suton partv light

7/27/2019 Suton PARTV Light

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PART VLIGHT

LIGHT SOURCESL.1. Projection Lanterns. Projection lanterns are of two

types, tungsten filament and carbon arc. For easein operation,the filament lampisbest, but formanyexperimentsthe intensely

FIG. 284.Lantern arranged or projection of apparatus.bright carbon arc gives superior results. Each type has itsadvantages,andboth should be available if possible.For some demonstrations inLightand formanyotherswhere asmallpiece of apparatusif projected may be seen by the entireclass,it hould bepossibletorimoveindependently theprojection367


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368 LIGHT [L-2lens, the slide holder, and the lens tube orbellows. Then nexttothe condensing ensesmaybe placeda shelforbench onwhichtheapparatus is supportedfor projection. The projectionlens canbe mountedon a separate stand. Onewayof accomplishingthisis shown in Fig. 284, where theprojection ensPissupported byan adjustable clamp andpieces of apparatusmaybeplacedon theshelf 2, whose heightis likewiseadjustable.The lantern should be equipped with a vertical projectionattachment,so thatobjects (suchasadish of water) thatmust bekept in a horizontal plane may be

projected (Fig. 285).The lantern is also very convenientfor illuminating circular openings,slits, and thin films in experiments on_______ diffraction and interference, the ob-jects being placedwhere he condens-inglenses form the brightest imageofthe source.L-2. IncandescentLamps. Lamps

with straight filaments such as areused in show-window ightingare available in several sizes andpower ratings.1 They are especially useful for diffraction andinterference experiments (L-73, etc.).Miniature lamps (flashlight) are made for operation frombatteriesorsmall transformers forvarious voltages up to 6 or 8 v.Colored Christmas-tree lights operating at 14 v are convenientfor some experiments.Automobile lamps are made in many styles, from the verybright double-filament headlight type (21, 32, and 50 cp) tosingle-filament lamps having a very short straight length offinelycoiledwire. Socketsfor these amps maybe obtained fromautomobile supply houses, or the lamps may be used withoutsockets by soldering a short length of lamp cord directlyto theterminals. A 6-v lampis averyusefulsource; it canbe runfroma storage batteryor a transformer, and whenoperatedat 10 v itfurnishes anintenselight,thoughits ife is shortened.A "point" source, sold under various trade names,is madebya tungstenarc in vacuum. It consists of a smalltungstenbead'For example, General ElectricT-6l,T-8; Westinghouse T-14.

'Alfrror.. "rr-Projecf/onlensI II I AppircfusI \t.AMn'ror.,TfIG. 285.Diagram of verticalprojection lantern.


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L-5] LIGHT PATHS MADE VISIBLE 369that is raised to incandescence by ion bombardment, giving avery intenseandsteadysource of smallarea.Photoflood lamps are available that are much more brilliantandmuch richerinvioletlightthanordinary amps, because theyare purposely overloaded at 110 v. They are excellent forproducing intense illumination aswellasforshowingby compari-son with ordinary lamps the increased whiteness produced by ahotter source. Anystandardlamp maybesimilarly overloaded,e.g., byan autotransformer E-238).A convenient holder for standard lamps can be made from apiece ofk-in. brasstubing,10 to 18in. long, threadedat one endtofit the base oftheordinarykeyed orkeylesslampsocket. Theconnecting cord enters through the brass tube. This holder canbe clamped in anypositionon an ordinarystand.L-3. NeonandArgon Glow Lamps. Inexpensive glow lampsequipped with the standard mediumbase for operation on 110-vdirect or alternatingcurrent are available. On direct current,they amount to polarity indicators. On alternating current,they giveflashesof double the a.c.frequency. Theycanbe usedfor stroboscopic illumination and as spectroscopicsources inthelaboratorybut arenotbrightenough forprojection in the lectureroom. The argon lamp has enough ultraviolet radiation forsomeexperiments onfluorescence (L-114).L-4. Sodium and Mercury Vapor Lamps. The recentlydeveloped sodiumvapor lampmakes possibleanintensesource ofmonochromatic radiation. It is valuable in demonstrations ofcolor perception, interference, diffraction, and spectroscopy.Quartzmercuryvapor lampsare excellent for showing effects ofultravioletradiation; they are, however, expensive. Lessexpen-sive glass lamps are obtainable or lectureand laboratoryuse, ortheymaybe constructed according todirections given elsewhere.'

LIGHT PATHS MADE VISIBLEL-5. Invisibility of Light. A woodenbox, 6 by 6by 18 in., isconstructedwithahingedtopandglassfront. Aholeabout2 in.indiameter scutineach endandapiece ofmailing tube nsertedin each hole to reduce stray light. The inside of the box is

paintedadeadblack. A lensoffocallength6to8 in.ismountedin one of the end apertures and masked down to about 1-in.1BALINKINandWELLS, Rev.Sci. In.truments,3,7, 1932.


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370 LIGHT [L-6aperture with a black diaphragm. The inside of the box iscoated with oil or glycerin to render the interior dust free. Acarbon or tungstenarc is placed at the outside focus of the lensand adjustedso thataparallel beampasses throughthecenter ofthe opposite aperture. No evidence of light can be seen in theinteriorofthebox. This condition will be alteredif awhitecardisintroduced nto the boxso asto interrupt the path ofthebeam.L-6. Optical Disk. A disk of metala footormorein diameterwhose surface is painted white and whose rim is graduated in

degrees is mounted on an axis about which it can rotate. Thedisk is supportedvertically bya stand thatalso carriesashield inthe form ofashort cylinder at rightangles to thedisk (Fig.286).Anopening is cut in the shield at the height of the center of thedisk, over whichmay be slipped shutters containing one ormorehorizontal slits. When a parallel beam of light is directed atthese slits, the paths of the separatebeams passed bythe slits aremade visible on the surface of the disk. Sections of mirrors,prisms,lenses,etc.,maybe clampedto the faceof the disk and heresult of reflection or of refraction made visible. Numerous

FIG. 286.---Opticaldisk.


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L-7] LIGHT PATTIS MADE VISIBLE 371experiments in geometrical optics are possible with thisapparatus.Asubstitute for the optical disk isthe following. Anopen arc(Fig.287) ismountedon the floor close to the wall and urned soas to send a beam of light upward. A piano-cylindrical lens Lis mounted below a slotted cover. The slots are closed withsliding covers so that as each cover is slowlywithdrawna ray is"drawn" up the wall. Cylindrical lenses and mirrors can beintroducedabove the cover and the pathsof the rays traced after reflection andrefraction.L-7. Optical Tank. A long troughwithglass sides and ends is useful for manyexperiments. It is filled with water towhich a small amount of fluorescein orother fluorescent material has beenadded,by means of which hepaths ofthe raysof -light are madevisible to the class. The \J/"size of the tank depends in part upon the *A Alenses and other accessories that are to be FIG. 28Trrange.used with it. Asuggested sizeis3 by3 by ment for showing lightpaths onwall.36 in. A convenient addition is a mirrorrunning the length of the tank just above it and tipped at450 so that the class can look down into the tank from aboveaswell as into it from the side. Such a tank isuseful for showingreflection and refraction phenomena; it can also be used forshowing scatteringandpolarization (L-127).The framesupporting he glass sides must be ofrigidconstruc-tion so as to carry the weight of the glass and water withoutappreciable distortion. Otherwise the stresses are apt to crackthe joints. The joints should be made watertight with someform of aquarium cement or glue that will not grow hard andcrackduringthe long intervals when the tank is idle and dry, orpeeloff andcause leakswhen the tankis filled with water.For reflection, refraction, and total internal reflection, a largeflat-sided battery ar filled with water containing a smallamountof fluoresceinwill serve nicelyto show the lightpaths to a wholeclass. Asheet offlashedopalglass (milk glass) set vertically atasmall angle with the beam will make it visible both below andabove the surface ofthe water.


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372 LIGHT [L-8L-8. Gauze Screen. A rectangular wooden frame 2 ft wide

and3 or4ft long is held inaverticalplaneby a clamp. Acrossthe frame arc stretchedwhite threadsor fine cottonstrings2 or3mmapart. When a lens isplacednear one end of the frameanda beam oflight directeduponit, the beam after passing the lenswill be caught and scatteredby the threads so that a large classcan see the light paths. By using a small section of a similarscreen before the lens, the incidentas well as the refractedraysare made visible. A simplification of thisplan is to use cheese-clothinplaceof the threads. Itcan be stretched ightby asten-ing it to rectangular metal rods at top and bottom, leavingthe sides unobstructed,or itmay be held taut inaniron hoopbylight helical springs. If two rectangular rods are screwed orclamped togetherwith the cloth between hem, at both top andbottom,theupper rods maybe held ina clamp, and theweight ofthe lower oneswill keep the cheesecloth stretched. A screen ofthis type is especially useful for showing lens action, chromaticaberration, spherical aberration, anddistortion. The optic axisof the lens should be in the plane of the gauze.

