The Electron microscope reference article from the English Wikipedia on 24-Jul-2004
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Electron microscope

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The electron microscope is a microscope that can magnify very small details with high resolving power due to the use of electrons rather than light to scatter off material, magnifying at levels up to 500,000 times.

Table of contents
1 History
2 Process
3 Types
4 Treatment
5 Disadvantages

History

The first electron microscope was built in 1931 by Ernst Ruska and Max Knoll at the Berlin Technische Hochschule. It was greatly developed through the 1950s and has allowed great advances in the natural sciences. The advantage of an electron beam is that it has a much smaller wavelength (see wave-particle duality), which allows a higher resolution - the measure of how close together two things can be before they are seen as one. Light microscopes allow a resolution of about 0.2 micrometres, whereas electron microscopes can have resolutions as low as 0.1 nanometers.

Process

High voltage electron beams from a cathode are focused by magnetic lenses on to the specimen. They are then magnified by a series of magnetic lenses until they hit photographic plate or light sensitive sensors - which transfer the image to a computer screen. The image produced is called an electron micrograph (EM).

Types

The Transmission electron microscope (TEM) produces images by detecting electrons that are transmitted through the sample, while the Scanning electron microscope (SEM) produces images by detecting secondary electrons which are emitted from the surface due to excitation by the primary electron beam.

Generally, the TEM resolution is about an order of magnitude better than the SEM resolution, however, because the SEM image relies on surface processes rather than transmission it is able to image bulk samples and has a much greater depth of view, and so can produce images that are a good representation of the 3D structure of the sample.

Treatment

Samples viewed under an electron microscope have to be treated in many ways:

Disadvantages

The samples have to be viewed in vacuums, as air would scatter the electrons. This means that no living material can be studied.
The samples have to be prepared in many ways to give proper detail, which may result in artifacts - objects purely the result of treatment, and this gives the problem of distinguishing artifacts from material, particularly in
biological samples.

There have been a few scientists, such as Dr Harold Hillman, who believe that such artefacts are responsible for all the structures observed in biological samples by electron microscopy, rendering the techniques useless for these materials. Mainstream scientists maintain that the results from various preparation techniques have been compared, and as there is no reason that they should all produce similar artefacts, it is therefore reasonable to believe that electron microscopy features correlate with living cells. In addition, higher resolution work has been directly compared to results from X-ray crystallography, providing independent confirmation of the validity of this technique. Recent work performed on unfixated, vitrified specimens has also been performed, further confirming the validity of this techique.

Electron microscopes are also very expensive to buy and maintain.

Wikipedia articles containing electron microscope images: