Fundamental Principles and Features of Polarizing Microscopes
① Characteristics of the Polarizing Microscope The polarizing microscope is an instrument used to identify the optical properties of the fine microstructure of substances. Any substance with birefringence can be clearly distinguished under a polarizing microscope. Some of these substances can also be observed by staining methods, while others cannot and must be examined with a polarizing microscope. The characteristic of a polarizing microscope is that it converts ordinary light into polarized light for specimen observation, so as to identify whether a substance is isotropic (single refraction) or anisotropic (birefringence). Birefringence is a fundamental property of crystals. Therefore, polarizing microscopes are widely used in mineralogy, chemistry and other fields.
In biology, many structures also exhibit birefringence and need to be distinguished using a polarizing microscope. In botany, it is used to identify fibers, chromosomes, spindle fibers, starch grains, cell walls, and to check whether crystals exist in cytoplasm and tissues. In plant pathology, the invasion of pathogens often causes changes in the chemical properties of tissues, which can be identified by polarizing microscopy. In human and zoological studies, polarizing microscopy is commonly used to examine bones, teeth, cholesterol, nerve fibers, tumor cells, striated muscle and hair.
② Basic Principles of the Polarizing Microscope (1) Single Refraction and Birefringence When light passes through a substance, if the optical properties and propagation path of light remain unchanged regardless of the illumination direction, the substance is optically isotropic, also known as a singly refractive body. Examples include common gases, liquids and non-crystalline solids. If the speed, refractive index, absorptivity, vibration direction and amplitude of light vary with the illumination direction when light passes through a substance, the substance is optically anisotropic, also called a birefringent body, such as crystals and fibers.
(2) Polarization of Light According to vibration characteristics, light waves can be divided into natural light and polarized light. Natural light vibrates in numerous planes perpendicular to the propagation axis, with equal vibration amplitude and identical frequency on all planes. After reflection, refraction, birefringence or absorption, natural light can be converted into light waves that vibrate in only one direction; this light is called polarized light. The simplest form is linearly polarized light vibrating along a straight line. When light enters a birefringent body, it splits into two linearly plane-polarized light beams A and B, whose vibration directions are perpendicular to each other, with different speeds, refractive indices and wavelengths.