L-9. Smoke Box. A large woodenbox, say 15by 15by 60in.,is equipped with a glassfront and hinged top. Windows are cutin the ends, and he box is paintedblack nside. Whenit isfilledwith smoke or ammonium chloride fumes, the light scatteredbythe particles will make the light paths visible. (For the methodofmakingammonium chloride smoke, seeA-S.) Lenses, mirrors,prisms, etc., maybe set inside the box, so that light paths maybeseen before and after reflection or refraction. The box is usefulfor showing lens combinations, as in telescopes andmicroscopes(L-54, 55).L-1O. Chalk Dust. Avery simple expedient for makinglightpaths visible is to clap two dusty blackboarderasers togetherabove the light beam. The chalk dust will make the beamvisible; but the region filledwith the dust is limited andvariable,and the dust soon settles.

PHOTOMETRYL-11. Inverse SquareLaw of IntensityofIllumination. Con-nect the output of a photocell (A-91) to the lecture-room galva-nometer. Remove the condensing lenses from a projection lan-


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L-13j PHOTOMETRY 373tern, preferablyof the arc type, and place thecell in aposition ogivemaximum galvanometer deflection when illuminatedby thelantern. Double and then triple the distance of the cell fromthe source, andnote the inversesquarerelation.The wayin which area increases as the squareof the distancefromapoint canbeillustrated with a wire framein theform of apyramidof square section (Fig. 288). Ifthe edge at the base isthree units, for example, then a square of edge two units can beplaced in the frame at one-third thedistance to the vertex and a square ofoneunit at two-thirds he distance.L-12. Joly Diffusion Photometer.Two blocks of paraffin (sold in grocerystores) about 2.5 by 5 by 0.5 in. with asheet of tin foil or cardboard betweenthem are clamped betweentwo strips ofthin wood and setverticallybetween hetwo:sources of light that are to be com-pared, so as to be seen edge on. The Fiu. 288.Wire frametosourcesaremoved until heblocksmatch illustrate nverse square lawof light intensity.in appearance.Thearrangementcanbeimprovedandmadeless perishable byplacing the paraffin blocks at the middle of a box open at bothends and painted black inside. The edges of the blocks areobservedthrougha glasswindowsetinoneside ofthebox. Theblocks are held between two glass plates, which prevent theparaffin from flowing in warm weather. This arrangementeliminates stray light and makes photometric comparisonspossible without darkening the room. The apparatus can beseen by a large class, andquantitative measurements are easilymade.L-13. Rumford Shadow Photometer. A laboratory supportrod is setvertically 1 ftin front of ascreen. The two sources oflighttobe compared areplacedso as to cast separateshadows ofthe rod on the screen andaremoved until the shadows appeartohave the same intensity. Electric light bulbs are satisfactorysources; they shouldbemountedon stands (L-2) so that they canbe moved about. The edges of the shadows may be hazy, andtheremaybemarkedcolor differences n the shadows,so that thismethod of comparison is not particularly accurate. The dis-


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374 LIGHT [L14tancesinvolvedare tobe measured from eachlightto the shadowcast by the other.L-14. Modified Bunsen Grease-spot Photometer. A tri-angularbox (Fig. 289a) ismadeoflightwood,the longside26in.,the short side 5 in., and the depth5 or6 in. Across the hypote-nuse of the triangle is stretched a piece of white paper PP onwhich is a row of 10 or 12 grease spotsmade with hotparaffin.A40-wbulb isplacedat SnearthesideBCtoilluminate hespotsfrom inside the box. The inside should be painted white orcoveredwith whitepaperorBristol boardto helpdiffuse the lightfrom S to all the grease spots. A lightoutsidethe boxplacedsoasto illuminate thepaper ismoved untilitsdistance s such as to

oooooooooooooo](c')FIG. 289a.Grease-spotphotometer.E;GEb)Fm. 289b.Box for observingpinhole camera effect.

makeone of thegrease spots nearlydisappear. Spotson one sideof this then look brighterand on the other side darker than thesurrounding area. This apparatus,which illustrates the princi-ple of grease-spot photometers, can be made quantitative inprinciple,thoughnot,perhaps, veryprecise,by calibrating twiththe aid of an illuminometer (foot-candlemeter).A Bunsen grease-spot photometer and a Lummer-Brodhunphotometer may be shown if desired, but they are suitable forobservation by only one student at a time. Modern photo-electric illuminometers andphotographic exposuremetersmaybeexhibited.L-15. Rectilinear Propagation of Light. An incandescentlampissurrounded byahousing, thefront of which s closedwitha sheetof heavypaperorcardboard. A projection lantern with-out lenses maybe used. By means of a steelwire 1 or 2 mm indiameter, a hole is pricked in the paper, and the light passingthroughit to a screen will produce thereonan invertedimageofthe lampfilament. Thebestsize forthe hole must be foundbytrial; toosmall ahole will notallow enough light to pass hrough,while too largea hole will notproduce a sharp image. As more


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L-16' PHOTOMETRY 37and more holes are pierced in the paper, each forms its ownimage of the filament, and these images may overlap. Theillumination of the screen is the result of the superposition of allthe different images of the source.The same effect may be shownby usinga 6-v, 32-cplampinasmallbox with a holein one side. Pinholes of different sizes aremadeina disk arrangedto turnso that anyhole can be broughtin front of the opening in the box. The light from eachpinholein turn falls on the screen to produce an image large enough forthe class to see. An empty chalk box is satisfactory. Thecracks can be closed with black gummed tape. Any of themethods described in L-6 to L-1O may be used for showingrectilinear propagation.I-16. Pinhole Camera. Two boxes are made, one to slidesnugly within the other (Fig. 289b). Oneend oftheinnerbox isclosedwith a piece of opalglass G. Theendof the outer box isclosed witha brass plate throughwhich is boreda 1-mm hole H.Theobserver looks throughthe end of the inner boxatEto seeaninvertedimage of externalobjects onthe glassplate G. The sizeof image maybe variedby slidingthe inner boxin and out. Thecamera can be usedinafully lighted oom provided that the endEis arrangedto fit overthe forehead and the bridge of the noseso as to exclude extraneous light.Onabright sunny day, thepinhole cameraeffectmaybe shownon a large scale. The room is darkened, and light is admittedonly througha hole 1 or 2 in. in diameterin a curtainorshutter.Invertedimages of outside objects or passers-by maybe seen onthe opposite wall. If the background is dark (e.g., grass, shrub-

bery, ora brickbuilding), an assistantdressed in white, walkingbackandforth within the proper area, perhapswaving his arms,makes a good object. With snow on the ground, he should ofcourse wear dark clothes. The size of the image depends uponthe distance of the screen fromthe hole.A photographof a local building made withacamerainwhichthe lens has beenreplaced byapinhole maybeexhibited eitherasalantern slide orby opaqueprojection. To make the pinhole, athin brassplate ismarked withapunchbya sharphammerblow,and the resulting bulge is filed until the first trace ofan apertureappears. With sucha pinhole, anexposureof about halfan hourin full sunlight is required to produce a photographof good


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376 LIGHT [L-17quality. Pinholesof reproducible size may be made by punc-turing aluminum foil with a sewing needle, noting the depth towhich the needle penetratesthe foil.L-17. Speed of Light. It is scarcely practicable to measurethe speedof lightas a demonstration experiment. However, byusingasmallmirror mountedonahigh-speed top,' anappreciableeffect over moderate distances may be shown by Foucault'smethod. The principle of Fizeau's toothed-wheel method maybe demonstratedby use of sound instead of light (see S-Si). Itmaybesufficientsimply to setupmirrorsand lightsource on thelecture table so that thestudentmayvisualizethe arrangementofoptical parts used in one or more of the standard methodsemployed for determining the speed of light.

REFLECTIONL-18. Plane Mirror. The virtual image formed by a planemirror may be demonstratedby superposing it on a real object.A light is placed n front of a sheetof glass, behind which, at anequaldistance, isplaced a glassof water. Theimageof the light

appearsto be in thewater. Tomakethe experiment visible to alargeclass, the apparatusshould bemounted on arotating tableso that itcan be turned. This also enables theclass to see by theparallaxeffect that the image is located behind the mirrorbut isfixedwith respect to it. Ifthe lecture room isbanked so that theseats in the rear are high, it is importantto locate the light lowenough and to have the sheet of glass tall enough so that theimage may be seen from the uppermostseats. A convenientsource of light is a candle or a small electric light run from atransformeror storage battery.AlargeletterF s cutfrom cardboard andmountedin front ofa planemirrorona rotatingtable. Theinterchangeof rightandleft is evident. With two mirrors set at an angle of 900, theimagecan be broughtback to proper orientation.L-19. Reflection at Normaland at GrazingIncidence. Thedifferencein theamount oflight reflected fromglassnear normalincidence and near grazing incidence can be readily shown byholding a large sheet of clear glass in the beam of light from a

1Experimental arrangementsare described in Harnwell and Livingood,"Experimental Atomic Physics," McGraw-Hill Book Company, Inc.,NewYork, 1933.


