. Light, a textbook for students who have had one year of physics. highly magnifying them, and they become a source of troubleonly in the most exacting work with telescope or nature of this fault will be considered later under thehead of diffraction, sections 72 and 73. 39. Spherical aberration.—Another fault is known asspherical aberration. Quite apart from the just-mentioneddifficulty of diffraction, and from chromatic aberration, therays coming through the edges of a lens are not brought to thesame focus as those coming through near the center. Thisfollows from the fact menti
. Light, a textbook for students who have had one year of physics. highly magnifying them, and they become a source of troubleonly in the most exacting work with telescope or nature of this fault will be considered later under thehead of diffraction, sections 72 and 73. 39. Spherical aberration.—Another fault is known asspherical aberration. Quite apart from the just-mentioneddifficulty of diffraction, and from chromatic aberration, therays coming through the edges of a lens are not brought to thesame focus as those coming through near the center. Thisfollows from the fact mentioned above, that when a sphericalwavefront is refracted at a spherical surface, it emerges nottruly spherical. Figure 52 illustrates this defect in an exag-gerated manner. The rays are drawn, but not the wravefronts. 92 LIGHT 0 is a point source, from which all the rays originate. Whenthey emerge from the lens, they do not converge to a singlepoint. Since the central part of the emergent wavefront, saythe part to which rays between those marked 5 and 7 belong,. Figure 52 is very nearly spherical, these rays will all intersect nearly ata single point f. Rays 4 and 5, however, cross before reachingf, at such a point as e, and the corresponding rays 7 and 8 ate. Rays 3 and 4 will cross still nearer the lens, as at c, 3and 2 at b, 2 and 1 at a, etc. Consequently, instead of havinga single point f as the image of 0, we may say that the imageis the line abcefecba, or rather, the surface formed by re-volving this line about the axis of the lens. This surface isroughly conical, with a point or cusp at f, and by far thegreater part of the light is concentrated at this point, whichwe commonly regard as the proper image. Nevertheless, muchlight fails to pass through f, and if a screen were placed atthat point we should see a sort of halo surrounding the brightcenter, caused by light which came to focus before reachingthe screen. The curve abeefecba is called a caustic, and thecorrespond
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