Monday, April 6, 2009

Electron Microscope

The electron microscope is an instrument that exploits the extremely short effective wavelengths of accelerated electron beams to form images in which fine detail is resolved. Optimal separations as small as 0.1 nanometers (10-10 m) can be observed; resolutions of 2 nm (2 x 10-9 m) are common. The electron microscope supplements the standard light microscope, the useful magnification of which is limited by the wavelength of visible radiation.

Electron microscopy is widely used in biological studies of the structure of individual cells and large molecules, and it has made a significant contribution to the understanding of the complexities of cellular structure. The applications of the electron microscope are being extended to clinical procedure in medicine such as kidney biopsies. In the physical science, applications include studies of the fine structure of metals and crystalline materials and the observation of electron diffraction pattern.

Transmission Electron Microscope

The transmission electron microscope is analogous in optical design to the light microscope; instead of a beam of light, however, a beam of electrons is made to pass through a thin sample. The source of the electrons is a gun that accelerates electrons through a potential in the range of 40 to 100 kilovolt (kV). The electron beam should be monochromatic (of single wavelength) and correspond to a uniform accelerating potential. Because electron scatter in the air, the electron microscope column must be kept at a vacuum of 10-4 torr or better. Electron lenses are appropriately shaped magnetic or electrostatic fields. The condenser lens focuses electrons at the specimen, the objective in termediate, and projector lenses produce the image that is observed on a fluorescence screen or recorded on a photographic plate.

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