. Light, a textbook for students who have had one year of physics. rystal structure and the nature of double refraction. Every crystal possesses certain regularities of structure,on account of which it can be easily split in certain , for instance, can he readily split into any one of theforms shown in figure 104. which differ in linear dimensions,but have exactly the same angles. Tin- important thing aboutcrystal structure is that the angles are fixed and invariable,while the dimensions of the faces may lie anything. In any oneof the three rhombohedral forms shown in the fig
. Light, a textbook for students who have had one year of physics. rystal structure and the nature of double refraction. Every crystal possesses certain regularities of structure,on account of which it can be easily split in certain , for instance, can he readily split into any one of theforms shown in figure 104. which differ in linear dimensions,but have exactly the same angles. Tin- important thing aboutcrystal structure is that the angles are fixed and invariable,while the dimensions of the faces may lie anything. In any oneof the three rhombohedral forms shown in the figure for cal-cite, each face bas two angles of 1**1 55 and two of 78° each rhombohedron, there are two opposite corners wherethree obtuse angles come together, as at A. A line drawn DOUBLE REFRACTION 207 through one of these corners, equally inclined to the three facesthat meet there, or any line parallel to it, is called the opticaxis. Note that the optic axis is defined only by its direction,and any line having that direction may be called the optic Figure 104 When a pencil of light enters such a rhomb of calcite it isseparated into two parts, so that there are two refracted beams,instead of only one as in the case of glass and other non-crys-talline media. One of the beams obeys the ordinary laws ofrefraction, the ratio of the sines of the angles of incidence andrefraction being always the same, no matter what the angle ofincidence may be. It is therefore called the ordinary ray,ami we shall represent it by the letter 0, for brevity. The otherdoes not follow the ordinary laws of refraction. The ratioof the sines changes as the angle of incidence changes, showingthat this beam travels through the calcite with differentvelocities in different directions, this being due, no doubt, tothe regular structure of the crystal. For this reason it is called the extraordinary ray and we shall rep-resent it by E. If we wish to speak ofan index of refraction for the extraordi-n
Size: 1909px × 1309px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectlight, bookyear1921
