Practical distinction between optically material

Figure 4. A practical distinction between optically uniaxial and optically biaxial drawn (or extruded) material. For optically uniaxial material, the area fraction exhibiting extinction between crossed circular polars is greatest when the normal to the plane of the thin section is parallel to the draw axis. For optically biaxial material, the greatest area fraction is observed in a section cut so that the angle between its normal and the draw axis is equal to half the optic axial angle of a monodomain.

This expression highlights an important distinction between EELS and optical experiment, namely that the virtual photon field associated with the fast electron is longitudinal ( 11 ) whereas the real photon field is transverse (E q) [3.23]. This difference does not affect the main point of the photon analogy namely that electron energy loss and optical experiments measure the same quantity e(a>) in most (and practically all the) cases. It does mean that the polarisation of the electric field acting on the atomic system is defined differently in both cases, a fact of obvious importance for anisotropic materials. 

The im< e mode produces an image of the illuminated sample area, as in Figure 2. The imj e can contain contrast brought about by several mechanisms mass contrast, due to spatial separations between distinct atomic constituents thickness contrast, due to nonuniformity in sample thickness diffraction contrast, which in the case of crystalline materials results from scattering of the incident electron wave by structural defects and phase contrast (see discussion later in this article). Alternating between imj e and diffraction mode on a TEM involves nothing more than the flick of a switch. The reasons for this simplicity are buried in the intricate electron optics technology that makes the practice of TEM possible. 

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