Cytology, General Histology and Embryology - V. K. Napkhanyuk 2002

Microscopes. Microscopy Technique
Microscope
Electron Microscope

Since numerous cellular structures lie beyond the resolution limit of a Light Microscope, they are studied using an Electron microscope. An electron microscope employs a beam of electrons with a shorter wavelength than that of a light microscope. At a voltage of 50,000 V, the wavelength of electromagnetic oscillations generated by the electron beam moving in a vacuum is 0.0056 nm. It has been calculated that under these conditions, the resolution limit can reach about 0.002 nm (0.000002 µm), which is 100,000 times greater than that of a light microscope.

Currently, There are two types of electron microscopes: transmission electron microscopes and scanning electron microscopes.

A transmission electron microscope makes it possible to obtain a two-dimensional image of the studied object.

A Scanning electron microscope provides a three-dimensional image. Its operating principle is based on scanning the object with an electron microprobe, which involves sequentially examining individual points On the surface using a sharply focused electron beam. When examining a selected area, the microprobe moves across its surface under METABOLISM/18.html">The Influence of deflection coils. This process is called scanning, and the pattern along which the microprobe moves is called a raster. The resulting image is displayed on a television screen, the electron beam of which moves synchronously with the microprobe.

Operating Principle of the Electron Microscope

The source of electrons in an electron microscope is the cathode, which is part of the electron gun, while electromagnetic coils are used as lenses. To ensure the unobstructed movement of electrons inside the microscope, high vacuum (10^-4 mm Hg) is created by special pumps.

Inside the microscope Column (tube), from which air has been evacuated, the following components are arranged sequentially: the cathode (a tungsten filament), the anode (a metal plate with an aperture in the center), magnetic lenses, a fluorescent screen, and photographic plates (a cassette).

An electric current passing through the tungsten filament of the cathode heats it, inducing electron emission. The high voltage applied to the cathode creates a large potential difference between the cathode and the anode, which drives the electrons toward the anode and further down the microscope column. The electron beam is first focused by a condenser magnetic lens. Most of the electrons passing through the object are not deflected. The electrons that have passed through the object are focused by a second magnetic lens—the objective lens—which produces a magnified image of the object. This image is further magnified by a third magnetic lens, known as the projector lens. Upon passing through the object, the electrons cause a phosphor-coated screen to fluoresce, thereby forming an image of the object on it.