Introduction to knowledge related to polarizing microscopy
Polarization microscope is a microscope that inserts a polarizer and a polarizer into the optical system of an optical microscope to examine the anisotropy and birefringence of the sample. The polarizing mirror and the polarizing mirror are both made of a polarizing prism or a Nicol prism of a polarizing plate. The former is installed between the light source and the sample, while the latter is installed between the objective lens and the contact lens or above the contact lens. In biological samples, muscle fibers, bones, and teeth exhibit anisotropy, while starch granules, chromosomes, and spindles exhibit birefringence, making them used in the chemical research of tissue cells. The light source can use single wavelength light. Due to the significantly weaker birefringence of biological samples compared to metallographic, rock, or crystalline materials, their interference colors are sometimes utilized through the addition and subtraction phenomena caused by sensitive polarization plates.
1, Natural light and polarized light
Light is an electromagnetic wave that belongs to the transverse wave (the direction of vibration is perpendicular to the direction of propagation). All actual light sources, such as sunlight, candlelight, fluorescent lamps, and tungsten filament lamps, emit light that is called natural light. These lights are the sum of the luminescence of a large number of atoms and molecules. Although the electromagnetic wave vibration direction emitted by a certain atom or molecule is consistent at a certain moment, the vibration direction emitted by each atom and molecule is also different, and the frequency of this change is extremely fast. Therefore, natural light is the sum of the light emitted by each atom or molecule, and it can be considered that the probability of its electromagnetic wave vibration in all directions is equal.
Natural light passes through certain substances in the window, and after reflection, refraction, and absorption, the vibration waves of electromagnetic waves are limited in one direction, while the vibration waves of other directions are greatly weakened or eliminated. This type of light that vibrates in a certain direction is called polarized light. The plane formed by the vibration direction of polarized light and the propagation direction of light waves is called the vibration surface.
Linear polarized light, circularly polarized light, and elliptically polarized light
1. Linear polarized light
Linear polarized light, due to the fact that the vibration direction of light is in the same plane, is also called plane polarized light. When viewed in the direction of light propagation, the vibration direction of this type of light is a straight line, so it is also called linear polarized light or linearly polarized light.
2. Circularly polarized light and elliptically polarized light
(1) The phenomenon of birefringence of light and the optical axis of crystals
When a beam of light enters an anisotropic crystal, it splits into two rays propagating in different directions. This phenomenon is called birefringence. Both beams of light that undergo birefringence are polarized light. One of these two beams of light always follows the law of refraction of light, and the propagation speed does not change when changing the direction of incidence. This beam of light is called an ordinary ray, represented by o; The other beam of light does not follow the law of refraction. When the direction of the incident light changes, its propagation speed also changes, and the refractive index of the light is different. This beam of light is called extraordinary light and is represented by e.
In anisotropic crystals, there are certain special directions in which birefringence does not occur. Ordinary and extraordinary light rays propagate in the same direction and speed, and these directions are called the optical axis of the crystal. A crystal with one optical axis is called a uniaxial crystal, and a crystal with two optical axes is called a biaxial crystal. For biaxial crystals, both beams of light after birefringence are very light.
(2) Wave chip
Wave plates, abbreviated as wave plates, can be used to change or test the polarization of light. When natural light is incident along the uniaxial crystal axis, no birefringence occurs. If the o-light and e-light generated when incident perpendicular to the crystal optical axis still propagate along the original incident direction, but with different propagation speeds and refractive indices, and the difference in propagation speeds is the greatest. If a thin film is cut in the direction parallel to the optical axis of a crystal, and the surface of the chip is flat with the optical axis, the resulting chip is called a wave chip. When polarized light is incident perpendicular to the optical axis of the wave plate, it forms o light and e light with the same propagation direction but different propagation speeds inside the wave plate.
