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field vector of the reflected light has a strong horizontal component. Sunglasses made Polarization by Double Refraction Solids can be classified on the basis of internal structure. Those in which the atoms are double- refracting or birefringent materials. Upon entering a calcite crystal, unpolarized light splits into two plane- polarized rays that travel with different velocities, corresponding to two angles of O that is the same in all directions. This means that if one could place a point source of light inside the The second plane-polarized ray, called the extraordinary (E) ray, travels with different speeds in different directions and hence is characterized by an index of E that varies with the direction of propagation. Consider again the point source within a birefringent material, as in Figure 38.34. The source sends out an optic axis, along which the ordi- nary and extraordinary rays have the same speed, corresponding to the direction for O " n E . The difference in speed for the two rays is a maximum in the direc- tion perpendicular to the optic axis. For example, in calcite, n O " 1.658 at a wavelength of 589.3 nm, and n E varies from 1.658 along the optic axis to 1.486 perpendicular to the optic axis. Values for n O and n E for various double-refracting crystals are given in Table 38.1. S E C T I O N 3 8 . 6 • Polarizaion of Light Waves 1229 Unpolarized light E ray O ray Calcite Figure 38.33 Unpolarized light incident on a calcite crystal splits into an ordinary (O) ray and an extraordinary (E) ray. These two rays are polarized in mutually perpendicular directions. (Drawing not to scale.) E O S Optic axis Figure 38.34 A point source S inside a double-refracting crystal produces a spherical wave front corresponding to the ordinary ray and an elliptical wave front corresponding to the extraordinary ray. The two waves propagate with the same velocity along the optic axis. |