Electron microscope

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An electron microscope is a type of microscope that produces an electronically-magnified image of a specimen for detailed observation. The electron microscope (EM) uses a particle beam of electrons to illuminate the specimen and create a magnified image of it. The microscope has a greater resolving power than a light-powered optical microscope, because it uses electrons that have wavelengths about 100,000 times shorter than visible light (photons), and can achieve magnifications of up to 2,000,000x, whereas ordinary, non-confocal light microscopes are limited to 2000x magnification.

The electron microscope uses electrostatic and electromagnetic "lenses" to control the electron beam and focus it to form an image. These lenses are analogous to, but different from the glass lenses of an optical microscope that form a magnified image by focusing light on or through the specimen. In transmission, the electron beam is first diffracted by the specimen, and then, the electron microscope “lenses" re-focus the beam into a Fourier-transformed image of the diffraction pattern for the selected area of investigation. The real image thus formed is a highly `magnified' image by a factor of several million, and can be then recorded on a special photographic plate, or viewed on a detecting screen. Electron microscopes are used to observe a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, the electron microscope is primarily used for quality control and failure analysis in semiconductor device fabrication.

An electron microscope's advantages over X-ray crystallography are that the specimen need not be a single crystal or even a polycrystalline powder, and also that the Fourier transform reconstruction of the object's magnified structure occurs physically and thus avoids the need for solving the phase problem faced by the X-ray crystalographers after obatining their X-ray diffraction patterns of a single crystal or polycrystalline powder. The transmission electron microscope's major `disadvantage' is the need for extremely thin sections of the specimens, typically less than 10 nanometers. For biological specimens it also requires biological sample special `staining' with heavy atom labels in order to achieve the required contrast, and then chemical fixation as well as encasing with a polymer resin to stabilize the biological specimen which is thin sectioned.


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