. A text book of physics, for the use of students of science and engineering . am is restricted to a narrow part nearthe principal axis, the more accurately will the refracted rays passthrough a point focus. For a wide beam, the outer rays will notpass through the same focus as those near the axis. 588 LIGHT CHAP With the arrangement of faces shown in Fig. 536, the beam afterpassing through the lens is divergent, each ray appearing to come from PF, the principal focus,wliieh in this case is a lens is said to be adiverging lens. Referring to the conventionas regards sign given on p
. A text book of physics, for the use of students of science and engineering . am is restricted to a narrow part nearthe principal axis, the more accurately will the refracted rays passthrough a point focus. For a wide beam, the outer rays will notpass through the same focus as those near the axis. 588 LIGHT CHAP With the arrangement of faces shown in Fig. 536, the beam afterpassing through the lens is divergent, each ray appearing to come from PF, the principal focus,wliieh in this case is a lens is said to be adiverging lens. Referring to the conventionas regards sign given on p. 567,we see that the focal length of aconverging lens is negative, whilethat of a diverging lens is positive. Classification of lenses.— Names are sometimes adopted Fig. by diverging lens. which are descriptive of the shapes of the lenses. The first three (illustrated in Fig. 537) are allconverging, and as shown, would all have the same focal length. Asimilar remark applies to the three diverging forms. We shall onlyuse the terms converging and Double Piano- Concavo-Convex Convex Convex DoubleConcave Piano-Concave Convexo-Concave Converging Lenses Diverging Lenses Fig. 537.—Classification of lenses. Refraction at single spherical surface.—In order to obtain the lensformula, the production of an image by a single refracting surfacewill be treated first, and the result then applied to an actual lens,which has, of course, two refracting surfaces. _ In Fig. 538 let A, in either diagram, be a point source of lightsif uated on the principal axis. A ray such as AB will, on enteringthe glass, be refracted along BD. In Fig. 538 (a) we have sin ABE _ sin i sinCBD sin r ~ ^and in Fig. 538 (b) we have sin ABC sin i sin EBD sin / ,/*? JLVI SPHERICAL REFRACTING SURFACES 589 Draw BP perpendicular to the axis ; then from either diagram,since all the rays are much nearer the axis than is shown in:.he diagrams, 0 = BPA u BP BP i + r = —v Y and .BP BP u rx BP
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