Why electron microscopes provide far higher resolution than optical microscopes
Because electron microscopes use an electron beam and optical microscopes use visible light, and the wavelength of the electron beam is shorter than the wavelength of visible light, the resolution of an electron microscope is much higher than that of an optical microscope.
The resolution of a microscope is related to the cone angle of incidence and wavelength of the electron beam passing through the sample.
The wavelength of visible light is about 300-700 nm, while the wavelength of the electron beam is related to the accelerating voltage. According to the principle of wave-particle duality, the wavelength of high-speed electrons is shorter than that of visible light, and the resolution of a microscope is limited by the wavelength it uses, so that the resolution of an electron microscope (0.2 nanometres) is much higher than that of an optical microscope (200 nanometres).
The application of electron microscopy technology is based on the optical microscope, which has a resolution of 0.2 μm, and the transmission electron microscope, which has a resolution of 0.2 nm, i.e., the transmission electron microscope is enlarged by a factor of 1,000 on the basis of the optical microscope.
How to increase the resolution of a microscope
The resolution of a microscope is the ability to clearly distinguish the minimum distance between two object points that can be distinguished by the microscope.
It is calculated by the formula σ = λ/NA
Where σ is the resolution; λ is the wavelength of light; NA is the numerical aperture of the objective lens (NA = nsina/2,n is the refractive index of the medium, a is the aperture angle, i.e., the sample to the objective lens aperture angle). It can be seen that the resolution of the objective lens is determined by the NA value of the objective lens and the wavelength of the illumination light source, the larger the NA value, the shorter the wavelength of the illumination light, the smaller the σ value, the higher the resolution.
To improve the resolution, i.e., reduce the σ value, the following measures can be taken
(1) Reduce the wavelength λ and use a short wavelength light source.
(2) Increase the n value of the medium to increase the NA value (NA = nsina/2).
(3) Increase the aperture angle a to increase the NA value.
(4) Increase the contrast between light and dark.
