. The Ontario high school physics. away, and we must use an objec-0 0, Qt tive with as great a focal length as pos-sible. The reason forthis will be evidentfrom Fig. 434. LetAC he a, ray fromthe upper part of the object looked at, passing through the centre C of the objective 0. Now the image of an object at a great distance is formed at theprincipal focus. If then F^ is the principal focus P^Q^ is the image,and if F^ is the principal focus P^Qo ^^ ^^^ image. It is clear thatP^Q^ is greater than P-^Q-^, and indeed that the size varies directlyas the focal length. Hence the greater the focal le
. The Ontario high school physics. away, and we must use an objec-0 0, Qt tive with as great a focal length as pos-sible. The reason forthis will be evidentfrom Fig. 434. LetAC he a, ray fromthe upper part of the object looked at, passing through the centre C of the objective 0. Now the image of an object at a great distance is formed at theprincipal focus. If then F^ is the principal focus P^Q^ is the image,and if F^ is the principal focus P^Qo ^^ ^^^ image. It is clear thatP^Q^ is greater than P-^Q-^, and indeed that the size varies directlyas the focal length. Hence the greater the focal length of theobjective the lai-ger will be the image produced by it. Further, since the celestial bodies (except the sun) are very faint,the diameter of the objective should be large, in order to collect asmuch light from the body as possible. A diagram illustrating the action of the telescope is given in The ob- <7| jective formsthe image at itsprincipal focusB, that \& OB= F, its focallength. This isfurther raagni-. FiG. 435.—The astronomical telescope. fied by the eyepiece E, which forms the image at -pq. B is justwithin the principal focus of the eyepiece, and so OE, the distancebetween objective and eyepiece, is approximately equal to F + fthe sura of their focal lengths. The magnification produced by the telescope is equal to Fjf,though we cannot here deduce this formula.* In the great telescope of the Lick Observatory the diameter of theobjective is 36 inches and its focal length is 57 feet. On using aneyepiece of focal length ti inch the magnification is 1368. Thediameter of the Yerkes telescope (belonging to the University ofChicago) is 40 inches and its focal length is 62 feet. * See Ganots Physics; or Watsons Physics, p. 492. OBJECTIVES AND EYEPIECES 365 418. Objectives and Eyepieces. In telescopes the objective usually consists of an aciirouiatic pair of lenses as shown in Fig. 411,the lenses being sometimes cemented together, at others timesseparated a
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