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The first electron microscope was built in 1931 by Ernst Ruska and Max Knoll.
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 below 1 nanometer.
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).
The Transmission electron microscope (TEM) produces 2D images (for example of cellss) while the Scanning electron microscope (SEM) produces 3D images or models.
As its name implies the TEM image is produced by detecting electrons that are transmitted through the sample.
By contrast the SEM usually monitors 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 thicker samples and gives better 3D contrast.
Samples viewed under an electron microscope have to be treated in many ways:
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 resulting in artifacts - objects purely the result of treatment, and this gives the problem of distinguishing artifacts from biological material.
The man who scrutinized this aspect most thoroughly is Dr Harold Hillman from London. Like Ignaz Semmelweis he paid dearly for this heresy.
Electron microscopes are also very expensive to buy and maintain.
Wikipedia articles containing electron microscope images: