Why electron microscopes have much higher resolution than optical microscopes
Because electron microscopes use an electron beam and optical microscopes use visible light, and the wavelength of the electron beam is shorter than the wavelength of visible light, the resolution of an electron microscope is much higher than that of an optical microscope.
The resolution of a microscope 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 nanometers, while the wavelength of the electron beam is related to the accelerating voltage. According to the principle of wave-particle duality, the wavelength of high-speed electrons is shorter than that of visible light, and the resolution of a microscope is limited by the wavelength it uses, so the resolution of an electron microscope (0.2 nanometers) is much higher than that of an optical microscope (200 nanometers).
The application of electron microscopy technology is based on the optical microscope, which has a resolution of 0.2 μm, and the transmission electron microscope, which has a resolution of 0.2 nm, i.e., the transmission electron microscope is enlarged by a factor of 1,000 on the basis of the optical microscope.
Although the resolution of electron microscope is much higher than that of optical microscope, it has some disadvantages:
1, in the electron microscope samples must be observed in a vacuum, so it is not possible to observe live samples. With the progress of technology, environmental scanning electron microscope will gradually realize the observation of live samples directly;
2, in the processing of the sample may produce a structure that was not originally present in the sample, which exacerbates the difficulty of analyzing the image thereafter;
3, due to the extremely strong electron scattering ability, prone to secondary diffraction and so on;
4, due to the two-dimensional plane projection image of three-dimensional objects, sometimes the image is not unique;
5, because the transmission electron microscope can only observe very thin samples, and it is possible that the structure of the surface of the material is different from the internal structure of the material;
6, ultra-thin samples (below 100 nanometers), the sample preparation process is complex and difficult, and there is damage to the sample preparation;
7, the electron beam may damage the sample by collision and heating;
8, electron microscope purchase and maintenance of the price are relatively high.
