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Pages 20 (5020 words)
1. You will be provided with a bright field image (Fig. 1), a dark field image (Fig. 2) and selected area diffraction pattern (Fig. 7) from the same sample. With the support of ray diagrams explain how the images were obtained. Your explanation must include comments about the location and use of the objective and selected area apertures.
A selected area aperture is inserted into the back focal plane of the objective lens to select the required beam. When the direct beam is selected, a bright field image is formed, and when the diffracted beam is selected, a dark field image is formed. Bright field image The given micrograph shows a bright field image of an MgO crystal. The crystal specimen appears dark with a bright background. The background appears bright because only the direct beam of transmitted electrons is selected and let to pass through the aperture. The surface topology and the raised texture on top of the crystal are clearly observable. This kind of image is obtained by placing the objective diaphragm or the selected area aperture in the back focal plane of the objective lens. The aperture allows only direct beam to pass through, while blocking the diffracted beam. The direct beam appears as a bright central spot. The aperture also maintains the collection angle. As seen in the ray diagram below, the objective aperture blocks the diffracted beam, allowing only the transmitted beam to reach the image plane. Dark field image The given micrograph shows a dark field image of an MgO crystal. The crystal specimen appears lighter than the background. The background is dark. The edges of the crystal are highly pronounced. ...
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