The two polarization filters of a polarizing microscope are positioned at 90 degrees to obtain the so-called "dark spot". At this point, the field of view is completely black. If the sample exhibits isotropy in optics (a single refractor), no matter how the stage is rotated, the field of view remains dark. This is because the direction of vibration of the linearly polarized light formed by the polarizing mirror does not change. According to Marius' law, the intensity of transmitted light is 0. If the sample has birefringence characteristics, the field of view will become brighter. This is because the linearly polarized light emitted from the polarizing mirror enters the birefringent body and produces two types of linearly polarized light (o light and e light) with different vibration directions. When these two types of light pass through the polarizing mirror, because e light does not comply with the refraction law and its polarization direction is not 90 degree with the polarizing mirror, a bright image can be seen in the field of view through the polarizing mirror.
1, Reflection of polarized light on anisotropic metal grinding surfaces.
Observing anisotropic crystals under orthogonally polarized light. Due to the different orientations of each grain in the metallographic grinding surface of optically anisotropic metals, i.e. the different positions of the "optical axis" of each grain, the polarization planes of reflected polarized light in each grain rotate at different angles. By using a polarizing microscope, grain contrast with different brightness can be observed in the eyepiece. Rotating the stage is equivalent to changing the angle between the polarization direction and the optical axis. Rotate the stage 360 degree and observe four bright and four dark changes in the field of view. This is the polarization effect of anisotropic crystals under orthogonal polarized light.
2, Reflection of polarized light on isotropic metal grinding surfaces
When isotropic metals are observed under orthogonally polarized light, due to their consistent optical properties in all directions, the polarization plane of the reflected light cannot be rotated. Linear polarized light is vertically incident on the isotropic metal grinding surface, and because the reflected light is still linearly polarized, it is blocked by the orthogonal polarizing mirror. Therefore, the reflected polarized light cannot pass through the polarizing mirror, and the field of view is dark, presenting an extinction phenomenon. The rotating loading platform also has no changes in brightness. This is the phenomenon of isotropic metals under orthogonal polarization. If studying isotropic metals under orthogonal polarization, a special method of changing the optical properties of the original crystal is required. Commonly used methods include deep etching or surface anodizing treatment. For example, some people use deep etching to observe the needle like martensite and original austenite grains in high carbon nickel chromium steel. Some people use this method to observe martensite, bainite, low-carbon martensite, and other fields.
3, Polarization analysis of non-metallic inclusions
The correct identification of non-metallic inclusions often requires the use of multiple detection methods to obtain accurate judgments. Among them, the metallographic method is a relatively simple and common approach, occupying an important position. Usually, optical properties are analyzed under a polarizing microscope using bright, dark, and polarized light fields.
