Optical axes

Interior acute angle between optic axes of biaxial mineral. 

Biaxial crystals offer the possibiUty of coincidence of the phase matching direction with one of the optic axes. This highly desirable situation, called non-critical phase matching, is quite tolerant of divergence of the incident beam from the most efficient phase matching direction. 

The plane of vibration of a linearly polarized beam emerging from a quarter-wave plate depends on the phase difference 8 between the interfering ray and is measured by the angle a between the optical axes of the quarter-wave plate and the analyzer ... 

The displacement of the Images Is in general so great as to canse the optic axes to converge to points eltoated within two or three feet from the nose, instead of the true distance of the objeete. The efifeot of tine... 

Biaxial crystals under similar optical conditions produce convergent light figures like that shown in Fig. 52 6, when the acute bisectrix of the optic axes lies along the line of vision and the vibration directions... 

Fig. 56. Dispersion of optic axes in orthorhombic crystals, a. p > u. b-d. Crossed axial plane dispersion.

The observation of convergent light figures may often provide confirmation of the orientation of the indicatrix. The plane of the optic axes is the ay plane, while the normal to this plane is the j8 vibration direction. For a positive crystal the acute bisectrix is the y vibration direction, while for a negative crystal it is the a vibration direction. 

Even when the crystals being examined are quite irregular fragments, it may be possible to obtain some information oil their symmetry, if certain types of dispersion of the optic axes are observed (see pp. 86-9). 

The value of measurements of the magnitude of the optic axial angle has been urged by Bryant (1932) and where dispersion of the optic axes occurs, the variation of the optic axial angle with light frequency is a highly characteristic feature which is valuable evidence for identity. (See Bryant, 1941, 1943.)... 

Figure 1. The biaxial indicatrix, whose principal axes correspond to the refractive indices na < n(i < ny. The optic axes are indicated by dashed lines and the optic angle is denoted 2V. N is the principal birefringence.

Attention must now be paid to the exponential factor, exp( 2nir (n iij)/A), in Equation 6.5, where (n it) is known as the complex refractive index of a substance. It can be seen that the effect of this factor upon the electromagnetic wave increases with the distance Irl that the light travels in that medium. In the general case of an anisotropic medium, n and are referred to as a specific set of axes, usually chosen to coincide with the optical axes of the medium. For example, the axes of maximum and minimum transmittance are selected for anisotropic absorption. The extinction f for an anisotropic medium is related to the extinction coefficient through Equation 6.9. 

Orientation of an optical element. The lines on the front face are parallel to the principal optical axes of the element. 

Extinction in one direction. Optic axes parallel and usually normal a... 

A linear birefringence involves the occurrence of an anisotropy between the components of the refractive index associated with linearly polarized monochromatic light whose polarization vector is directed along two perpendicular optical axes. In the examples discussed here the principal optical axis lies parallel to an external applied field, and... 

Crystals may have two optic axes, not necessarily perpendicular, in which case they are termed biaxial. Orthorhombic, monoclinic and triclinic crystals are biaxial hexagonal, tetragonal and trigonal crystals are uniaxial cubic crystals are isotropic. In the following discussion attention is confined to uniaxial crystals. 

The Babinet compensator is a calibrated optical device comprising two wedge-shaped pieces of quartz arranged so that their optic axes, and the light path, are... 

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