Electron Microscope

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The electron microscope, instrument that produced the first magnified image showing ‘three-dimensional' and highly magnified image of a small object. It directs a beam of electrons rather than light through a specimen. The beam of electrons is created from an electron gun. This beam then travels through the length of the microscope cylinder, which contains the lenses, the specimen chamber, and the image-recording system. Two types of electron lenses are used, electrostatic and electromagnetic. They create electric and electromagnetic fields to both concentrate and move the beam.

The electron microscope requires that the electron beam be in a vacuum, because electrons cannot travel far in air at atmospheric pressure. Pumps empty the column and specimen chamber of the electron microscope. Living specimens cannot be examined with an electron microscope, since they will not survive in a vacuum.

The magnification in magnetic electron microscopes is determined by the strength of the current passing through the electric and electromagnetic lens coils. Changing the current through the objective lens coil focuses the image. In the optical microscope the image is determined by absorption of light by the specimen; in the electron microscope the image results from a scattering of electrons by atoms of the specimen. Since an atom, with a high atomic number, have more electrons than a light atom, it appears darker.

As the beam passes through a specimen, each tiny differences in the structure of the specimen causes a variation in the electron stream. The image produced is then projected onto a fluorescent screen or recorded on film. The electron microscope, with its tremendous resolving power, can magnify specimens over 50,000 times.
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