Atomic microscope Similarities and Dissimilarities of the Optical Microscope Imaging Principle
Based on the concept of electron optics, an electron microscope is a device that visualizes the intricate details of substances at extremely high magnifications using electron beams and electron lenses as opposed to light beams and optical lenses.
The shortest distance that an electron microscope can bridge between two neighboring spots is used to describe its resolving power. The resolution of transmission electron microscopes in the 1970s was roughly 0.3 nanometers (the resolving power of the human eye is about 0.1 millimeters). Now that the electron microscope's maximum magnification exceeds 3 million times, compared to the optical microscope's maximum magnification of only about 2000 times, it is possible to directly observe the atoms of some heavy metals and the orderly atomic lattices in crystals using the electron microscope.
When German scientists Knorr-Bremse and Ruska modified a high-voltage oscilloscope with a cold cathode discharge electron source and three electron lenses in 1931, they were able to obtain an image that was magnified by more than ten times, confirming the viability of magnified imaging using an electron microscope. After Ruska's advancements, the electron microscope's resolving power reached 50 nanometers in 1932, nearly 10 times that of the optical microscope at the time. As a result, people started to pay more attention to the electron microscope.
In order to correct the rotational asymmetry of the electron lens, Hill in the United States utilized an astigmatizer in the 1940s. This innovation helped the electron microscope's resolving power advance and eventually reach the level of today. A transmission electron microscope with a resolution of 3 nanometers was successfully created in China in 1958, and a big electron microscope with a resolution of 0.3 nanometers was produced there in 1979.
Although the electron microscope's resolving power is much greater than that of the optical microscope's, it is challenging to observe living things because the electron microscope must operate in a vacuum and the irradiation of the electron beam will cause radiation damage to the biological samples. It also needs to be further researched how to improve the brightness of the electron cannon and the caliber of the electron lens, among other things.
An essential measure of electron microscopy is resolving power, which depends on the incident cone angle and electron beam's wavelength as it passes through the material. While the wavelength of electron beams is correlated with the accelerating voltage, the wavelength of visible light ranges from 300 to 700 nanometers. The electron beam wavelength is roughly 0.0053–0.0037 nanometers when the accelerating voltage is 50–100 kV. 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 significantly greater than that of the optical microscope because the wavelength of the electron beam is much shorter than the wavelength of visible light.
