The resolution limit of optical microscope is about 0.2 μ m, which is equal to the magnification of 1500-2000 times. To achieve a larger magnification, we need to use electron microscope or tunnel scanning microscope.
The magnifying glass can refocus the light to achieve the magnifying effect. The combination of the magnifying glass can get the optical microscope; the limit of the optical microscope is limited by the wavelength, so it is impossible to enlarge infinitely.
Generally, the resolution limit of a fixed wavelength optical microscope is half of the wavelength of light, and the wavelength of visible light is between 400-760nm, so the resolution limit of an optical microscope is 200nm (0.2 μ m). For objects smaller than 0.2 μ m, the optical microscope will not be able to distinguish them, just as the tactile resolution of the human hand cannot exceed the minimum distance between the tactile cells.
The magnification is a subjective term, which is defined as the ratio of the size of the object seen by the human eye to the actual size when the apparent distance is 25cm. The resolution of the optical microscope is 0.2 μ m, equivalent to the magnification of 1500-2000 times, which is enough for us to see the structure of general cells.
If we use shorter wavelength electromagnetic wave, we can achieve greater magnification, but this is beyond the wavelength range of visible light; in 1931, the British physicist Lucca invented the electron microscope, according to the principle of wave particle duality, the electron beam has shorter de Broglie wave length, so it can achieve smaller resolution.
The acceleration voltage of the electron corresponds to its own wavelength. When the voltage is 100kV, the wavelength of the electron beam is about 0.004nm (the actual resolution can only reach 0.2nm), which is also far less than the wavelength of visible light. Therefore, the resolution limit of the electron microscope is far beyond that of the optical microscope, and it can achieve a maximum magnification of 3 million times, and it can distinguish viruses, mitochondria, DNA and other small objects.