Electron MicroscopeOptical MicroscopeImaging PrinciplesSimilarities and Differences
Electron microscope is an instrument that uses electron beam and electron lens instead of light beam and optical lens according to the principle of electron optics, so that the fine structure of the material is imaged under very high magnification.
The resolution of an electron microscope is expressed in terms of the small spacing between two neighbouring points that it can resolve.In the 1970s, the resolution of a transmission electron microscope was about 0.3 nanometres (the resolution of the human eye is about 0.1 millimetres). Nowadays, the large magnification of electron microscope is more than 3 million times, while the large magnification of optical microscope is about 2,000 times, so that the electron microscope can directly observe the atoms of certain heavy metals and crystals of the atomic dots in the arrangement of the neat array.
Although the resolution of the electron microscope has been far better than the optical microscope, but the electron microscope needs to work in vacuum conditions, so it is difficult to observe the living organisms, and the irradiation of the electron beam will make the biological samples by irradiation damage. Other problems, such as the brightness of the electron gun and the improvement of the quality of the electron lens also need to continue to study.
The resolving power is an important index of an electron microscope, which is related to the cone angle of incidence and wavelength of the electron beam passing through the sample. The wavelength of visible light is about 300-700 nm, and the wavelength of the electron beam is related to the accelerating voltage. When the accelerating voltage is 50 to 100 kV, the wavelength of the electron beam is about 0.0053 to 0.0037 nm. Because the wavelength of the electron beam is much smaller than the wavelength of visible light, so even if the cone angle of the electron beam is only 1 per cent of the optical microscope, the resolving power of the electron microscope is still far superior to the optical microscope.
Electron microscope consists of three parts: mirror tube, vacuum system and power supply cabinet. The barrel mainly has electron gun, electron lens, sample holder, fluorescent screen and camera mechanism and other components, these components are usually assembled from top to bottom into a column; vacuum system consists of mechanical vacuum pump, diffusion pump and vacuum valves, etc., and through the pumping pipeline connected to the barrel of the mirror; power supply cabinet consists of a high-voltage generator, the excitation current stabiliser and a variety of regulatory control units.
Electron lens is an important part of the electron microscope barrel, it is symmetrical to the axis of the barrel of the space electric or magnetic field so that the electron track to the axis of the formation of the focusing of the role of the glass convex lens to make the role of the beam of light focusing is similar to the role of the lens, so it is called an electron lens. Most modern electron microscopes use electromagnetic lenses, which focus the electrons by a strong magnetic field generated by a very stable DC excitation current through a coil with a pole shoe.
Electron microscopes can be divided into transmission electron microscopes, scanning electron microscopes, reflection electron microscopes and emission electron microscopes according to their structure and use. Transmission electron microscope is often used to observe those with ordinary microscopes can not distinguish the fine structure of the material; scanning electron microscope is mainly used to observe the solid surface morphology, but also with the X-ray diffractometer or electron spectrometer combined to constitute the electron microprobe, used for the analysis of the composition of the material; emissive electron microscope is used for the study of the surface of the self-emission of electrons.
