Scanning electron microscopes (SEMs) are useful for studying surface structures. The sample surface is covered with a thin film of gold. The beam excites electrons on the surface of the sample. These secondary electrons are collected and focused on a screen. The result is an image of the topography of the specimen. The SEM has great depth of field, resulting in an image that seems three-dimensional. Electron microscopes reveal organelles that are impossible to resolve with the light microscope. However, electron microscopes can only be used on dead cells. Light microscopes do not have as high a resolution, but they can be used to study live cells. Microscopes are major tools in cytology, the study of cell structures. Cytology combined with biochemistry, the study of molecules and chemical processes in metabolism, to produce modern cell biology. Cell biologists can isolate organelles to study their functions.
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Scanning electron microscope, 1,000,000 g