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L-20} REFLECTION 377lantern. The glass is held near the lantern,setmidwaybetweentwo walls, and the light is reflected first to a spot on one wallnearlyinline withtheoriginal beam(grazing incidence) and hento another spot on the opposite wall almost behind the source(normal incidence). Theintensity ofthe reflected light at graz-ing incidence ismany timesthat at normal incidence.If it is desired to keep the positionof the illuminatedspotunchanged, the lantern may be turned around through 180while the mirror is turned through 90. Onemay compare theintensityof beams reflectedby planeglass andby a well-silveredmirror. At normal ncidence, there is great contrast; at grazingincidence, almostnone. A single sheet of glass, half clear andhalf silvered, is convenient for rapid comparison.L-20. Multiple Reflections in Plane Mirrors. Two planemirrors, which may be hinged together, are placedon a rotatingtable so that they stand vertically. Between them is placed asmall source of light such as a flashlight or automobile taillight.The multiple images can be seen by a large class if the source isplaced close to the vertex of the dihedral angle formedby themirrors. When a sheet of clear glass held in a slotted block isset vertically betweenthe two mirrors, so as to form approxi-mately an isosceles triangle with them, the number of imagesvisible is greatly increased. By raising the front edges of thetwo mirrors and slightly tilting the sheet of glass backward,the images canbe madevisible to those in the raised seats of thelecture room. If the single light source is now replaced orsupplemented byseveral differentlycolored Christmas-tree lights,some very beautifuleffects can be produced, and the principleof the kaleidoscope canbe shown to the entire class at once.Byplacing wo mirrorsexactlyat right angles to one another,it ispossible "tosee yourself as others see you." Right and eftare interchangedwithout distortionso that the student has theillusion of looking at himself in a plane mirror, but he may besurprised at the behaviorof his image if he winks his right eye!The number of reflections increases as the angle between thetwo mirrors s reduced. If wo mirrorsareplacednearlyparallel,the number of reflections is very large ("barbershop effect").Place a bright headlight bulb midway between two parallelmirrors, andobserve the multiple reflections througha peep-holemadeby removing the silver from asmallarea of one mirror.


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378 LIGHT [L-21L-21. Plane MirrorsSpecial Combinations. If two plane

mirrors makean angleof90 with oneanother,thenarayoflightincidentupon one of them is turned through 180 and reflectedback parallelto itself regardless of the direction of incidence, solongas the incident ray is in a plane normalto both mirrors. Ifthree mirrors are mutually perpendicular (like the three facesforming one corner of a cube), then a rayoflight incident onthesystemwill bereflectedparallel toitselfregardless of thedirectionof incidence. Thisprinciple isusedin manyroad signswheretheletters or othercharacters aremade ofmanymirrors ofthis type.Suchreflectors are available at automobile supply stores.L-22. LawsofReflection. Thefact thatwhenlightisreflectedfrom aplanemirrortheangleof reflectionis equaltothe angle ofincidence may be shown with any mirror and source of light.However, it is moreconvenient to use an opticaldisk (L-6). Abeam of lightfallsupona mirrorwhose planeis perpendicular tothedisk. By turning the disk andwithitthemirror,it is shownthat the reflected beam turns through twice the angle throughwhich the mirror is turned.

L-23. Concave MirrorPhantom Bouquet. This experimenthasmany forms. Themethod nall is to superpose part or all ofthe real image of an object 0Eye (called the "bouquet"), formedShield by a concave spherical mirror, onsome other object C (called theFIG. 290.Optical arrangement or "vase") so that apersonviewingshowing phantombouquet. .the combination will have the

illusion that it is all real (Fig. 290). For satisfactory esults,the imageI of the bouquetproduced bythemirrormust actuallyfall in theproperpositionwithrespectto thevase Cso thatparal-laxwillnotdestroy he illusion. Theobservermust be ableto seeonly Cand he imageof0 andnot0 tself, which sthereason ortheshield. The illumination ofthebouquet0 and of the vase Cmust be such as to preserve the illusion. Both should be illumi-nated in corresponding manners by the sametype and intensityoflight. Thus 0willbelightedfrom the side next to the mirrorbyL1, while C will be lightedfrom the side next the observer bythe lightL2, all lights tobe hidden if possible. Themirrorshouldbe as large as possible,but it is equally mportant touse a mirror

"2.


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L-26} REFLECTION 379withalargeratioofdiameter ofocal ength. Thus thebrillianceis ncreased andtheangularfield coveredby the image senlarged.Generally the object is put just below the center of curvatureof the mirror, and the part seen is just above it. The class isasked first to see the illusion and then to locate the bouquet.L-24. A blackened porcelain receptacle is mounted on theunder side of the top of a wooden box, andjust above it, in fullview on top of the box, is mounteda second (white) receptacle.The interior of the box is painteddeadblack. The blackrecep-tacle contains a 40-w tungsten lamp; the white one is empty.Aconcave mirror is set behind the boxsoastoformareal imageof the lamp above the emptysocket. Whenproperly adjusted,the arrangementgives the illusion that the lamp is situated inthe upper socket; butwhenthe lampis turned off byan externalswitch, the image disappears.L-25. Concave MirrorsCaustics. Large cylindrical mirrorsthatare excellent for showing caustics andotherphenomena to alarge class can be cheaply and easily madefrom thin strip metalfastenedto a wooden form. Cutout ofplywood theshapeof themirrordesiredspherical, parabolic, etc. To the curved edge ofthis form screw or nail a strip of thin nickel or tinned iron sheetabout 2.5 in. wide. If now the inside surface of the board ispaintedwhite or coveredwith a white paperor Bristol board, thecaustic formed by the mirror is easily seen. A semicircularmirrorof 6-to 7-in, radiusandaparabolic mirrorofcorrespondingsizeareconvenient. When illuminatedby a bright source, they canbeseenby c?P't7!T7ginmirrora large class.

L-26. Ellipsoidal Mirror. The par- c4amP30co I,allel beam formed by a converging enslens using incident light previously Fia.291.Ellipsoidalmirror.reflected from the distant focus of anellipsoidal mirror is more intense than if the same source is atthe nearer focus of the ellipsoid (which is also the focus of thelens) because reflection,in the first case, concentrates he radia-tion from 1800 into a muchsmaller cone.Cut outofplywood anellipse50 cm longby 30 cmwide, whichhas a focal ength of about5 cm (Fig. 291). Mount an electric-light socket accuratelyat each focus. Fasten a strip of monelmetal or other highly reflecting material 17 cmwide around the


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380 LIGHT [L-27edgeof theboard,leaving anopeningabout 3 cmwideatoneend.Placea condensing lens opposite the open end, with onefocus attheadjacentfocusoftheellipse. Screwinto eachsocket asingle-filament lamp (e.g., Westinghouse T-14, 6 v), and cover withblackpaint hat side of the most distant lampthat faces the lens,toinsurethat the only lightthat reachesthe lens from the distantlamp arrives by reflection from the ellipsoid. By operating thetwo lamps alternatelywith the aid ofa two-way switch, it willbefoundthat themore intense beamoriginates at the distant lamp.For a parabolic mirror formedby rotating mercury, see M-143.

L-27. Convex Mirror. The reduced, virtual image seen inconvex mirrors is doubtless familiar to most students. Distor-tion of image isfrequentlyobserved when parts of an object are\\\\\\\

n. ieeI Sljo,7L focusbc3'll lensFio. 292.Arrangementfor viewing highly distorted image in convex mirror.at different distances from such a mirror. Anamusingexampleofthisdistortion sseen when reflection takesplace fromaconvexsurface of high curvature, such as a small steel ball (Fig. 292).The ball, in. to 1 in. in diameter, is held 3 or 4 in. awayfromthe observer's ye. The image isviewedwithastrong ens. Theobserver's nose akes on ludicrousproportions. Forconvenience,lens and ballmaybeheldinawire frame;if a 4-in, lens and i-in.ball areused,they may be separated 3 in. The shorter the focallengthof the lens, the more exaggerated the effect.

REFRACTIONL-28. Refraction. Theimageof astraightwire isprojectedona screen. Near thewire ontheside next to theprojection ens isheld a thick rectangularblock of glass making a considerableangle withthe normal tothe light beam. A part ofthe imageofthewire onthescreen willbe displaced. If thickglass is lacking,

aparallel-sided glass vessel ofwater (projection cell) maybeused.Refraction oflightby watermaybe shown with theopticaltank(L-7). The ripple-tank method (S-49) of showing refraction


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L-32J REFRACTION 381of surfacewaves is useful for emphasizing the relationof ref ae-tion to wave velocity.L-29. Refraction by Shadow with Cube of Glass. A cube orlargerectangularblock ofglassis placed ona sheet ofwhite paper(Fig. 293). A strip of cardboardor metal ofthe same heightasthe block of glass and twice as long is set verticallyagainst it asshown. A straight-filament lamp is placedhorizontally with itsfilament parallel to the edgeofthe metalstrip and insucha posi-tion as to throw a shadow of the pro-jecting edge of the strip at the lowercorner ofthe block. Thelight refractedby the block will fall much nearer theobstacle, and the edge of the shadowwithin the cube will be easilyseen. Theindex of refraction of the glass can becalculated from the dimensions. The Fio. 293.The shadow ofthe vertical shield is shorterfilament and refractmg edges maybe set in the glass thaninthe air.vertically and the whole apparatusmounted on a rotating table so as to be more easily visible tothe class.L-30. Refraction Model. An axle3 or4 in. long rolls on twoindependentwheels 1 in. in diameter. It will roll with littlefriction down a slightly inclined board. If it isrolled at an angleacross the line of separationbetweena plain boardand a cloth-covered one, it will change its direction, as a ray of light isrefracted. "Total internal reflection" can also be shown.L-31. Refraction by Gases. Thechange of index ofrefractionof air with temperaturemaybeshown byallowing the lightfroma strongly illuminated pinhole to cast the shadow of a Bunsenburner onthescreen (11-137). When hegas is lighted, the rising,turbulent airwill be madeclearly evidentonthescreen farabovethe flame. Anyhotobject will show the same shadow effect bythe convection currentsrising from it.A related experiment may be performed by holdinga warmobjectin the optical path ofone arm of a Michelson interferome-ter. The interference fringes are displaced in the neighborhoodof the object (L-72).

