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Electron Microscopy - Coursework Example

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Electron Microscopy

Therefore, a lens will have varying focal lengths for light of diverse wavelengths. Spherical aberration also occur in the Electron Microscopes when electrons passing through the side of the lens are refracted greater than those passing along the axis.2 (Lam, 2009); while Diffractive aberrations are brought about by the deviations from geometrical optics caused by the wave nature of light.3 Consequently, the result of each aberration is to deform the image of each point in the object in a specific way, leading to a total loss of quality and resolution in the image.1 b. Where appropriate include in your answer how the aberrations are influence by aperture size. Wherever light passes into an aperture, diffraction occurs in order that a parallel light beam is converted into a series of cones that are seen as circles. For light of a certain wavelength, the central spot's diameter is inversely proportional __________________________________ 1. Beanland, R., Goodhew, P. J. and Humphreys, F. J. 'Prelims', Electron Microscopy and Analysis, 1:1, 0 – 19. 2. Lam, R,L. Electron Microscopy, Institute of Technology Press, Wayward, 2009. to the aperture's diameter from which the diffraction is existing. Thus, the smaller the aperture is, the larger is the circle's (airy disc) central spot. Very small apertures can be used in order to create the Airy disc clearly visible, however, the same effects occur at the point of relatively larger apertures of the microscope. To make the image of every point as small

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as possible, aperture should be as large as is feasible.3 The only way of minimizing monochromatic aberrations is to restrain the electrons to paths that is very near the optical axis, by applying a small objective aperture. Spherical aberration can also be reduced if small aperture is use; although can make the diffraction-limited resolution worse. c. From the answer provided in lb comment on the best aperture size (small, medium, large) for spherical, chromatic, diffraction and astigmatism. High quality image formation in an optical system appears when all of the rays start at a single object point cross to a single image point, or congruently, when the geometrical wavefront in the space of image has a spherical shape that is centered on the image point. 4 The background intensity generated by spherical aberration can be adjusted by limiting the angular aperture of its objective lens. To do this, the objective aperture has to be set at the back focal plane of its objective lens. This way, aperture works to decrease the background intensity brought about by the spherical aberration and therefore, increases the contrast of the image points. When the size of its objective aperture is reduced, the more scattered electrons are stopped, thus contrast _________________________ 3. J.C.H. Spence, Y.M. Huang, O. Sankey, Acta Met. 41, 1993, 2815. 4, Zach, J., and Haider, M. Optik 99, 1995, 112. improves. The Objective aperture sizes range within 25 and 75 µm.; so, the smaller the aperture, the greater would be the aperture contrast, since more scattered electrons will be prevented from reaching the image.5 Diffraction is due to the wave nature of electrons and aperture size of final lens. The only means to reduce diffraction problems is by way of increasing the final aperture size. Astigmatism on the other hand, results from magnetic lenses which do not have

Summary

la. Briefly describe how the main lens aberrations arise in electron microscopy. Electron microscopy is a technique in microscopy that offers higher magni-fication, higher resolution and greater depth of field in viewing objects. When the main lens aberrations arise in electron microscopy1 the high quality of resolution, light focusing to the point of object and depth of field is not achieved…
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Electron Microscopy essay example
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