Polarized Microscopy With A Retardation Plate
Polarized light microscopy exploits optical properties of anisotropy to reveal detailed information about the structure and composition of materials, which are invaluable for identification and diagnostic purposes.
This interactive tutorial simulates the effect on specimen colors of adding a 530 nanometer retardation plate between the polarizer and analyzer in a polarizing microscope. To operate the tutorial, first select a sample using the Choose A Specimen pull-down menu. Next, use the Angle slider to rotate the specimen between the polarizer, analyzer, and retardation plate. The blue arrow buttons allow rotation of the specimen in 10-degree increments. As the specimen rotates, birefringent crystallites change color to reflect their interaction with the polarized light and retardation plate.
Isotropic materials, which include gases, liquids, unstressed glasses and cubic crystals, demonstrate the same optical properties in all directions. They have only one refractive index and no restriction on the vibration direction of light passing through them. Anisotropic materials, in contrast, which include 90 percent of all solid substances, have optical properties that vary with the orientation of incident light with the crystallographic axes. They demonstrate a range of refractive indices depending both on the propagation direction of light through the substance and on the vibrational plane coordinates. More importantly, anisotropic materials act as beamsplitters and divide light rays into two parts. The technique of polarizing microscopy exploits the interference of the split light rays, as they are re-united along the same optical path to extract information about these materials.
To help in the identification of fast and slow beams, or to improve contrast when polarization colors are of low order, such as dark grey, accessory (or retardation) plates can be inserted in the optical path. These will cause color changes in the specimen, which can be interpreted with the help of a polarization color chart. These charts show the polarization colors provided by optical path differences from 0 to 1800-3100 nanometers together with birefringence and thickness values. The wave plate produces its own optical path difference. When the light passes first through the specimen and then the accessory plate, the OPDs of the wave plate and the specimen are either added together or subtracted from one another in the way that "winning margins" of two races run in succession are calculated. They are added when the slow vibration directions of the specimen and accessory plate are parallel, and subtracted when the fast vibration direction of the specimen coincides with the slow vibration direction of the accessory plate. If the slow and fast directions are known for the accessory plate (they are usually marked on the mount of commercially available plates), then those of the specimen can be deduced. Since these directions are characteristic for different media, they are well worth finding out and are essential for orientation and stress studies.