How to calibrate the polarizer of a polarizing microscope
In practical operation, the vibration directions of the upper and lower polarizing mirrors of a polarizing microscope should be orthogonal to each other, or in the east-west or north-south direction, and each should be consistent with the horizontal and vertical directions of the crosshair of the eyepiece. Sometimes only one lower polarizer is used for observation, and the vibration direction of the lower polarizer must be determined, so the polarizer must be calibrated during operation.
1. Detection of crosshair in eyepiece
Check whether the crosshair of the eyepiece is orthogonal and whether it is consistent with the vibration direction of the upper and lower polarizing mirrors. Simultaneously select a piece of biotite with extremely complete cleavage, move it to the center of the crosshair of the eyepiece, align the cleavage seam parallel to one of the crosshairs, record the scale number on the stage, and then rotate the stage to make the cleavage seam parallel to another crosshair, record the scale number on the stage. The difference between the two scale degrees is 90 °, indicating that the crosshairs are orthogonal.
2. Determination and correction of the vibration direction of the polarizing mirror
The reason for using biotite to check the vibration direction of the polarizer is that biotite is a widely distributed transparent mineral that is highly characteristic under single polarization. Firstly, find a clear and cleaved piece of biotite, move it to the center of the crosshair of the eyepiece, push out the upper polarizer, rotate the stage once, and observe the color change of biotite. Because biotite absorbs the vibration light in the cleavage direction * strongly, when the biotite color reaches its deepest, the direction of the cleavage seam is the vibration direction of the lower polarizer.
3. Correction of orthogonal polarization of upper and lower polarizing mirrors
After aligning the direction of the lower polarizer, remove the thin film and push it into the upper polarizer to observe whether the field of view is completely black, that is, whether it is in an extinction state. If it is all black, it indicates that the vibration directions of the upper and lower polarizations are orthogonal to each other. Otherwise, the upper polarizer needs to be calibrated by rotating it until the field of view becomes * dark. When rotating, the stop screw of the upper polarizer must be loosened first, calibrated correctly, and then tightened.
