Basic working principle of polarizing microscope
1, Monorefractive and birefringence:
When light passes through a substance, if the properties and path of light do not change due to the direction of illumination, this substance has "isotropy" in optics, also known as a single refractor, such as ordinary gases, liquids, and amorphous solids; If the speed, refractive index, absorption and polarization, amplitude, etc. of light passing through another material vary depending on the direction of illumination, this material has "anisotropy" in optics, also known as a birefringent material, such as crystals, fibers, etc.
2, Polarization phenomenon of light:
Light waves can be divided into natural light and polarized light based on their vibration characteristics. The vibration characteristics of natural light are that there are many vibration surfaces on the vertical axis of light wave propagation, and the amplitude distribution of vibration on each plane is the same; Natural light, through reflection, refraction, birefringence, and absorption, can produce light waves that vibrate only in one direction, which are called "polarized light" or "polarized light.".
3, The generation and effect of polarization:
The important components of a polarizing microscope are the polarizing device - the polarizer and the detector. In the past, both were composed of Nicola prisms, which were made of natural calcite. However, due to the limitation of large crystal volume, it was difficult to achieve large polarization areas. Polarization microscopes used artificial polarizers instead of Nicol mirrors. Artificial polarizers are made from crystals of quinoline sulfate, also known as graphite, and have a green olive color. When ordinary light passes through it, it can obtain linearly polarized light that vibrates only in a straight line. A polarizing microscope has two polarizing mirrors, one of which is located between the light source and the object being tested and is called a polarizing mirror; Another device located between the objective lens and the eyepiece is called a "polarizing mirror", which has a handle that extends outside the lens barrel or middle attachment for easy operation, and has a rotation angle scale on it. When the light emitted from the light source passes through two polarizers, if the vibration directions of the polarizer and the polarizer are parallel to each other, that is, in the "parallel polarizer position", the field of view is brighter. On the contrary, if the two are perpendicular to each other, that is, in an orthogonal calibration position, the field of view is completely dark. If the two are tilted, the field of view indicates a moderate degree of brightness. From this, it can be seen that the linearly polarized light formed by the polarizing mirror can pass through completely if its vibration direction is parallel to the vibration direction of the polarizing mirror; If skewed, only a portion will pass through; If it is vertical, it cannot pass through at all. Therefore, when using a polarizing microscope for inspection, the principle is to ensure that the polarizing mirror and the inspection mirror are in an orthogonal inspection position.
4, Birefringent body under orthogonal bias position:
In the case of orthogonality, the field of view is dark. If the object being tested exhibits an isotropic single refractor in optics, no matter how the stage is rotated, the field of view remains dark. This is because the vibration direction of the linearly polarized light formed by the polarizing mirror remains unchanged and perpendicular to the vibration direction of the polarizing mirror. If the object being tested has birefringence characteristics or contains substances with birefringence characteristics, the field of view in the area with birefringence characteristics becomes 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 with different vibration directions. When these two types of light pass through the polarizing mirror, because the other beam of light is not orthogonal to the polarization direction of the polarizing mirror, the human eye can see bright images through the polarizing mirror. When light passes through a birefringent material, the vibration directions of the two types of polarized light formed vary depending on the type of object.
When the birefringent body rotates the stage in an orthogonal manner, the image of the birefringent body undergoes four changes in brightness during 360 ° rotation, and darkens every 90 °. The dimming position is the position where the two vibration directions of the birefringent body are consistent with the vibration directions of the two polarizers, known as the "extinction position". When the object being tested rotates 45 ° from the extinction position, it becomes the brightest, which is called the "diagonal position". This is because when the polarized light reaches the object at a deviation of 45 °, some of the light can be decomposed and pass through the polarizer, making it bright. Based on the above basic principles, polarization microscopy can be used to determine isotropic single refractors, anisotropic birefringents, and substances.
5, Interference color:
In the case of orthogonal offset detection, using mixed light of different wavelengths as the light source to observe the birefringent body, when rotating the stage, not only the brightest diagonal position appears in the field of view, but also the color can be seen. The reason for the appearance of colors is mainly caused by interference colors, and of course, the object being tested may not be colorless and transparent. The distribution characteristics of interference colors are determined by the type and thickness of the birefringent material, which is due to the dependence of the corresponding delay on the wavelength of different colored light. If the delay in one region of the object being tested is different from that in another region, the color of the light passing through the polarizing mirror will also be different.
