Transmission electron microscopy, Biology

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Transmission Electron Microscopy

The source of illumination in a TEM is a tungsten filament that emits electrons. A vacuum is created to prevent the collision of electrons with anything else except the specimen. The electron beam is focused by different electromagnetic coils (lenses). First electrons are collected and focused in an electromagnetic 'condenser' lens and then they are collected again by an 'objective' lens. The electromagnetic objective lens produces a magnified image of the object which is received by 'ocular or projection' lens. The final image is formed either on a photographic plate or on a phosphorescent screen. The image results from differential scattering of electrons from molecular constituents of the cell.

If you compare the functioning of a TEM with a LM, you will observe that while there are many similarities between a LM and a TEM. there are also many differences. The image formation in a LM depends on the degree of light absorption whereas in a TEM it depends on electron scattering. The lenses of LMs are glass lenses and focusing is done by moving them nearer or farther away from the specimen. In TEMs all the lenses are electromagnetic coils and focusing is done by manipulating the amount of current flowing through the magnetic coils. The final image in a LM is seen by the eye whereas in a TEM it is formed on a phosphorescent screen.


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