L-32. Mirage. A piece of sheetmetalseveral feet long and3to 6 in. wide is supported on stands horizontally above thelecture table. It sheatedbyarow ofgasburners placedbeneath


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382 LIGHT [L-32it, spaced closely enough so that the heating is fairly uniform.Apipeclosed at one end and drilledwitha row of closely spacedsmall holes (M.-58) maybeusedin place ofthe separateburners.Thepipemustbeatleastaninchin diameter, and the holesmustbesmall;otherwisemost of thegaswill escapebefore reaching thefar end ofthepipe. Atone endatabout the heightofthe metalplate is placeda 40-w lamp (Fig.294). Close to the end of theplate is heldapiece offine ground glassor lightpaperBonwhichsmall figures of trees, houses, etc., have been drawn with India

Eye*Light ABI4S __W_5 5_L.JL5_ZFia.294.Thepathof thelightiscurved by thelayerof hotairabove he metalplatetoproduce theeffect of mirage.ink. The figuresshould beaboutaninchhigh, and he bottomofthe figures or the foreground should be in line with the metalplate. Between the light and the plate may be set a secondground-glass screen A to diffuse the light. If now the eye isplaced about on a level with the plate, a "mirage" willbe seen.By careful adjustment,the smallfigureswillbe seen inverted nthe "lake" above the metalplate.The gas-heated plate of the preceding paragraph may bereplaced by an electrically heated unit (Fig. 295). The unitB B B B B

FFIG. 295.Electrich

F Ieater forproducing mirage.consists ofa woodenframeFFabout 3 in. squareand 9 ft long, tothe top of which are fastened five or more bridges B,B of thecross section shownin Fig. 296a. Thebridge proper is cut fromheavybrass and carries onits top wostripsofasbestos apeeach1 in. wide. Beneath the bridge is a blockof asbestos or otherinsulatingmaterial that supportsfour nichrome heater strips ofsuch width and thickness that their resistance will allow them tobe connected directly o the 110-vpower ineandstill give agooddeal of heat. The top of the wooden frame is covered withasbestosto keep it from scorching or catching fire. Toprevent


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L-33] REFRACTION 383sagging, both the heater and the asbestos strips are kept undertensionby the spring and levermechanism shown in Fig. 296b.A rectangularframeLLis fastenedto the mainframeFFby anaxle rod at D. The asbestos stripsare attachedto the upperrodof this frame,and the heater stripsare attached to an insulatingblockNthat is connected to the lowerrodoftheframebysprings.It is evident that the springswill keep both sets of strips taut.To preventtheheaterstripsfromtouchingeachotherand causinga short circuit,asbestos stringis woven between them at appropriate intervals. Toconfinethe heated air andto overcome thedisturbanceof cross drafts and convection currents,boards some8 or 10 in. wide and8 or 9ft long are placed at either side of theunit. Thismakes atroughofquietheatedair3or4 in. deep.

(c)

,4sbes/os BNecter

(b)Fin. 296.Details of heater shown in Fig. 294: (a) bridge supporting heaterelements; (b) spring and leverarrangement or keeping strips taut.L-33. Refractive IndexChristiansen Filters. If a liquidmixture s adjustedto have the same index ofrefraction asthatofa piece of glass immersed in it, light willnot be reflected at theinterface between the two, and the glass will not be visible.Thus ablock of glasslowered into sucha liquid byablack hreadseems to disappear, leaving only the threadvisible. A rod of

glass dipped into the liquid and withdrawnseems to drip liquidglass. A parallel-sided glass vessel containing a quantity offinely powdered glass is opaquebut becomes transparent whenthe liquidisadded. However, since the two indices ofrefractioncan be the same for only one wave length, the powder-liquidmixture can be made into an excellent Christiansen color filter)Soft glass should be used, and the composition of the liquidadjustedby trial. The glassis boiled in aqua regiato free it ofgrease and dirt and thoroughly washed with water. It is thendried and placed in a glass vessel. Enough carbon disulfide i''WooD, R. W., "Physical Optics," 3d ed., p. 115, The MacmillanCornpany, New York, 1934.


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384 LIGHT {L-34added towet the glass,and hen benzeneis poured n,a ittle at atime. As the mixturebeginstobecome transparent,it transmitsfirst red, then yellow, and so on. A permanent setup may bemade by drawing down the neck of a flask after the powderedglass has beenput into it. The liquid mixture, adjusted totheproperproportions byprevious trial, isthenadded. The flask ispackedin a freezing mixtureto reduce the vapor pressure of theinflammable liquidandwrapped ina owel to guardagainst injuryin the event ofanexplosion. The neck of the flask isthensealedoff. Thecold flask isopaquebut asit is warmedup, eitherwith asmallflame or with the hands,it exhibits colors.The following liquids are suitable: cedar oil; glycerin; carbondisulfidewith benzene, amyl acetate,or phenyl phthalate. Theproportionswill dependon the index of refraction of the glassas wellas on that of the liquids themselves. Pyrexglassisnotsuitable on account of its high index; soft glass should be used.Clearfused quartzisvery satisfactory.L-34. Total Internal Reflection. Lightincidenton the end ofa curved rod ofglass orfusedquartz will,bytotal internal reflec-tion, be carried to the other end without loss throughthesides.The ends of the rod should be squareand preferablypolished.Thelightsource should besmall andbrightandplaced as closetothe end of the rod as possible. By using flashlight or small6-vbulbs, the filamentmaybeputnear theend of the rod. Anotherway to illuminate the rod is to focusthe light from an intensesource upon its end. The rod should be passed through tight-fittingholes intwoor three screens so that nolight can reach thefar end of the rod except throughit. A whitesurface, such as asheet ofheavy paper, is held near theexitendoftherod, tilted sothat the class canseeit, and is illuminatedby ightemergingfromtheendof the rod. Most glass absorbsviolet much morethan itdoes red, so that evenwith white light as a source the lightthatemerges is yellowish.L-35. Critical Angle and Total Internal Reflection. In Fig.297 is showna convenient arrangement or demonstrating efrac-tionand total internal reflection. A largeglass jar with parallelsides is filled with water containing a little fluorescein. Aplanemirror M1 reflects a slightly convergent beam upon the watersurface where it is refractedat A. At B it is totally reflectedto a second planemirror M2 mounted on a spindle so that it can


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L-37] REFRACTION 385beturned abouta horizontal axis byahandleprojectingupward.By changing the positionof M2, the angle of incidence of thebeam at the water-airsurface canbe changed, and critical angleand total internal reflection demonstrated. The path of thebeam, both in the water and in the air, is made more visible byplacing asheetof flashedopal glassvertically in the tank,soas tomake a small acute anglewithbeam emerging from the sur-face of the water can be seento be colored on account ofdispersion by the water.L-36. IlluminatedFountain.A cone or funnel-shaped vessel(Fig. 298) with a watertightwindowW, anentrance ubeA,and an exit tube B, is arrangedto throw a stream of waterinto the sink. Astrongbeamofthe window to the exit tube.reflection along thecurved stream of wateruntilitbreaksupintodrops. Thedemonstration isenhanced byplacingcolor filters nthe path of the light to color the jet. Red, green, and amberfilters are especially effective. If no color filters are available, acarbon arc can be used close to the window W, and the lightfocused by means of the heavycondensing lenses only. Onaccount of the chromatic aberra-tion of the lens, the color of the

streammay be changed by mov-ingthe arc backandforth througha a smalldistance. Screens shouldbe interposed to cut out straylight.L-37. Total Internal Reflection at Glass-air Interface. Twosheets of thin glass (lanternor microscope slides) separated bystripsof paper are fastened ogetherwith waterproof cement, sothat theyhavean air space between hem. When theplatesareimmersed in water in an optical tank (L-7) and turned at theproper angle to an incident light beam, they will exhibit totalinternal reflection.

the direction of the beam. The

FIG. 297.Water tank and mirrors toshow total internal eflection.lightisfocusedby lensesthroughThis light is carriedby internal

FIG. 298.Light follows the waterjet byinternalreflection.


