The imaging principle of electron microscope and optical microscope is basically the same, but the difference is that the former uses electron beam as light source and electromagnetic field as lens. In addition, because of the weak penetration of electron beam, the specimen used for electron microscope must be made into ultra-thin sections with a thickness of about 50nm. This kind of slice needs to be made with an ultra-thin slicer. The magnification of an electron microscope can reach nearly one million times. It consists of five parts: illumination system, imaging system, vacuum system, recording system and power supply system. If subdivided, the main parts are electronic lens and imaging recording system, which consists of electron gun, condenser, sample room, objective lens, diffraction mirror, intermediate mirror, projection mirror, phosphor screen and camera placed in vacuum. An electron microscope is a microscope that displays the inside or surface of an object with electrons. The wavelength of high-speed electrons is shorter than that of visible light (wave-particle duality), and the resolution of microscope is limited by the wavelength used, so the theoretical resolution of electron microscope (about 0. 1 nm) is much higher than that of optical microscope (about 200 nm). Transmission electron microscope (TEM), short for transmission electron microscope, projects an accelerated and concentrated electron beam onto a very thin sample, and the electrons collide with atoms in the sample to change direction, thus producing solid angular scattering. The scattering angle is related to the density and thickness of the sample, so images with different brightness can be formed, which will be displayed on imaging devices (such as fluorescent screen, film and photosensitive coupling element) after amplification and focusing. Because the de Broglie wavelength of electrons is very short, the resolution of transmission electron microscope is much higher than that of optical microscope, which can reach 0. 1~0.2nm, and the magnification is tens of thousands to millions of times. Therefore, transmission electron microscope can be used to observe the fine structure of samples, even the structure of only one column of atoms, which is tens of thousands of times smaller than the smallest structure that can be observed by optical microscope. TEM is an important analytical method in many scientific fields related to physics and biology, such as cancer research, virology, materials science, nanotechnology, semiconductor research and so on.