AL Pysics/pticl instruments/p.1 PLK VICW K.T. CHNG SIXTH FRM CLLEGE Form Six AL Pysics pticl Instruments pticl instruments Mgniying glss Microscope Rercting telescope Grting spectrometer Qulittive understnding o ow opticl instruments work (using simple ry digrms only). Mgniying powers o mgniying glss, microscope nd rercting telescope considered s rtio o visul ngles sutended y te imge nd te oject (s otined rom simple ry digrms). Two-lens type only. Formtion o imge t lest distnce o distinct vision. Two-lens type only. Formtion o imge t ininity. Qulittive explntion o te unctions o te collimtor nd te telescope using ry digrms. Use in simple spectrl nlysis. For te purpose o te prcticl exmintion milirity wit use o concve nd convex mirrors, converging nd diverging lenses, nd prisms is expected ut ull instruction on procedure will e given nd knowledge o prticulr metods in not required. A. Bsic Concept 1. Te norml eye cn ocus on oject t ininity (te r point o te norml eye). In tis cse te eye is relxed or sid to e unccommodted. 2. Te eye cn see n oject in gretest detil wen it is plced t certin distnce rom te eye, known s te lest distnce o distinct vision, wic is out 25 cm, or norml eye. Te point t distnce rom te eye is known s its ner point. 3. Visul ngle:- consider n oject plced some distnce rom te eye, nd suppose θ is te ngle in rdin sutended y it t te eye. Te lengt o te imge ormed y te eye is proportionl to te ngle sutended t te eye y te oject (i.e. te visul ngle). R oject L θ θ imge ** (i) A & B, sme size visul ngle o A - visul ngle o B - As >, B ppers lrger tn A. (ii) P & Q, ctul size o P > ctul size o Q owever, visul ngle sutended y P & Q re te sme ence, P & Q ppers to e te sme size. A B (i) A B P Q P&Q θ (ii)
AL Pysics/pticl instruments/p.2 4. Angulr Mgniiction:- - Some instruments (e.g. telescope nd microscope) re designed to increse te visul ngle, so tt te oject viewed cn e mde to pper muc lrger wit teir elp. - Beore tese instruments re used, te visul ngle te oject sutended =. - Ater te instruments re used, te visul ngle te inl imge sutended =. - Te ngulr mgniiction, M, o te instrument is deined s M = / (mgniying power o te instrument) B. Simple Microscope or Mgniying glss 1. A microscope is n instrument used or viewing ner ojects. Wen it is in norml use, te imge ormed y te microscope is t te ner point o te eye. 2. Wit te unided eye, te oject is seen clerest A wen it is plced t te ner point. 3. A converging lens L is used s mgniying glss to view te oject. Te oject is moved until its imge is t te ner point o te eye nd te imge sutends n visul ngle t te eye. L 4. Hence, te ngulr mgniiction o te ' mgniying glss ' I F M ' = = = 5. Te rtio / is equl to te rtio o imge distnce v to te oject distnce u, i.e. v/u. 1 1 6. By lens ormul, + = 1 u v v u = v 1 ence, ' = v u = 1 = ( + 1) ence, numericlly M = +1 7. From te ormul, it ollows tt lens o sort ocl lengt is required or ig ngulr mgniiction. ** Wt we ve just consider is te norml use Top point o te mgniying glss, were te imge o imge ormed is t te ner point o te eye nd te eye is ccommodted. In ct te imge my e ormed t oter point nd still tere is te mgniying eect. ne specil cse is tt te F L C
AL Pysics/pticl instruments/p.3 imge is ormed t ininity, owever, wic is not norml use o mgniying glss, te eye is undergoing te lest strin nd is unccommodted. In tis cse te oject must e plced t te ocus, F, o te lens. Suppose tt te ocl lengt o te lens is. Te visul ngle o te imge sutended t te eye is ten /. Hence, te ngulr mgniiction, M, is given y M = = = Notice tt te ngulr mgniiction vries etween / nd / + 1 wen te imge is ormed etween ininity nd te ner point. Exmple:- Q.1 Wic o te ollowing two ojects would give rise to te lrger imge on te retin o te umn eye? () A mn o eigt 2.0 m t rnge o 0.20 km, or () te Moon, wic sutends n ngle o 9 mrd t te Ert s surce? Q.2 An oject 10 mm ig is to e viewed troug simple mgniying glss o ocl lengt 50 mm. (1) Imge t ininity () Were sould te oject e plced? () Wt ngle does te inl imge sutend t n eye plced very close to te lens? (c) Wt ngle would te oject sutend t te ner point, 250 mm rom te eye? (d) Wt ngulr mgniiction is otined? (2) Imge t ner point Repet Q.() - (d) o (1). Is tere ny dvntge to e otined in orming te inl imge (i) t ininity, (ii) t te ner point, (iii) closer tn te ner point? iscuss. C. Compound Microscope u ojective eyepiece I 2 L 1 1 v F e I 1 1 2 L 2 2