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386 LIGHT [L-38L-38. Transmission andReflection nearCritical Angle. The

image of abrightlyilluminatedslit is projected on a screen. Inthe path of the light is placeda rectangularglass container ofwaterin whichis immersed theslidewith the air film described inthepreceding paragraph. When thisslide is set so that the angleof incidence is about 49, part of the light incident on it will betransmitted,andpartwillbe totallyreflected off to theside whereit may be caught on a second screen. The colors of the twoimages are complementary in the region of the critical angle.Theexperiment ismost satisfactory ftheentire beam falls on theslide and if the light isnearlyparallel. In anotherarrangement,two 90 prisms are placedwith their hypotenuses together, theentrapped air film producing the results described. In eithercase, the resultingbeams may, if desired, be dispersed by otherprisms and lenses to show two spectra whose colors are com-plementary. As the device is turned in the neighborhoodof thecriticalangle, the colorsare transferred rom onespectrumto theother.L-39. "Mystery Ball." Coat a ball heavily with soot.Immerse t in a jarofwater. The sootserves to keepan air filmaroundthe ball. It appears brightandsilvery whensubmergedbecause of reflection of light at the water-airinterface.L-40. Comparison of Total Internal and MetallicReflection.A test tube is partlyfilled with mercury, closed with a stopper,and immersed in a jar of clear water in the path of a beam oflightfrom a lantern. Whenviewed from an angleof about 100from the incident beam, the light reflected from the mercuryappears quite dull in comparison with the light totallyreflectedat the glass-air surfacejustabove the mercury.L-41. Reflections from Diamond. A pencil of light is pro-jectedtoward the class througha small hole in a large sheet ofwhite cardboard. In the beam is placed as large a diamondasprocurable, and the reflections, both internal and external,producea beautifulpattern on the cardboard.L-42. Dispersion by a Prism. Since index of refraction is afunction of frequency, it is possible to break up white lightinto the familiar colors of the spectrum. Light from a straight-filament source or a stronglyilluminated slit S is focused on ascreen by a projection lensP (Fig. 299). A prism of glass or ahollow prism filled with carbon disulfide is introducedinto the


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L-46] REFRACTION 387beam beyond the projection lens. A continuous spectrum isthus formed on the screen.L-43. Rainbow. A vertical slit is illuminatedby a beam ofparallel lightfrom the lantern asin hepreceding experiment. Inthe beam from the slit is placed a glass beakerfilled with water.Twospectra may be seen on a screen placedbetween the slit andthe beakerand to one side.The projection lens is removed from an arc lantern, and theposition of the arc with respect to the condenser is adjusted toproduce a parallel beamwhich is directed towardthe class. Thebeampasses througha 4-in, hole in the middle of a white card-boardscreen 3 ft squareandfallson asphericalflaskof water4 in.indiameter,set2 ft from the screen. Ablackdisk isplaced ust

RedI V'b,'ef

beyond the flask to prevent glare. On the screen there appearsa circular spectrum, with the red outermost, representing aprimaryrainbow.L.-44. Rainbow Droplets. Coata sheet ofglass withsoot fromburning turpentine. Spraythesurfacewith waterdropletsfromanatomizer, and lluminate the surface by a strongparallel beamoflight. Thewaterdroplets donotwettheplate;hence theyarespherical because of surface tension. When they are viewedfrom an angle of about 410, the droplets glistenlike individualcolored jewels. Drops about 0.5 mm in diameter show best.They should be allowed to fall on the plate perpendicularly, astheywifi otherwise bounce off.L-45. Artificial Rainbow. Form a horizontalspectrumas inL-42. Place a tube of water 1.5 in. in diameterbetween prismand screen so that the tube is perpendicular o one edgeof theprism. A "rainbow" in the form ofaverticalcircle is castuponthe screen or the walls of the room.

L-46. Scattering ofLight"SunsetExperiment." A circularopening ina sheet of metal is stronglyilluminated with the lightfrom a projection lanternand an enlarged image of the opening,

Fm. 299.Formationofspectrumbyaprism.


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388 LIGHT [L-47the "sun," is projected on a screen. The room should bedarkened and stray light eliminated. Between the circularopening and the projection lens is placed a glass vessel withparallel sides 5 in. apart, which is nearly filled with a measuredquantity of clear water (distilled ifnecessary). The clear waterscatters very little light, so that the "sun" iswhite or perhapsyellow. Then 10 ml of concentrated solution of photographichypo (sodium thiosulfate) and 3 to 8 ml of 5-normal solutionof either hydrochloric or sulfuric acid are added for each 500 mlof water. The exact amount and concentration of the acid arenot critical. In about a minute, colloidal sulfur will begin toform, and the beam passing through the liquid will look brightblue by scattered light. The number and size of the scatteringparticles increase so that in from 2 to 8 mm, depending on thetemperature,even red light will no longer be transmitted. Thetemperatureof the water should be between 20 and 30C formost satisfactory esults.

LENSESL-47. ImageFormation byLenses. Anumber offundamentalexperiments showing the relation between image and objectdistances andfocal ength may be performed by castingthe realimageof a lampfilamentupon a screen, using various distancesand variousconverging lensesof different focal lengths. Imagesof different size are cast by different lenses, keeping lamp-to-screen distance constant. Conjugate foci may be shown forconstant lamp-to-screen distance by setting a givenlens succes-

sively at two predetermined points. Finally, after casting asharp image of the lamp filament (or other object) upon thescreen, move the lens away from the objectuntil the image isblurred. Thenpickupasharp magebetween ens andscreen, bymovinga piece of white cardboard or opal glass along the lightpath to the correct image position.L-48. Actionof a Lens. Enclose a bright source (automobileor clear-glass incandescent lamp) in a tubular housing withaperturethesize of a converginglens. Adjustthe lens to cast animage (nottoo large) of the filament on a screen. Movethe lensaside, andclosetheendofthehousingwithametal foilcontainingmany pinholes. Many images of the filament will thus be


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L-49j LENSES 389formed on the screen (L-16). Now move the lens back to itsformerposition, andshow howit collects the many images intoasingle image.L-49. Chromatic and Spherical Aberration. A large thickspherical lens, preferablypiano-convex, is most satisfactory fordemonstratingaberrations. The lens should be provided withseveral diaphragms, one allowing only the centralportionof thebeamto pass, asecond allowingonlyanannularportionnear theedge of the lens to pass, and a third havingtwo pairs of smallroundorvarious-shaped openings, onepairnearer thecenter thanthe other pair. A smallopening at the focus of the condensinglensesofaprojection lanternisthe lightsource. The lensLtobestudied is placedfarenough from thisopening to be illuminatedall over and so that it forms a real image of theopening. Thebeam of light after leaving the lens may be made visible by thetrough,thegauze screen,orthesmokebox (L-7, 8, 9). Thegauzescreen willprobablyshow the colors best. Anothermethod is tohave the beamfromthe lens pointed towardthe class, interrupt-ing it with amovable translucentscreen or a sheet of ground orflashed opalglass or a smoothwhitepaper that is not too heavy.Bymoving he screen,thechanging cross section ofthebeammaybe shown in amannerthat enables the student to graspthe ideaofimageformation. The distance between he focus ofthe rayspassing through the center of the lens (first diaphragm) and thatofthe rayspassing through he annulardiaphragmnear the edgeof the lens (second diaphragm) is a measure of the sphericalaberration. Withthe latterdiaphragm, a ring of lightis formedonthescreen and sclearly seen to befringedwithcolor, redwhenthe screen ison the side of the focus toward the lens and bluewhenit is ontheopposite side. Tipping he lensseemsto tie thisring into a bowknot. The central beam and the four-openingdiaphragmhelp to show the formationof aberrations. By turn-ing the lens with first one side and then the other toward thelight, the effect of relativepositionon aberrationscanbe demon-strated. With a piano-convex lens, the differenceismarked ifthe objectand the imagedistances are very different. By usinglargemagnification (sourceplaced only very slightly beyondtheprincipal ocus),thedistancebetween thefocifortherimrays andforthecentralrayscan be madeas greatas2ft,usinganordinarylantern condensing lens.


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390 LIGHT [L-50Instead of an illuminatedopening, a small concentrated-fila-

mentautomobile orflashlightbulb run atratedvoltage orslightlyovermay be used. Nodiaphragm s then needed except to limitthe beam so that stray light will not blind the class. Anothersatisfactorysource is a200-wclear-glass incandescent lamp,withthe plane of its filament perpendicular to the optical axis of thesystem. Thelamp should be enclosedto reducestray light.L-5O. Effect of Medium on Focal Length. Place a lens in aparallel beam of light, and determine its focal length. Thenimmerse thelensin atankof water (L-7), and find thenewfocallength. Since the focal length of a lens immersed in wateris approximately four times that of the same lens in air, it isimportantto use a lens of short focal length. If a long tank isnotavailable, the experiment maybe performed with the lens ina small vessel, such as a projection cell, with parallel sides.

* L1 II(ci) pb*tb) pL ' Screet?Fin. 300.Arrangementoflenses and diaphragms for producing distortion: (a)barrel-shaped; (b) pincushion.L-51. Astigmatism and Distortion. A coarse wire mesh or alantern slide ruled witha rectangularnetwork of horizontalandvertical ines is usedas an objectand is illuminatedby lightfromthe condenser lens ofa antern. Large condenser lenses of rather

short focal engthare then usedto form amuchenlarged image ofthe mesh on the screen. If the lens is turned about an axisparallelto either set of wires or rulings, there will be found twopositions of the screen, for one of which onlythe horizontal inesare in focus; for the other, onlytheverticallines are infocus. Aconcave mirror will show a similar effect. A set of radial linesmay be used instead of therectangularmesh.L-52. Distortion. Apieceofwire gauzeoraglassplateGruledwith a rectangularnetwork s placedin thebeam of light from alantern, perhaps 5 cm from the condenser lenses. An image ofthe mesh is cast on a distant screen by a double convex lens L.Whena diaphragm Displacedinfront ofthe lens,as inFig. 300a,


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L-55j OPTICAL INSTRUMENTS 391barrel-shaped distortionof the imageresults. Ifthe diaphragmis placed on the other side of the lens (Fig. 300b), and severaltimes its focal length away, pincushion distortion of the imagewill be seen. The size and location of the diaphragmmust bedetermined by trial. In the first case, the hole should be lessthan 1 cm indiameter; n the secondcase,asomewhat largerholeworks better. The sheet in which the hole is made should belarge enough to shield the screen from stray light from thelantern.The singlelensmaybereplaced bytwo lenses,6to10cmapart,whose combined focallengthis about the equivalent ofthe singlelens. The twotypes of distortion can be shown asbefore, but ifthe diaphragmis placed between the two lenses, there is nodistortion.

