Why can electron microscopy magnify objects by a million times?
Put a flea under the magnifying glass, then move the magnifying glass to the right distance, and you should see a flea several times bigger than before. That is a picture of a flea that has been enlarged. If there are two magnifying glasses, place them on top of each other, and adjust to an appropriate position to observe the flea, the flea’s image will become even larger.
Optical microscopy used in the biological laboratory is taking advantage of this principle to manufacture. In the optical microscope, there is a tube that is not very long, called a lens. At the two ends and inside the tube are fitted with a few crystal lenses that become magnifying glasses. In general, the more lenses, the longer the lens, the greater the magnification. Then can you freely increase the number of lenses to increase the magnification or not? Although increasing the number of lenses increases the magnification, the increase results in a deterioration of the image quality, i.e., the enlarged image becomes dimly dim and unrecognizable. It’s the true shape.
To improve the magnification of the microscope, much effort has been made to study the structure of lenses and crystal grinding technology, etc. When the magnification reaches around 2500, there is no way to increase it anymore. This is because optical microscopes have to rely on visible light to reflect objects. If the object to be observed has a wavelength size of visible light, it will pass around when the light shines on the object, not receiving the image produced by the reflected light. So we also have no way to consider the form of the object.
Through long-term research, electron waves have been discovered. Because the electron has a negative charge, it has wave motion like a light when it is attracted by high voltage electricity and produces high-speed motion. The higher the positive voltage, the faster the electron’s motion speed, the shorter its wavelength. When the positive voltage is 50 kilovolts, the electron wave wavelength is only 1 / 100,000 to 1 / 180,000 of the visible light wavelength. A microscope is made in a way that utilizes electron waves, its ability to perceive much higher than an optical microscope, which can increase the magnification by a few thousand times, even more than a million times microscope is called an electron microscope.
The simplest electron microscope comprises electron guns, objectives, projection glasses, and fluorescent screens, etc. The electron gun is composed of a V-shaped filament and a metal plate in the middle with a small hole (anode). When electricity is passed through, the hot filament emits electrons attracted by the positive voltage of the anode into accelerated motion. A part of the high-speed electron passed through the hole in the anode metal plate’s center, forming an electron beam. Because it features electron waves, it is similar to the light source of an optical microscope.
There is a light source, but a zoom lens is also required. Lenses in an electron microscope are a type of electromagnetic lens. It has two iron plates with small concentric holes and different magnetic poles combined. A coil of alternating current generates its magnetism, so it is called an electromagnetic lens. The magnetic field in the small hole can cause the generated electron beam to shift. It is similar to the phenomenon of refraction that occurs when light rays pass through a glass lens. So it resembles a glass lens, can also exert a magnifying effect. When the electron beam passes through the object to be observed, it is projected on a fluorescent screen through the objective and the magnified projection glass. Then, the object image is displayed. The magnification of the electromagnetic lens is extremely large, can reach several hundred thousand times. Three electromagnetic lenses can magnify an image 200 thousand times to over a million times.
Because the electron microscope has super high magnification and very fast analytical capabilities, it is widely used in metallurgy, biology, chemistry, physics, medicine, etc.