AL Pysics/pticl instruments/p.4 1. Te mgniying power o single lens, M, is greter te smller te ocl lengt o te lens. However te ocl lengt o mgniying glss cnnot e too smll, ecuse grinding lens o sort ocl lengt is very diicult. 2. Compound microscope:- - Compound microscope is composed o two seprted lenses. - Te lens nerer to te oject is clled te ojective; te lens troug wic te inl imge is viewed is clled te eyepiece. - Te ojective nd te eyepiece re ot converging nd ot ve smll ocl lengts. 3. Working Principle:- - Wen te microscope is used, te oject is plced t sligtly greter distnce rom te ojective tn its ocl lengt. - A lrge inverted rel imge is ten ormed t I 1. - Te eyepiece is djusted so tt I 1 is nerer to te eyepiece tn te ocus F e nd tus lrge virtul imge is ormed t I 2. - I te inl imge I 2 is ormed t te lest distnce o distinct vision,, rom te eye, te microscope is sid to e in norml use. 4. ngulr mgniiction, M = / were is te visul ngle sutended y te oject t te unided eye. ence M 2 2 = =. Astronomicl Telescope ojective C 1 1 I T F e 2 eyepiece E C 2 1 2 top point o inl imge 1. An stronomicl telescope mde rom lenses consists o n ojective o long ocl lengt nd n eyepiece o sort ocl lengt. 2. Te telescope is in norml djustment wen te inl imge is ormed t ininity. 3. - Te ojective lens collects prllel rys rom te distnt oject so it orms n imge I t its ocus. -As F e nd re t te sme plce. - Te inl imge is ormed t ininity. 4. Te ngulr mgniiction o te telescope M = /
AL Pysics/pticl instruments/p.5 Since te telescope lengt is very smll compred wit te distnce o te oject rom eiter lens, we cn tke te ngle sutended t te unided eye y te oject s tt sutended t te ojective lens, s sown. ence M 2 = = 1 1 2 For ig mgniiction, te ojective sould ve long ocl lengt, 1, nd te eyepiece sort ocl lengt, 2. ------------------------------------------------------------------------------ ** Eye-Ring o Telescope A E F e B 1 2 eyering = imge o AB in E - Wen oject is viewed y n opticl instrument, only tose rys rom te oject wic re ounded y te edge o te ojective lens enter te instrument. - Wit given ojective, te est position o te eye is one were it collect s muc ligt s possile rom tt pssing troug te ojective. - Tis position is were te imge o te ojective AB in te eyepiece ormed. - Te smll circulr imge is clled te eye-ring. - It is te est position were te eye cn collect te mximum mount o ligt entering te ojective rom outside so tt it s wide ield o view. E. Spectrometer * ligt source collimtor telescope crosswire pltorm 1. Te spectrometer is designed to produce nd mke mesurements on te spectr o ligt sources nd is used wit dirction grting (or prism). 2. Te instrument consists o
AL Pysics/pticl instruments/p.6 (i) ixed collimtor wit movle slit o djustle widt (to produce prllel em o ligt rom te source illuminting te slit), (ii) turntle (ving circulr scle) on wic te grting or prism is plced nd (iii) telescope (wit vernier scle) rottle out te sme verticl xis s te turntle.
AL Pysics/pticl instruments/p.7 3. Preliminry djustments: (i) Eyepiece - move in te tue contining te cross-wires until te cross-wires re clerly seen. - An imge ormed on te wires will ten e distinct. (ii) Telescope - A distnt oject is viewed troug te telescope nd te distnce o te ojective rom te cross-wire eyepiece djusted y tum-screw until tere is no prllx etween te imge o te distnt oject nd te cross-wires. - Prllel rys entering te telescope re now rougt to ocus t te cross-wires. (iii) Collimtor - Te telescope is turned into line wit te collimtor, nd te slit, illuminted wit sodium ligt, is moved in or out o te collimtor tue until tere is no prllx etween te imge o te slit nd te cross-wires. - Te slit is t te principl ocus o te collimtor lens wic is producing prllel em. (iv)levelling te tle. 4. Appliction: - mesurement o te wvelengt (wit dirction grting) - Simple spectr nlysis (wit dirction grting) - mesurement o te rerctive index o glss prism