What is the difference between electron microscope and optical microscope in observing objects?
Optical microscopes are very different from electron microscopes, with different light sources, different lenses, different imaging principles, different resolutions, different depths of field, and different sample preparation methods. Optical microscope, commonly known as light microscope, is a microscope that uses visible light as the illumination light source. Optical microscope is an optical instrument that uses optical principles to magnify and image tiny objects that cannot be distinguished by the human eye, so that people can extract microstructure information. It is widely used in cell biology. An optical microscope generally consists of a stage, a condenser illumination system, an objective lens, an eyepiece and a focusing mechanism. The stage is used to hold the object to be observed. The focusing mechanism can be driven by the focusing knob to make the stage move roughly or finely, so that the observed object can be imaged clearly. The image formed by the optical microscope is an inverted image (upside down, left and right interchanged). Electron microscopes are the birthplace of high-end technical products. They are similar to the optical microscopes we usually use, but they are very different from optical microscopes. First, optical microscopes use a light source. The electron microscope uses an electron beam, and the results that can be seen between the two are different, and the magnification is different. For example, when observing a cell, the light microscope can only see the cell and some organelles, such as mitochondria and chloroplasts, but only The existence of its cells can be seen, but the specific structure of the organelles cannot be seen. Electron microscopes, on the other hand, can see the finer structures of organelles in more detail, and even macromolecules like proteins. Electron microscopes include transmission electron microscopes, scanning electron microscopes, reflection electron microscopes, and emission electron microscopes. Among them, scanning electron microscopy is more widely used. Scanning electron microscopy is widely used in material analysis and research, mainly used in material fracture analysis, micro-area composition analysis, surface morphology analysis of various coatings, layer thickness measurement and microstructure morphology and nanomaterial analysis. Combined with X-ray diffractometer or electron energy spectrometer, it constitutes an electron microprobe, which is used for material composition analysis, etc. Scanning Electron Microscope, abbreviated as SEC, is a new type of electron optical instrument. It consists of three parts: vacuum system, electron beam system and imaging system. It uses various physical signals excited by a finely focused electron beam to scan the surface of the sample to modulate imaging. The incident electrons cause secondary electrons to be excited from the sample surface. What the microscope observes is the electrons scattered from each point, and the scintillation crystal placed next to the sample receives these secondary electrons, and modulates the intensity of the electron beam of the picture tube after amplification to change the brightness on the screen of the picture tube. The deflection yoke of the picture tube keeps scanning synchronously with the electron beam on the sample surface, so that the phosphor screen of the picture tube displays the topographic image of the sample surface. It has the characteristics of simple sample preparation, adjustable magnification, wide range, high image resolution and large depth of field. Application performance of transmission electron microscope: 1. Analysis of crystal defects. All structures that destroy the normal lattice period are collectively referred to as crystal defects, such as vacancies, dislocations, grain boundaries, and precipitates. These structures that destroy the periodicity of the lattice will lead to changes in the diffraction conditions of the area where the defect is located, so that the diffraction condition of the area where the defect is located is different from that of the normal area, so that the corresponding difference in brightness and darkness is displayed on the phosphor screen. 2. Organizational analysis. In addition to various defects, different diffraction patterns can be produced, through which the structure and orientation of the crystal can be analyzed while observing the microstructure. 3. In situ observation. With the corresponding sample stage, in situ experiments can be performed in TEM. For example, strain tensile samples were used to observe their deformation and fracture processes. 4. High-resolution microscopy. Improving the resolution so that the microstructure of matter can be observed more deeply has always been the goal that people are constantly pursuing. High-resolution electron microscopy utilizes the phase change of the electron beam, which is coherently imaged by more than two electron beams. Under the condition that the resolution of the electron microscope is high enough, the more electron beams used, the higher the resolution of the image, even Can be used to image the atomic structure of thin samples.
