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发表于 2008-8-7 09:46:19
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Angular momentum L about a point O is defined by L = p 
rwhere p is the linear momentum and r is the position vector of the body from O. For a circular orbit around O, the angular momentum is orbital and L = pr = mvr. Newton's Second Law in terms of angular momentum becomes, dL/dt =  where  is the applied torque about the same point O. In classical mechanics, in the absence of an external torque, the angular momentum of a body is conserved. In a central force field, i.e. when the force experienced by a body is always directed to pass through O, that is the force F is directed along r, L is also conserved (this is the so-called central field theorem). In quantum mechanics, the orbital angular momentum of an electron in an atom is quantized via L = (h/2)[l(l +1)]1/2 where l is the orbital quantum number of the electron, l = 0,1,2,...,(n–1), h is Planck's constant and n the principal quantum number.
Anion is an atom that has gained negative charge by virtue of accepting an electron or electrons. Usually atoms of nonmetallic elements can gain electrons easily to become anions. Anions become attracted to the anode (positive terminal) in ionic conduction. Typical anions are the halogen ions, F-, Cl-, Br-, I-.
Anisotropy (optical) refers to the fact that the refractive index n of a crystal depends on the direction of the electric field in the propagating light beam. The velocity of light in a crystal depends on the direction of propagation and on the state of its polarization, i.e. the direction of the electric field. Most noncrystalline materials, such as glasses and liquids, and all cubic crystals are optically isotropic, that is the refractive index is the same in all directions. For all classes of crystals excluding cubic structures, the refractive index depends on the propagation direction and the state of polarization. The result of optical anisotropy is that, except along certain special directions, any unpolarized light ray entering such a crystal breaks into two different rays with different polarizations and phase velocities. When we view an image through a calcite crystal, an optically anisotropic crystal, we see two images, each constituted by light of different polarization passing through the crystal, whereas there is only one image through an optically isotropic crystal. Optically anisotropic crystals are called birefringent because an incident light beam may be doubly refracted. Experiments and theories on “most anisotropic crystals”, i.e. those with the highest degree of anisotropy, show that we can describe light propagation in terms of three refractive indices, called principal refractive indices n1, n2 and n3, along three mutually orthogonal directions in the crystal, say x, y and z called principal axes. These indices correspond to the polarization state of the wave along these axes. Crystals that have three distinct principal indices also have two optic axes and are called biaxial crystals. On the other hand, uniaxial crystals have two of their principal indices the same (n1 = n2) and only have one optic axis. Uniaxial crystals, such as quartz, that have n3 > n1 are called positive, and those such as calcite that have n3 < n1 are callednegativeuniaxial crystals. |
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