OPTICAL INSTRUMENTSL-53. Microscope. The action of a microscope can beexplained with the aid of wall chartspublished bythe makersofmicroscopes to illustrate the formation of the image in their

respective instruments.L-54. Model Microscope. The essential optical parts of amicroscopemay be demonstrated on a moderately largescale byusingagood-quality lens of short focallengthanda argereadingglass. The first lens (objective) is placed close to some smallbrightly illuminatedobject so as to produce an enlarged realimage ofit. The readingglass (eyepiece) is set a short distanceabove this mage. Amirrorabove the second lens inclined at450enables those looking at the system to see a greatly enlargedvirtual imageof the illuminatedobject.The wholesystemmaybe mountedon a rotating stand so thatthemirrormaybeturned towardeach memberof the class. It ispossible to make a number of minute effects visible to severalstudentsatoncebyuseofthisopticalarrangement, .g.,Brownianmovements(A-Si), Newton'srings (L-71), etc.L-55. Telescope. Astronomical, terrestrial, and Galileantelescopes canbeshowndiagrammatically. They mayalsobesetupfor demonstrationbysuitablecombinationsof lenses mountedon an optical bench. Each student maylookthrough the ensesindividually; but ifprojection is usedand animage s formed onascreen so that the whole class canseeit, therays oflightentering


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392 LIGHT [L-56the objectiveand leaving the eyepieceare no longer parallel, andthispoint in the operation of the telescope is lost.L-56. Telephoto Lens. The simple camera lens is easilydemonstratedbyformingareal image ofsome objectonascreen;telephoto orotherspecialcamera ensescanbedemonstrated lso.Theobject, illuminatedwith the condensersystemofaprojectionlantern, maybe awire mesh orcrosshatch ruling1? (Fig. 301). Acircular holeina diaphragm D limits the area tilized. The lensL1isplacedatitsfocaldistance infront ofthe object, thusgivingaparallel beam, so that the lensL2can be atanydistance romL1.It forms an image of the mesh and diaphragmin its own focalplane;this image is formed on a screen, and the diameterof the*___ __

Screen

.T LFIG. 301.Principleofthetelephoto ens.

image of the diaphragm opening and the distance from L2 tothe screen are measured. When a concave lensL3 is inserted inthelightpathbetweenL2and the screen,thescreenmust bemovedfarther back to focus the image, but the magnification is muchgreater. The diameter of the imageand the distance from L2 tothe screen are again measured; it is foundthat the magnificationratio greatlyexceedsthe lens-to-screenratio, whichis thepurposeofa elephoto ens. Thewire mesh used asobjecthelps to securea sharp focus, while the diaphragmprovides an easily visibleimage.L-57. Lens Combinations. Several fundamentalprinciples oflens combinations can be illustratedbyapairof lenses so mountedthat principal foci, principal planes (magnification of +1), andthe symmetrical conjugate planes (magnification of 1) of thesystem are outside of the combination. Such a system can beconstructed by mounting two projection-lantern condensinglensesof about 18-cm focal length ina 10-cmstovepipe about 60


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L-60] THE EYE 393cmapart,planesidesout. A lampfilamentplaced about 70 cmfrom one end produc3s an erect real image of the same size atthe same distancefrom the otherend. Ifplaced20 cmfromoneend, an inverted real image of the same size will appear atthe same distance fromthe otherend. Theprincipal foci of thesystem are, of course, midwaybetweenthe two positions of theobject and of the image respectively.

THE EYEL-58. Blind Spot. Every human eye has one spot that isblindthe portion where the optic nerve enters the eye. Todemonstrate this fact, a white circle is mounted on the black-board, anda whitecross of about the same size is moved slowlyacross the board toward the circle. rfhe student closes oneeyeand fixes his gazeonthe circle, seeing the crossoutofthe cornerof his eye. In the positionwhere the image of the cross fallson the blind spot, it disappears. Alternatively, each studentmay locate his ownblindspot bymovingabout acard held10 in.

from his eye and marked with a black circle and black cross.If his attention is fixedon the circle, he will find a spot wherethe crossdisappears. Circle andcross should be 3 in. apart.L-59. Inversion ofImage on Retina. Threeholesaredrilled inabrassplate witha No.60 drill atthe cornersof asmall riangle2 mm on each edge. The plate covers one end of a tube 1 in.long; the other end is closedwith a similarplate having a singlehole throughit. Ifthethree holes are heldclose to the eye, withthe triangle standingon itsbase, thepattern ontheretina will bethree circular patchesoflight in a riangle standingon its vertex.The system of holes limits the light entering the eye to threepencils originating so closeto theeyethat theycannot beinvertedby the lens. Hence distant objects viewed through the instru-ment will appear "rightside up,"whereas the orientationof thetriangular pattern demonstrates conclusively that the imageonthe retina is inverted.L-60. Fluorescence of Retina. Turn toward the class in adark room a strong ultravioletsource (quartz mercury lamp orarc) screened to exclude all visible light (L-114). Call atten-tion to the luminous haze that covers the field of view of eachindividual.


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394 LIGHT [L-61L-61. Visual Fatigue. The eye quickly becomes fatigued

after looking steadily for a few seconds at a single color. It ispossibletoproduce theeffectinafully lighted oom byprojectinga bright spot of color on a screen. After the spot has beenobservedsteadilyforafew seconds,it s extinguished, anda spotof the complementary colorappearsin its place. It isimportantnot toshift the eyesduring heprogress oftheexperiment. Red,green, yellow,andblue filters give the besteffects.L-62. Color Blindness. Slideswitha largenumber of coloredrectangles areobtainable bywhich the usualtypes of color blind-ness may be tested. Directions are providedwith the slides.Each member of the class is askedto writedown the numbers ofthose rectangles which resemble a test color; the numbersmaythen be quickly checkedagainstthe correctnumbers. In manycases, students discover color blindness of which they wereunawareprior to the test. Color-blindness test charts are alsoavailable but are better suited to individual than to class use.L-63. Chromatic Aberration of the Eye. A purple filter ismountedin front of a clear-glasstungsten lamp having straightfilaments. The eyehas difficulty in focusing simultaneously onboth the redandtheblue mages. Theeffect cannotbeseen wellfrom a distance; norisit the same for all individuals.L-64. Astigmatism. Astigmatism is usuallytested by observ-ingachartbearing aset of radialblack lines. To thenormaleye,these lines appear equal in intensity, but to an astigmaticeyethey appear in varying degrees of sharpness, depending uponthe direction of the axis of astigmatism.L-65. Eye Models. A spherical flask is filled with slightlymilky water to represent the eyeball. A large lens is placednear the flask so as to bring the image of a bright light to afocus on the far side of the flask. Verticalmotion of the lightshows that the imageonthe "retina" is inverted. Motionof thesource toward the "eye"shows thenecessity for accommodation,andadjustment of the lens position showssuchaccommodation.Moving the lens so that the focus is beyond or in front of the"retina"shows near-and far-sightedness, and these defects canbe corrected byproperselection and combination of lenses.

Several apparatuscompaniesmakeamodel oftheeye that canbe shown in the lecture, but it is more valuable as a aboratoryapparatuswhere the student himself can work with it.


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L-67] INTERFERENCE AND DIFFRACTION 395L-66. Eyeglasses. A study of lenses and the defects of the

eye maybemademore vividbyborrowingtheglassesofmembersof the class and testingthem in the light from the lantern. Thelenses are held near the screen anddrawnback fromit to locatethe position of best effect. The type of lens can be inferredfrom the image formed.Using the projection lens, throw on the screen an imageof aslide showing concentric circles crossed by radial lines, makingthis original image very sharp. Now place over the projectionlens another lens, say a diverging one. The image becomesblurred,as itwould look to apersonwith faultyvision. Correctitbyplacing infront of the diverginglensa converginglens ofthesame power. Repeat, usingconverging, diverging, andcylindri-cal lenses,each ime bringing the imageback tosharpfocuswithalens of the opposite kind. Spectacle lenses of 1 or 2 diopterspowerarerather nexpensiveand can be secured nmatchedpairseitherfroma ocal opticianordirectfrom themanufacturers. Asa by-product of this demonstration, some students may beinduced to have their eyes examined and secure much neededglasses.

