What principle does an electron microscope use to magnify objects?
The resolving power of an electron microscope is expressed by the minimum distance between two adjacent points that it can resolve. In the 1970s, the resolution of transmission electron microscopes was about 0.3 nanometers (the resolving power of the human eye is about 0.1 millimeters). The maximum magnification of electron microscopes now exceeds 3 million times, while the maximum magnification of optical microscopes is about 2,000 times. Therefore, atoms of certain heavy metals and the neatly arranged atomic lattice in crystals can be directly observed through electron microscopes.
The resolving power is an important indicator of an electron microscope, 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 to 700 nanometers, and the wavelength of the electron beam is related to the accelerating voltage. When the accelerating voltage is 50 to 100 kilovolts, the electron beam wavelength is approximately 0.0053 to 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.
The electron microscope consists of three parts: lens tube, vacuum system and power cabinet. The lens barrel mainly includes components such as an electron gun, an electron lens, a sample holder, a fluorescent screen, and a camera mechanism. These components are usually assembled into a cylinder from top to bottom; the vacuum system consists of a mechanical vacuum pump, a diffusion pump, a vacuum valve, etc., and is pumped through The gas pipeline is connected to the lens tube; the power cabinet is composed of a high-voltage generator, an excitation current stabilizer and various adjustment control units.
The electron lens is the most important component of the electron microscope barrel. It uses a spatial electric field or magnetic field that is symmetrical to the axis of the barrel to bend the electron trajectory toward the axis to form a focus. Its function is similar to that of a glass convex lens to focus the beam, so it is called electrons. lens. Most modern electron microscopes use electromagnetic lenses. The strong magnetic field generated by a very stable DC excitation current passing through a coil with pole shoes focuses the electrons.
