forms an image with curved sides. Caused by the lens surface having different magnification at the marginal and central portion of the image. This is reflected as variation in magnification for different axial distances, resulting in distortion. Figure: Distortion 5. Astigmatism Point object appears distorted and star shaped. Image can never be focused sharply. The image points are off the axis. A spreading of the image is observed along the lens axis. Corrected by:- Minimized by using convex and concave lenses of suitable focal lengths and separated by a distance.  6. Coma The image of the point object is comet shaped. Similar to spherical aberration i.e. failure to bring all light rays from the object to one point. The object points are situated off the axis. Correction can be made by using aplanatic lens due to which lateral magnification is same for all rays of light. Figure: Coma Aberration 7. Lateral color This causes an off-axis image of a point object to be spread out into a tiny spectrum or spread of color. This is because one wavelength is magnified greater than other.