INTERFERENCE AND DIFFRACTIONL-67. InterferenceColor of ThinFilms. Interference colorsin a thin film canbeprojectedso that a large class mayseethem.AGooch funnel orafiat metalringisdipped inasoapsolutionandis then heldinaclampso that the filmformedacross itsmouth isvertical. It is moreconvenient to clampthefunnel orringin theproper positionand then to forma soapfilm uponitby drawing

across its mouth a piece of paper dipped in soap solution. Abeam of light from an arc or projectionlantern is broughtto afocus just beyond the film, anda greatly enlarged imageof thefilm is thenprojectedonthescreen withafast ensL (Fig. 302) ofshort focal length, placedso that the angle between its axis andthe normalto the film is equal to the anglebetween he normaland the axis of the incidentbeam. As the water runsoutof thefilm, thefilm grows thinneratthetop (which is the bottomoftheimageon the screen). Newspectralordersappearuntil the filmis so thin that it becomesblack at the top. Soon after the topofthefilm turnsblack, itgenerally breaks. Thedurabilityofthefilmmaybe ncreasedbydecreasing ts diameter, so thatoneshould


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396 LIGHT [L-68use as small afunnel as is consistent with visibility on thescreen.To diminish convection currentsin the funnel,the end ofit maybe closed with a cork throughwhicha small hole is cut to pre-serve pressure equilibrium. The number of visible spectralorders is increased by interposing a red filter betweenthe lightsource and the film. If a gentle air jet is played on the film,the colors are mixed in abeautifulmanner.L.-68. The colors produced by interference of light in a soapbubble may be shown by holding the bubble in front of anextended source of light in a darkenedroom. The source isprovidedby housing a 100-w lamp inside a box in one end ofwhichapieceofflashedopalorground glass sfitted. Thebubble

should be held near the groundglass but should have as dark abackground as possible. Undera strongmonochromatic sourceoflightoramercuryvapor lamp (L-4), the colorsare replacedbyalternate light and dark rings.L69. The Boys rainbow cup is a hemispherical shell that canbe rotated about a vertical axis. When a soap film is formedacross theopen top of thecupand the latter s turned rapidly, thefilmbecomes thinner toward the center on account of centrifugalforce, andcircular coloredfringes are formed. Rotation may becontinued until the black spot appears at the center. If themotion is quickly stopped, the black spot may break up intoseveral irregular pieces. The film should be protected from aircurrents by a transparent cover. The color pattern may be

projectedas previously described (L-67).L-70. Interference in Thin Air Films. Two large pieces ofplate glass are laid on top of one anotheron the table. Abroad

FIG. 302.Methodofprojecting interference colorsin hin films.


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L-71} INTERFERENCEAND DIFFRACTION 397sodium flameor a mercuryarc is placed so that it may be seenreflected in the glass. The interference fringes are generallyseparatedwell enough to be seen easily.If an optical flat or its equivalent s available, pieces of glassmay be laid on it, andfrom the contour of the fringes observed,the character of the irregularities of the glass surfacemay beinferred. If some of the test pieces arealso optically flat, theirstraight even fringes maybe compared with the crookedonesofthe othersamples to show the optical method oftestingsurfaces.L-71. Newton's Rings. When a plano-convex lens of verysmallcurvatureisheldwith its convexside againsta planepieceof glass, a thin film of air is included betweenthe two pieces ofglass, the thickness of the filmincreasing with increasing distance

from thepoint ofcontact. The colored rings formed in this waymaybeprojectedbythesameoptical arrangementas that shownin Fig. 302, by placing the lens and plane glass in the positionoccupied by the soapfilm (L-67). When a red and a blue filterarealternatelyplaced nthe incident ightbeam, it isveryeasyforthe class to see the change in sizeof the rings due to change ofwave length.Amoreelegantbutmoreelaboratewayof illuminating the ringsystemisthat showninFig. 303. A strongarc or 1000-wprojec-tion lampS is focused bythe condensinglensLon avariableslit.Thelightfromthisslitpassesthrougha ensL1andalargecarbondisulfide prism P to form a strong spectrum at ABC. TheNewton's-rings apparatusisplacedat N near the center of thisspectrum,andbymeans of afast ensL1amagnified image of thering system s formedon the screen. NowbyturningtheprismPbackandforth,different colorswillfall on.Nso that the rings willchange color and size. By not making the spectrumtoo large,and by using a fairly wide slit, fast lenses and a large liquid

ArBFio. 303.Projectionf Nowton's rings.


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398 LIGHT {L-72prism the effect will be bright enough for a fairly large class tosee.L-72. Michelson Interferometer. The Michelson interferom-cter can be demonstrated to large classes. The interferometeris first adjusted for monochromatic fringes and then for white-lightfringes bygetting the light paths equal. The beamfrom astrongsource (carbon arc) is focused on the first mirror, usingawatercell to reduce the heat. With a rathershort focal length,high speed lens (4 in.,f4.5) from a camera, the fringes may beprojectedona creen. Incase the direction of the beamemergingfromthe interferometer is not thatdesired or projection, aplanemirror of good quality may be interposed. The fringes may beeasilyseen iftheyareprojectedonaflashed-opalscreen facing theclass. If a hot wire or the hot stick of a match that has beenlightedandblownout is placed in one arm ofthe interferometer,thedistortionproduced bychange of index of refractionoftheairdue to heatingwill be evident (L-30).L-73. Single-slitDiffraction. Each student in the class mayobserve the diffraction of light througha narrow slit by simplyholding two adjacent fingers directly in front of one eye andobserving a straight-filament lamp through the narrow aperturethus produced. The fingersmust be held parallel to the filamentandvery close together (see also L-82).L-74. Diffractionby StraightEdge. Pieces of razor blade arecemented between sheets of cellophane to eliminate danger ofinjury and are distributed to the members of the class. Auenclosed carbon arc isset inthe front oftheroom pointing owardthe class. If the edge of abladeisheldclose to the eyeso as tocut off part of the light from the arc, interference fringes due todiffraction will be observedfar into the shadow. Screening thearc with redglass may improvethe visibility of the fringes. Inthis case, stray light should be eliminated, since the intensity isreduced by the glass.L-75. DiffractionbyaFeather. A brightsourceisfocused bya condensing lens L1 (Fig. 304) on a vertical slit. The lens L2forms an image of the slit at A, wherea verticalobstacle like atripodrodinterceptsthe light. A featheror othersimilarobjectis placedat such aposition0 that the lensL2 forms an enlargedimage of it on the screen. This image willbe formed by lightdiffracted by the feather and hence deflected so as to pass the


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L-77] INTERFERENCE AND DIFFRACTION 399rod, which interceptsall of the light in the direct beamso thatwithout the feather in position the field is dark. A 1000-wlantern bulb willgive sufficient light so that a largeclass can seetheeffect. The slitmaybe to in. wide and the obstacle in.wide. The slit is put at the place where the condensinglens ofthe lantern forms the image of the filament. The lensL2 shouldbeachromatic. The finerthe texture oftheobjectat0 the morstriking heresult;feathers areespeciallygood,but finewasteor acoarse transmission gratingwill serve.Small feathers mounted between microscope slides may bepassed aroundthe class so that each studentmay look throughafeatherat a bright point source to observe diffraction.

Screet-,FIG. 304.Projectionofimagebydiffracted light.L-76. Diffraction Box. Many diffraction demonstrations aregreatlyfacilitatedby a permanent, thoughsemiportable, diffrac-tion box (peep show) 30 cmsquareandsome 5 m long. At oneend is a door, throughwhich the source, lens, anda color screenmay be adjusted. About 75 cm from this end is a partition inwhich the small aperture is mounted;it is accessible through adoor in the side of the box. A holder for diffraction specimensandmasking screen ismounted onasmall ripod that is movableto any part of the box. The other end of the box is providedwith an interchangeable holder for aneyepiece or viewingscreen.Distances hathave beenusedwithgoodresultsare the following:slit to diffracting object, 202 cm; object to eyepiece, 237 cm;width of object, 0.55 cm; this gives 1.2 cm for the width of thediffracted image when ight of wavelength5461 (mercury arc) isused.L-77. Fresnel Diffraction. A bright light A (Fig. 305) isfocused bymeans of lens Bon apinhole atC. Thelightspreadsout, illuminating the screen G. At some intermediatepoint are

placed such objects as pins, screws, fine wires, and small slits.Their shadows on the screen show strikingdiffraction patterns.Aslit at C willgive more light thana pinhole and is satisfactory


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400 LIGHT [L-78for pins, slits, and edges but does not give two-dimensionaldiffraction andso is notgood forscrew threads,holes, androundobjects. A masking screen to eliminate unnecessary light isadvisable.L-78. Diffraction about Circular Object. A bright light A isfocused on the pinhole C (Fig. 306). A small round object suchas acoinorsteelball is heldat N;itmaybehungbyfinewires,oritmaybefastenedtoasheet of clear glass. AtS is a card withasmall hole in it. The light diffracted around the edge of theobject will makeit look as if it were surrounded with a ring oflightwhenS is at the properplace. Ifa largehole is cut in thecard at S and covered with thin paper with a small hole in its

center, the observer cansee the geometrical shadow oftheobjectand so is all the more surprised to see the ring of light. Thisshadow also helps when ining up the parts. Redandblueglassfilters may be used to show the effect of change of wave length.The distances are variable overa wide range. Thus CNmaybe10ft, andNS2ft. With asteelball ora coin asobject,CNmaybe 3 ft, and NS 10 ft. A telescopeor magnifying eyepiece mayimprovethe results. If the object is viewed from the properdistance along the axis of its geometrical shadow, its centerappears bright as if lightactually passed throughit.L-79. CrossedGratings. Each student as he comes into theroom isgiven a small square of silk, throughwhich to view anautomobile headlight lamp, to observe two-dimensional diffrac-tionbycrossedgratings. Certainfeathersshowtheeffect. Fine-mesh wire screen also serves well. A refinement is described inA-109.

