What is the theoretical limit of magnification of an optical microscope?
The resolution limit of an optical microscope is approximately 0.2 micrometers, equivalent to a magnification of 1500-2000 times; To achieve greater magnification, an electron microscope or tunneling scanning microscope must be used.
Magnifying glasses can refocus light and achieve magnification effects, and using a combination of magnifying glasses can result in an optical microscope; The limit of optical microscopes is limited by wavelength and cannot be infinitely magnified.
Generally, the resolution limit of a fixed wavelength optical microscope is half of the wavelength of light, with visible light wavelengths between 400-760nm. Therefore, the resolution limit of an optical microscope is 200nm (0.2 micrometers). Objects smaller than 0.2 microns cannot be distinguished by optical microscopes, just like the tactile resolution of a human hand, which cannot exceed the small distance between tactile cells.
And magnification is a subjective term, defined as the ratio of the size of an object seen by the human eye to its actual size at a clear visual distance of 25cm. The resolution of an optical microscope is 0.2 micrometers, which is equivalent to a magnification of 1500-2000 times. This is enough for us to see the structure of ordinary cells clearly.
If we use electromagnetic waves with shorter wavelengths, we can achieve greater amplification, but this is already beyond the wavelength range of visible light; In 1931, British physicist Lusca invented the electron microscope. According to the principle of wave particle duality, electron beams have shorter wavelengths of de Broglie waves, allowing for smaller resolutions.
The acceleration voltage of electrons corresponds to their own wavelength. When the voltage is 100 kV, the wavelength of the electron beam is about 0.004nm (the actual resolution can only reach 0.2nm), which is much smaller than the wavelength of visible light. Therefore, the resolution limit of electron microscopes far exceeds that of optical microscopes, and can achieve a magnification of up to 3 million times, which can distinguish small objects such as viruses, mitochondria, DNA, etc.
