Anisotropism (anisotropy) characterizes substances that exhibit physical properties with different values when measured in different directions. The specific response of these substances to an internal or external stimulus also differs according to the kind of stimulus and the manner and direction or directions of its application. Anisotropism to some kinds of stimuli is observed in all crystals, in many strained, noncrystalline substances, and in aggregates and accumulations of discrete particles or larger bodies in which there exists at least some degree of parallelism of a linear or planar fabric or of structural elements. Optical anisotropism is exhibited by light-transmitting substances through which light of a particular frequency travels with different velocities for different directions of transmission. Such substances doubly refract transmitted light and are described as birefringent. Optically, anisotropic opaque substances reflect or absorb light in varying degrees...
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