. Light, a textbook for students who have had one year of physics. Figure 50 the mirror, the ray OP will be reflected in the direction PI,where, by the laws of reflection, the angles OPO and IPI areequal, and the intersection of this line with the plane confocalto the plane of 0 will give the image I. It follows at oncethat equation (13) holds for mirrors as well as lenses. Butin the case of a mirror, the image is inverted if it lies on thesame side as the object, erect if on the opposite side. An important application of the principles just stated isillustrated in figure 51. Suppose there are
. Light, a textbook for students who have had one year of physics. Figure 50 the mirror, the ray OP will be reflected in the direction PI,where, by the laws of reflection, the angles OPO and IPI areequal, and the intersection of this line with the plane confocalto the plane of 0 will give the image I. It follows at oncethat equation (13) holds for mirrors as well as lenses. Butin the case of a mirror, the image is inverted if it lies on thesame side as the object, erect if on the opposite side. An important application of the principles just stated isillustrated in figure 51. Suppose there are two stars, prac-tically at an infinite distance, in the direction from the lensindicated by the letters C and D, the arrows indicating the. -Ffff-f-f-*. +4» Figure 51 direction in winch the light is propagated from them, in planewaves. If either star lay on the prolongation of the axis BM,its light would be focussed at the principal focus F. Other-wise, if the angles CBM and DBM are small, not more thana few degrees, the image of each star will lie in what is calledthe principal focal plane of the lens, a plane through F per- 00 LIGHT pendicular to the axis. For plane waves, every line perpen-dicular to the wavefronts is a ray. Therefore, if we drawthrough the center of the) lens a line perpendicular to each setof incident wavefronts, these will be the undeviated rays fromthe two stars; and the points where these lines meet the prin-cipal focal plane, c and d, will be the images of the two angle cBd, subtended by the images from the center of thelens, is then equal to the angle subtended by the stars them-selves from the center of the lens—or indeed from any terres-trial point since the distance of the stars
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectlight, bookyear1921
