Light-emitting body of a homogeneous body: When a light wave propagates in a homogeneous body, it vibrates in any direction, and its propagation speed does not change, and the refractive index values ​​are equal. Therefore, the light body of the homogeneous body is a sphere, as shown in Figure 2-8-2.
Figure 2-8-2 Light body of homogeneous body
One-axis crystal light body: The intermediate crystal mineral crystals have the same horizontal crystal axis units and the same optical properties in the horizontal direction. These minerals have maximum and minimum two main refractive index values, which are represented by the symbols Ne and N0, respectively. When the direction of vibration of the light wave is parallel to the z-axis, the corresponding refractive index value is Ne, and when the direction of vibration of the light wave is perpendicular to the z-axis, the corresponding refractive index value is N0. When the direction of the vibration of the light wave is oblique to the z-axis, the corresponding refractive index magnitude is gradually changed between Ne and N0, and is represented by a symbol. Obviously, the one-axis crystal light body is a spheroid with the z-axis as the rotation axis.
The axis of rotation is the optical axis of the mineral. The values ​​of Ne and N0 are different in different minerals. If Ne>N0, the photometric body is a long-axis spheroid, as shown in Fig. 2-8-3a. The light-emitting body, the corresponding mineral is called a one-axis crystal positive mineral. If Ne < N0, the light body is a flat spheroid, as shown in Figure 2-8-3b, such a light body is called a one-axis crystal light-emitting body, and the corresponding mineral is called a-axis crystal light. Sex minerals. In these two types of minerals, the value of Ne changes with the angle between the incident light and the z-axis. It is often expressed by Ne', but the value of N0 is always constant, so Ne is called extraordinary light, and N0 is called constant light.
It can be seen from the above that the one-axis crystal light body is a spheroid, and its rotation axis is the Ne axis regardless of the positive and negative optical properties, and its horizontal axis is the N0 axis. Ne and N0 represent the maximum and minimum refractive indices of a monolithic mineral, called the primary refractive index. The relative size of the main refractive index Ne and N0 determines the optical positive and negative of the monoaxial mineral. The difference between Ne and N0 is the maximum birefringence of the monoaxial mineral.
When identifying transparent minerals under a polarizing microscope, all the faces of the mineral crystals in different directions (ie, the light-weight body cuts in different directions) are encountered. The main cut planes of the one-axis crystal light have the following three types: the vertical optical axis cut surface. As shown in Fig. 2-8-4a, the cut surface is a circular cut surface with a radius equal to N0. When the light wave is perpendicular to the cut surface, no birefringence occurs, and the vibration direction of the incident light wave is not changed. The corresponding refractive index is equal to N0, and the birefringence is obtained. It is equal to zero. The one-axis crystal light body has only one such circular section. The second main section is the plane of the parallel optical axis: as shown in Fig. 2-8-4b, the elliptical section has a length and a radius of Ne and N0, respectively. When this tangential plane is incident, birefringence decomposition occurs to form two polarized lights, and the vibration direction must be parallel to the length and radius of the elliptical section, respectively, and the corresponding refractive index is equal to the length ellipses N0 and Ne' of the elliptical section, respectively. The birefringence is the length of the elliptical section The difference in radius is the maximum birefringence of the monoaxial mineral. This section is the main section of the monoaxial crystal, which contains the optical axis, and there are countless. The third section is the section of the oblique optical axis: figure 2- 8-4c, which is an elliptical section whose length and radius are N0 and Ne', respectively. When the plane of the light is perpendicular, the birefringence is decomposed into two polarized lights. The vibration direction is parallel to the length and radius of the elliptical section, and the corresponding refractive index is equal to The radius of the ellipse is N0 and Ne'. The birefringence is equal to the difference between N0 and Ne', and its magnitude is gradually changed between zero and the maximum birefringence. In the length of the elliptical section of any oblique optical axis of the one-axis crystal light body, There is always one that is N0.
