Electron Microscope EM

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Electron Microscope EM

Electron Microscope (EM)

The advantage of EM over light microscope is the high resolution it offers to about 3 Å (1 Angstrom unit = 10^-7 mm) compared to about 0.2 mµ the light microscope offers. EM is useful to study the ultrastructural details of bacteria and also to visualize non-cultivable micro-or-ganisms. There are two types of EM, the transmission EM and the scanning EM.

In TEM tungsten filament is used to emit electrons which are accelerated by an electrical field in the interior of the microscope which is kept under vacuum because air has a tendency to scatter electrons.

The lens system used in EM is the same as in an inverted light microscope with two differences, namely, the use of electromagnetic lenses to focus the electron beam instead o glass lenses and the use of an electronic gun instead of an incandescent lamp.

Focusing of the electron beam is done by varying the magnetic field of electromagnetic lenses.

The electron beam is concentrated by the use of one or two condenser lenses. An objective lens initially magnifies the image which is further enlarged by the use of one or more projector lenses in the same way the eye-piece lens does in a light microscope.

The enlarged image can be flashed on a fluorescent screen which is then photographed. The photograph (electron micrograph) is later interpreted to bring home the characteristic features.

The specimen used for TEM studies has to be processed prior to microscopy. This is done by staining thin specimens (80 nm or less) with heavy metals such as uranium, osmium or lead. When these dense elements are present, electrons are scattered from the beam resulting in dark images on the fluorescent screen or in electron micrographs.

Individual cells may be affixed to thin electron micrographs. Individual cells may be affixed to thin electron-transparent films and stained with heavy metals stains. However, tissues must be fixed in a suitable fixative

Basic Optical Features

A. Inverted Light Micrscope	B. Transmission Electron Micrscope

A	B
1. Lamp	1. Electron gun
2. Condenser lens	2. Condenser lens
3. Specimen Plane	3. Specimen plane
4. Objective lens	4. Objective lens
5. Intermediate Image plane	5. Projector lens system
6. Ocular	6. Fluorescent screen
7. Photographic plate or the eye	7. Intermediate image planes
	8. Binocular viewer

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Algae Protozoa of Microorganisms Bacteriophges Fungal Viruses Microscopy Compound Light Microscope Bright Field Microscope Dark Field Microscope Fluorescence Microscope Ultraviolet-UV Microscope Phase Contrast Microscope Electron Microscope (EM)

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