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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.
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. ...
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