Axial crystal light body: Low-grade crystal group mineral crystal belongs to biaxial crystal mineral. The three crystal axis units of such mineral crystals are not equal [a≠b≠c] indicating their three-dimensional spatial heterogeneity. These minerals have three main refractive index values: large, medium and small. They are respectively equivalent to three vibration directions perpendicular to each other, and the symbols Ng, Nm, and Np often represent three major refractive index values ​​of large, medium, and small. When the light wave vibrates in other directions, the corresponding refractive index value is gradually changed between Ng, Nm, Np, and is generally represented by the symbols Ng' and Np'. Obviously, the biaxial crystal light body is an ellipsoid with three axes, as shown in Figure 2-8-5. In the biaxial crystal light body, three mutually perpendicular axes represent the three main optical directions of the biaxial crystal mineral, and are called optical spindles, which are referred to as spindles, that is, Ng axis, Nm axis, and Np axis. A section containing two spindles, called the main surface [main section]. The biaxial crystal light body has three mutually perpendicular main axis faces, that is, an NgNp face, an NgNm face, and an NmNp face. Because the biaxial crystal light body is an unequal triaxial ellipsoid, a series of elliptical cut surfaces can be continuously formed on the side of the light body by the Nm axis (Fig. 2-8-5b). One of the radii of these sections is the Nm axis, and the other radius is between Ng and Np, and a circular section with a radius equal to Nm can be found between them. On the other side of the light body, another circular cut surface [2 - 8 - 5b] can also be cut out. When the two circular cut planes of the light wave are incident, no birefringence occurs, and thus the two directions are the optical axis directions, which are indicated by the symbol "OA" (2-8-5b and 2-8-6). Through the center of the light body, only two circular cut surfaces can be cut out, that is, there are only two optical axes, so it is called a two-axis crystal.
The face including the two optical axes is called the optical axis, and is represented by the symbol "AP". The direction perpendicular to the optical axis plane through the center of the luminous body is called the optical normal. The acute angle between the two optical axes is called the optical axis angle and is represented by the symbol "2 V ". The bisector of the acute angle between the two optical axes is represented by the symbol "Bxa"; the bisector of the obtuse angle between the two optical axes is called the obtuse bisector, and is represented by the symbol "Bxo". The optical sign of the biaxial crystal mineral is determined according to the relative sizes of Ng and Nm. "Ng-Nm>Nm-Np is positive light. At this time, the Nm value is closer to Np, and the circular section with the radius of Nm is closer to the Np axis (Fig. 2-8-6). The two lights of the vertical circular section The axis is close to the Ng axis. The acute bisector [Bxa] of the two optical axes must be the Ng axis. Conversely, when Ng-Nm
The positive light crystal corresponds to the NgNm surface of the major axis; the negative optical crystal corresponds to the NmNp surface of the major axis. When the light wave is perpendicular to such a plane, birefringence occurs and is decomposed into two polarized lights. The vibration directions are parallel to the Nm and Ng axes or the Nm and Np axes, respectively; the refractive indices are equal to Ng and Nm or Nm and Np, respectively. The birefringence is equal to Ng-Np or Nm -Np and its magnitude is between zero and maximum. But whether the light is positive or negative, the birefringence of the vertical Bxa section is always smaller than the birefringence of the vertical Bxo section. The planes of the parallel optical axis plane, the vertical Bxa and the vertical Bxo are equivalent to the major plane of the biaxial crystal light body, that is, the tangent plane belonging to the vertical principal axis. Oblique cross section (Fig. 2-8-7g, h): the non-vertical main axis, and the cross section of the vertical optical axis belongs to the oblique cross section. There are a myriad of these cut surfaces, which are elliptical cut surfaces (non-spindle faces). The oblique cross-sections can be roughly divided into two types (1) oblique cross-sections of the vertical principal faces (ie, vertical faces NgNp, NgNm faces, NmNp faces). The cut surface is called a semi-arbitrary cut surface (Fig. 2-8-7g). In the ellipse length and radius of this cut surface, there is always one radius which is the major axis [Ng or Nm or Np], and the other radius is Ng' or Np'. The circular section of the axis is also a special case of this type of section. (2) Any oblique section (Fig. 2-8-7h). The length and radius of the ellipse are Ng' and Np' respectively. The light wave is perpendicular to such oblique section incidence (Chen When both the optical axis and the main axis are in any direction, birefringence occurs and the two polarized lights are decomposed. The vibration directions are parallel to the ellipse length and the radial direction respectively; the refractive index values ​​are equal to the long and short radii respectively. The birefringence is equal to the difference between the long and short radii, and its size The change is between zero and maximum.
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