Some Introduction to Polarizing Microscope
A polarizing microscope is a microscope that inserts a polarizer and an analyzer into the optical system of an optical microscope to check the anisotropy and birefringence of a sample]. Both the polarizer and the analyzer are made of polarizing prisms or Nicol prisms of polarizing plates. The former is installed between the light source and the sample, and the latter is installed between the objective lens and the eyepiece or on the eyepiece. In biological samples, muscle fibers, bones and teeth have anisotropy, and starch grains, chromosomes and spindles have birefringence, so they are used in the chemical research of tissue cells. The light source is preferably single-wavelength light. Since the birefringence of biological samples is significantly weaker than that of metallographic, rock or crystal, sometimes the interference color is also used by the addition and subtraction phenomenon caused by the sensitive analyzer.
1. Natural light and polarized light
Light is a kind of electromagnetic wave, which belongs to 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 lamps, are called natural light. These lights are the sum of light emitted by a large number of atoms and molecules. Although the vibration direction of the electromagnetic waves emitted by an atom or molecule at a certain moment is the same, the vibration direction of 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, which can be regarded as The vibration of its electromagnetic wave has equal probability in all directions.
When natural light passes through certain substances in the window, after reflection, refraction, and absorption, the vibration waves of electromagnetic waves are limited to one direction, and the electromagnetic waves vibrating in other directions are greatly weakened or eliminated. This kind of light vibrating in a certain direction is called polarized light. The plane formed by the vibration direction of polarized light and the light wave propagation direction is called the vibration plane.
2. Linearly polarized light, circularly polarized light and elliptically polarized light
1. Linearly polarized light
Linearly polarized light is also called plane polarized light because the vibration direction of the light is in the same plane. Looking at the direction of light propagation, the vibration direction of this light is a straight line, so it is also called linearly polarized light or linearly polarized light.
2. Circularly polarized light and elliptically polarized light
(1) Birefringence of light and the optical axis of the crystal
When a beam of light is injected into an anisotropic crystal, it will split into two beams that propagate along different directions. This phenomenon is called birefringence. The two beams of light that are birefringent are polarized light. One of the two beams of light always obeys the law of refraction of light, and the propagation speed does not change when the incident direction is changed. This light is called ordinary light, denoted by o; the other beam does not obey the law of refraction. When , its propagation speed also changes accordingly, and the refractive index of light is different. This beam is called extraordinary light and is represented by e.
In anisotropic crystals, there are certain special directions in which birefringence does not occur, ordinary rays and extraordinary rays travel in the same direction and speed, and these directions are called the optical axis of the crystal Crystals with an optical axis It is called a uniaxial crystal, and a crystal with two optical axes is called a biaxial crystal. For biaxial crystals, the two rays after birefringence are both extraordinary rays.
(2) wave chip
The wave plate, referred to as the wave plate, can be used to change or test the polarization of light. When natural light is incident along the optical axis of a uniaxial crystal, birefringence does not occur. If the o-ray and e-ray generated when incident perpendicular to the crystal optical axis still propagate along the original incident direction, but the propagation speed and refractive index are different, and the difference in propagation speed is the largest. If a thin slice is cut in a direction parallel to the optical axis of the one-axis crystal, the surface of the wafer is parallel to the optical axis, and the wafer made in this way is called a wave plate. When polarized light is incident perpendicular to the optical axis of the wave plate, a wave plate is formed. o-rays and e-rays traveling in the same direction but at different velocities. If the wave plate is thicker, it is an integer multiple of the wavelength of the o-ray and e-ray, and this wave plate is called a full-wave plate. And so on, there are half wave plates and 1/4 wave plates and so on.
(3) Formation of circularly polarized light and elliptically polarized light
When a beam of natural light is incident perpendicular to the optical axis of a uniaxial crystal, the two beams of polarized light whose vibration planes are perpendicular to each other are incoherent. Because natural light is produced by different molecules and atoms in the light source, there is no fixed phase difference, so no interference occurs. But when a beam of monochromatic polarized light passes through a birefringent material[/url], the two beams of polarized light produced can be coherent. It is equivalent to the synthesis of two mutually perpendicular vibrations of the same period.
