Elements of natural philosophy (Volume 2-3) . Couclusion in lei plane faces, is not ultimately deviated, but remains pa- to its first direction. The ray D D\ being supposed to traverse a second same true formedium bounded by parallel plane faces, and of which any number^ t/ -T J. > media bounded the refractive index is m, will undergo no deviation; by parallel planeand the same may be said of any number of mediaface9,bounded by such faces. If, now, the spaces betweenthe media be diminished indefinitely so as to bring theminto actual contact, there will still be no deviation, an
Elements of natural philosophy (Volume 2-3) . Couclusion in lei plane faces, is not ultimately deviated, but remains pa- to its first direction. The ray D D\ being supposed to traverse a second same true formedium bounded by parallel plane faces, and of which any number^ t/ -T J. > media bounded the refractive index is m, will undergo no deviation; by parallel planeand the same may be said of any number of mediaface9,bounded by such faces. If, now, the spaces betweenthe media be diminished indefinitely so as to bring theminto actual contact, there will still be no deviation, andwe find that a wave will emerge from a medium, ar-ranged in parallel strata, parallel to its position before entrance. 12 178 NATURAL PHILOSOPHY. To find therelative index ofrefraction; § 20. Let us next sup-pose a ray to traversetwo media A and B,bounded by plane pa-rallel feces, the mediabeing in contact, andhaving their refrac-live indices denotedby m and m respec-tively ; we shall have,by calling m, the in-dex of refraction of Fig. the second, or denser medium in reference to the first, Equationsapplicable to tbodeviations; sin <p =sin <p == m sin <p m sm <p n 1 (7) it Multiplying these Equations together, there will result Result ofoperations; tt m m = — r^\ m (T) Rule. Example; Result That is to say, to find the index of refraction in the caseof a ray passing from any one medium to another, dividethe index of the second by that of the first referred to avacuum. The index thus obtained is called the relativeindex. Example. What is the relative index of air and crownglass, the light entering the latter from the former? Thetabular index of crowm glass is 1,52, and that of air is1,0003, whence 1,5200 At^ = 1,52. 1,0003 ELEMENTS OF OPTICS. 179 § 21. If a ray pass from amedium to another more dense,the index m will be greaterthan unity, and from equation(7), we shall haveisin sin <p; and if sin <p be taken a maxi-mum, or the angle of incidencebe 90°,
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