Polarizing Microscope: Its Basic Principles and Properties
① Characteristics of the Polarizing Microscope
A polarizing microscope is an instrument for identifying the optical properties of the microstructures 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 microscopic examination, so as to identify whether a substance is singly refracting (isotropic) or birefringent (anisotropic). Birefringence is a fundamental property of crystals. Therefore, polarizing microscopes are widely applied in mineralogy, chemistry and other fields. Many structures in biology also exhibit birefringence and need to be distinguished by polarizing microscopy. 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 pathogenic bacteria often changes the chemical properties inside tissues, which can be identified by polarizing microscopy. In human and animal histology, polarizing microscopy is commonly used to identify bones, teeth, cholesterol, nerve fibers, tumor cells, striated muscle, hair and so on.
② Basic Principles of the Polarizing Microscope
(1) Isotropy and Birefringence When light passes through a substance and the optical properties and light path remain unchanged regardless of the incident direction, the substance is optically isotropic, also known as a singly refracting body, such as ordinary gases, liquids and non-crystalline solids. When light passes through another substance, the velocity, refractive index, absorptivity, vibration and amplitude of the light wave vary with the incident direction. Such substances are optically anisotropic, also called birefringent bodies, e.g. crystals and fibers.
(2) Polarization of Light Light waves are classified into natural light and polarized light according to their vibration characteristics. Natural light vibrates in numerous planes perpendicular to the propagation axis, with identical amplitude and frequency in all planes. After reflection, refraction, birefringence or absorption, natural light can be transformed into light waves vibrating along only one direction; this light is called polarized light. The simplest form is linearly polarized light that vibrates along a straight line. When light enters a birefringent body, it splits into two linearly plane-polarized beams A and B as shown in the figure. Their vibration directions are perpendicular to each other, and they differ in velocity, refractive index, wavelength and other parameters.
