Basic principles of polarizing microscopy
(1) Single refraction and birefringence: When light passes through a certain substance, if the nature and path of the light do not change depending on the direction of illumination, the substance is optically "isotropic", also known as single refraction. Objects, such as ordinary gases, liquids and amorphous solids; if light passes through another substance, the speed, refractive index, absorptivity of the light and the vibration and amplitude of the light skin are different depending on the direction of the irradiation. This substance is in optical The above has "anisotropy", also known as birefringent bodies, such as crystals, fibers, etc.
(2) Polarization phenomenon of light: Light waves can be divided into natural light and polarized light according to the characteristics of vibration. The vibration characteristic of natural light is that it has many vibration planes on the vertical light wave transmission axis. The amplitude of vibration on each plane is the same and its frequency is also the same. Natural light can vibrate in only one direction through reflection, refraction, birefringence and absorption. The light wave is called "polarized light" or "polarized light".
The simplest is linearly polarized light that vibrates only in a straight line. When light enters a birefringent body, it is divided into two linear plane polarized lights, A and B, as shown in the figure. The vibration directions of the two are perpendicular to each other, but the speed, refractive index, and wavelength are different.
(3) The generation and function of polarized light: The most important components of a polarizing microscope are the polarizing devices-polarizer and analyzer. In the past, both were composed of Nicol prisms, which are made of natural calcite. However, due to the limitation of the large size of the crystal, it is difficult to obtain polarization over a large area. Recently, polarizing microscopes use artificial polarizers. To replace the Nicol Shun lens. Artificial polarizers are made of quinoline sulfate crystals, also known as Herapathite, and are green-olive in color. When ordinary light passes through it, it can obtain linearly polarized light that only vibrates in a straight line. A polarizing microscope has two polarizers. One is installed between the light source and the object to be inspected and is called the "polarizer"; the other is installed between the objective lens and the eyepiece and is called the "analyser". It has a handle to reach into the lens barrel or the middle The outer side of the attachment is convenient for operation, and there is a scale for the rotation angle. When the light emitted from the light source passes through two polarizers, if the vibration directions of the polarizer and analyzer are parallel to each other, that is, in a "parallel analyzer stand" situation, the field of view will be the brightest. On the contrary, if the two are perpendicular to each other, that is, in the "orthogonal correction position", the field of view is completely dark. If the two are tilted, the field of view shows moderate brightness. It can be seen from this that if the vibration direction of the linearly polarized light formed by the polarizer is parallel to the vibration direction of the analyzer, it can pass completely; if it is deflected, only a part of it can pass; if it is perpendicular, it cannot pass at all. Therefore, when using a polarizing microscope for inspection, in principle, the polarizer and analyzer should be in the orthogonal analyzing position.
