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In this article we will discuss about the parts of electron microscope with its construction.
1. The electron microscope is an instrument which utilizes the short wavelength of an electron beam, rather than light waves, to obtain very high magnification and resolution of minute structures for which a light microscope is inadequate.
It contains an electric gun whose beam is refracted and focused onto a specimen by an electron lens system. The image of the specimen is magnified and projected onto a stage or fluorescent screen.
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2. On the basis of several earlier researches in Physics, Knoll and Ruska (1931) of Berlin were the first to develop an electron microscope. With the help of their electron microscope, they could magnify objects up to 12,000 times.
By an improved type of electron microscope developed by Borries and Ruska (1938), they could obtain pictures of 20,000 times magnification. In this the magnification of objective coil was 100 and the projector coil was 200. The resolution of this microscope was 100Å. The electron microscopes with 4-10Å or even better resolution are now available.
3. A magnification as high as 1,60,000 times can be achieved by using intermediate coils between the objective and projector coils of the electron microscope.
4. Construction:
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An electron microscope consists of an electric gun, microscope column, electromagnetic coils, a fluorescent screen and some other accessories described below:
(a) The electron gun is located at the top of the body of microscope. It is the source of electrons. It is made up of a tungsten filament surrounded by a negatively biased shield with an aperture. The electron beam is drawn off through this aperture.
(b) The microscope column or central column is made up of an evacuated metal tube. It protects the person operating the microscope from X-rays that are generated when the electrons strike the surface of the metal tube.
(c) The electromagnetic coils or lenses include projector coils, objective arid condenser. In each coil, the coils of electric wire are wound on a hollow metallic cylinder. The magnetic field, produced by passing the electric current through the magnetic coil, functions as a magnifying lens.
(d) The fluorescent screen is used for observing the magnified image of the object. It remains coated with a chemical which, on being excited, forms the image as on the screen of television.
(e) Some other essential accessories of the electron microscope include high voltage transformers (for developing high voltage current for the electron gun and electromagnetic coils), vacuum pumps (for maintaining high vacuum inside the microscope column), a water cooling system (for prevention from overheating of various parts), a circulating pump, a refrigeration plant and also a filter system.
All these parts require elaborate arrangements and contribute to the massive size of the electron microscope.
6. The image formation in this microscope occurs by the scattering of electrons. The electrons strike the atomic nuclei and get dispersed. These dispersed electrons form the electron image. By projecting on a fluorescent screen or photographic film, this electron image is converted into a visible image of the object.
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7. The electron beam in this microscope is made by accelerating electrons through a potential difference of from 1-1500 kV in an electron gun.
8. Only dried specimens are studied by electron microscope. Living cells cannot be studied with this microscope because they possess water which causes large scale scattering of electrons.
9. Ultrathin sections (10-50 nm thickness), which are more than 200 times thinner than those routinely used for light microscopy, are cut for electron microscopy. These are cut with the help of diamond or glass knives of an ultra-microtome.
