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Defects in images by lens

November 24, 2012  by: kanduri narasimha  Points: 12   Category: General Science  Earning $0.15   Views: 436

In this article, we can find out the explaination of defects in images by lens. In this article, spherical abberation and chromatic abberation are explained.

         

When light rays are incident on a lens, the rays that near to the principle axis are called paraxial rays. The rays near to the periphery of the lens is called marginal rays. Lens formula is applicable to paraxial rays. According to this lens formula, point object forms point images when object is illuminated by monochromatic light. But, point image is not expected to form because all rays are not paraxial rays. So, in such cases we get monochromatic abberations. Monochromatic abberations are different types. They are

i)Spherical abberation:


When light rays are incident on the lens, if the rays incident on the peripheral of the lens then they focus on the principle axis near to the lens. That means height of the lens on which incidence of light increases then the angle by which focus on the principle axis increases. If a point object is placed on the principle axis illuminated by monochromatic light, then we can find a line image when the screen placed horizontal to the principal axis. This is called longitudinal spherical abberation. When the screen is perpendicular to the principle axis then we can find out circle image is formed. This is called lateral or transversal spherical abberation.

At image'I1' for marginal rays, we get circle having brightness at its centre and brightness decreases when it reaches to the edge of the circle. At image 'I' for paraxial rays, brightness is at the edge but it decreases when it reaches to the centre. In between these two images, we can find circle image which having uniform intensity of brightness. This circle is called as circle of least confusion.

Methods to reduce spherical abberation:


By using stops, we can cease either paraxial rays or marginal rays. So, we can remove spherical abberation to maximum extent.




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