Electron Microscope Optical Microscope Imaging Principle Similarities and Differences
An electron microscope is an instrument that uses electron beams and electron lenses instead of light beams and optical lenses to image the fine structures of substances at very high magnifications based on the principle of electron optics.
The resolving power of an electron microscope is represented by the minimum distance between two adjacent points that it can resolve. In the 1970s, transmission electron microscopes had a resolution of about 0.3 nanometers (the resolving power of the human eye is about 0.1 millimeters). Now the maximum magnification of the electron microscope exceeds 3 million times, while the maximum magnification of the optical microscope is about 2000 times, so the atoms of some heavy metals and the neatly arranged atomic lattices in the crystal can be directly observed through the electron microscope.
In 1931, Knorr-Bremse and Ruska of Germany refitted a high-voltage oscilloscope with a cold cathode discharge electron source and three electron lenses, and obtained an image magnified more than ten times, which confirmed the possibility of electron microscope magnified imaging . In 1932, after Ruska's improvement, the resolving power of the electron microscope reached 50 nanometers, about ten times the resolving power of the optical microscope at that time, so the electron microscope began to receive people's attention.
In the 1940s, Hill in the United States used an astigmatizer to compensate the rotational asymmetry of the electron lens, which made a new breakthrough in the resolving power of the electron microscope and gradually reached the modern level. In China, a transmission electron microscope was successfully developed in 1958 with a resolution of 3 nanometers, and in 1979 a large electron microscope with a resolution of 0.3 nanometers was manufactured.
Although the resolving power of the electron microscope is far better than that of the optical microscope, it is difficult to observe living organisms because the electron microscope needs to work under vacuum conditions, and the irradiation of the electron beam will also cause the biological samples to be damaged by radiation. Other issues, such as the improvement of the brightness of the electron gun and the quality of the electron lens, also need to be further studied.
Resolving power is an important indicator of electron microscopy, which is related to the incident cone angle and wavelength of the electron beam passing through the sample. The wavelength of visible light is about 300-700 nanometers, while the wavelength of electron beams is related to the accelerating voltage. When the accelerating voltage is 50-100 kV, the electron beam wavelength is about 0.0053-0.0037 nanometers. Since the wavelength of the electron beam is much smaller than the wavelength of visible light, even if the cone angle of the electron beam is only 1% of that of the optical microscope, the resolving power of the electron microscope is still far superior to that of the optical microscope.