L-80. Diffraction by Red Blood Corpuscles. Smear a dropofblooduniformly on amicroscope slide, andprotect itwith a coverglass. Place the slide close to the eye, and ookatabright point

B

FIG. 305.ProjectionofFresnel diffraction.


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L-82] INTERFERENCE ANDDIFFRACTION 401source of light. Beautiful diffraction rings are observed. Toprojectthe pattern, place theslide 5 to 10 cmaway from a pointsource, and form its image on a screen by a projection lens of30-cm focal length. In pernicious anemia, it is important toknow the average size of blood corpuscles, which can be deter-minedby measuring the diffraction rings.'L-81. Halos. A 3-in, hole is cut in the center of a piece ofwhite cardboardabout2ftsquare. Abeamfrom awell-shieldedpointsourceoflightisconvergedonthishole,so thatit disappearswithin it. If necessary, to reducestray light, the beam can beabsorbed in a black box or by black cloth placed behind thescreen. In the beamishelda glass plate 1 ft square, which hasbeen allowed to collect a thin layer of fine dust but which isotherwise clean. Its distance from the cardboard screen isadjusted until circular halos appear on the cardboard screen

around the hole. The distance from plate to screen is severalfeet. The effect may be temporarilyenhanced by breathingontheplate. Instead of dust, a very thin sprinkling of lycopodiumpowder may be used, and the plate covered with anothersheetof glass, th3 two being bound together.Several of the previous experiments may be combined so thatstudents may observe them one afteranother as they enter theclassroom. The setup is that for diffraction arounda circularobject (L-78). On one sideis a silk handkerchief on a frame, onthe other the dusty sheet of glass. The same source of lightserves for all. The halos contrast strikinglywith the crossed-grating effect; they help to illustrate the powder as contrastedwith the crystal type of x-ray pattern. This contrast may alsobe shown by the rotating crossed grating(A-109).L-82. Diffraction by Single and DoubleSlits. Each student isprovidedwitha small piece of photographic plate, on the foggedemulsion of which have been ruled a single line and a pair of1P7JPER, A., Med. J. S. Africa, August, 1918.

Fia.306.Diffraction around acircular obstacle.


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402 LIGHT [L-83parallel scratches, the latter0.2 to 0.5 mmapart. Avery brightstraight-filament lamp is set in view of the whole class, who canthen individually observe single-slit diffraction and the coloredinterference fringes from double slits. If the upper half of thefilament is coveredbyaredfilter and he lowerhalfbyablueone,the variation in diffraction patterns and in fringewidths is verymarked. The filtersshouldbe as close to the lightas safetywillpermit. Verygood filters for thisand otherexperiments can bemade from one or two thicknesses of colored cellophane. Thecellophane is clear enough to see through easily, and it can bebentaround hesource so that thestraightfilamentcan beviewedfrom all directions.The rulingof the double slits is greatly facili-tated by the use of a tool made for the purpose.A brass rod is drilled with two holes in the end,into which steelphonograph needles are fastened.The rod is then flattenedin such a way that thedistances of the needles from the fiat sidedifferbythe desired separationbetween slits (Fig. 307).

Fm. 307. With this tool and a straightedge, it is a simplematter toruleequallyspaced double slits on anoldphotographic exposed plate. If the rulingsaremade about toplate. in. apart, the plate can, after ruling, be turnedover, marked into squares witha glass cutter, andbrokenup sothat each student can have a section.L-83. Projection of Single-and Double-slit Patterns. Withthe exceptionof grating spectra, most diffractionexperiments aretoo faint to project so that a large class can see them. In thegrating, the lightis ncreasedbyhaving manyslits, and he effectsof all are brought together by the projection lens. The sameresultmaybeattained forsingleanddoubleslitsas follows. Onafogged photographic plate (lantern-slide size is convenient) isruleda largenumber of parallel but unevenlyspaced slits. Onanother plate is ruled a large number of pairs of slits, thespacing of each pair being constant.1 For protection, theseplates may be covered witha clear glass andbound like lanternslides. A bright source is focused on a slit (Fig. 308), and bymeans of an achromatic lens asharp enlarged imageof the slit is1But like the previousplate, hesuccessive pairs ofrulingsareseparatedin random fashion.


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L-85] INTERFERENCE AND DIFFRACTION 403thrown on a screen. Ifnow the ruled plates are introducedat Pnear the lenswith theirrulingsparallel to the illuminated lit, theimage on the screen will be that of a single slit for the first plateand that of a double slit for the second plate. Knowing thedouble-slit spacing, the distance between lensandscreen,and hedistance between fringes on the screen, the wave length ofthe light may be calculated roughly. Frequently the red andthe blue edges of the patternare distinct enough to permit thecomputationof both wave lengths. The relation between thesingle- and the double-slit patterns is clearly shown, especially ifthe plates are held so that as they aremoved up and down firstone and then the other is before the lens.The intentional inequalityof the spacing of the lines on thefirst plate destroysthe regularityof the pattern due to n slits

within the single-slit envelope sufficientlyto wipe it outwithoutaffecting the width of the single-slitpattern produced andsuper-posedby the individualslits. The same cause operates in thesecond plate through inequality of the spacing between thesuccessivepairs of equally spaced lines.L-84. Fresnel Biprism. AFresnelbiprism may be usedin aspectrometerfor a "peep-show" type of demonstrationor in acarbon-arc projection antern for projection on a screen. In thelatter case, the arc should be retracted, and the slide holderpushedforward untilaslitin the slide holder is illuminatedby theimage of the arc formedbythecondenser lens. Theimageoftheslit isfocused on the screen bythe projection ens; thebiprism splacedbetween the slit and this lens andadjusteduntil the bestpattern s obtainedon the screen. Thepattern s similartothatof a double slit.L-85. Diffraction Grating. By means of an achromatic lens,an enlargedimageof abrightly lluminatedsingle slit isprojectedon a screen (Fig. 308). Near thelens onthe side toward the shis inserted a coarse grating. Fine copper screen of 100 to 200

PFIG. 308.Projectionofdiffraction patternsfrom slitsorgratings.


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404 LIGHT L-86wires per inch is satisfactory;finer mesh is still better. Suchcoarse gratingscanalso be madeby photographing adrawingofequally spaced lines. The spectrum will be visible for severalorders on eachside. If a gratinghaving several thousandlinesper inch is used, it cannot be placed near the lens but can beplaced nthe convergingbeamoflightnot oofarfrom thescreen.The spectrumwillshow up clearlyif a fairly largereplicagratingis usedso that it ransmitsplentyoflight through the ruledarea.Wavelength of light can be calculated approximately rom theknown grating space, the measured grating-to-screen distance,and thedistance rom thecentralimageoftheslit toanyparticu-lar color in any order of the spectrum.L-86. Resolving Power. The two filaments of a double-fila-ment automobile headlight lamp are so spaced as to be at thelimitof resolution ofanormaleye,whensomewhat dark-adapted,at a distance of about 25 to 30 ft. Hence, students in thefronthalf of an average classroom will be able to see both filaments,those in the rear half will see only one. Both filaments arelighted simultaneously, and the brightness for best seeing isadjustedwith a rheostat.One way to help students to see what is meant by resolvingpower is to show slides of the "Navicula" made with the ultra-violet microscopein green (5461)and hen in theultraviolet ightof the mercury-arc line 3650.1

COLORAND RADIATIONL-87. Color. When an object appears colored (barringpsychological or physiological tricks like those described in L-61

andL-94), three conditionsmustbefulfilled: the colormust be inthe illuminating source; the object viewed must be capable ofreflecting or transmitting the color; the detecting deviceeye,photocell, photographic platemust be sensitive to the color.The followingdemonstration illustrates the firstandsecond con-ditions; it canbemodifiedto demonstrateall three (seealso colorblindness, L-62). Arrange where they will not conflictwith oneanotherthefollowingsources ortheirequivalents: amercuryarc,a sodium flame or arc, a deep-red (not orange) lamp like thoseused for photographic work, a white desk light. A real blue-violet light is good if available; it can sometimes be made by1These slides aremadebyA. P. II.TrivelliofEastman KodakCo.


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L-88J COLOR AND RADIATION 405using filters. As the class comes into the room, have themexamine colored cardboard, coloredyarns,oranythingelseunderthe several lights, the white one last of all. If the class is toldbeforehand to comewearing gaudytiesor to bring otherbrightlycolored objects, so much the better. For the uninitiated, theeffects are surprising. The room must be darkened, and thelights must not interfere with one another. However, this iseasilyaccomplishedby screening the sources in separatestalls orby putting some on the floorunder tables and the others on thetables.

The preceding method may be modified by using the projec-tion lantern as a spotlightwith colored filters. Ona large pieceofbeaverboard arearrangedanumberofsquares ofcolored paper,cloth, or other material such as flowers or green leaves. If onewishesto take the troubleto cut large lettersoutof constructionpaper and spell out in their appropriatecolors the words red,orange, blue, green, black, andwhite, it is easier for the class toremember the colorsof the different letters inwhite light. Inex-pensive filters may be made by mounting colored cellophanebetween sheets of glass like lantern slides. Two or three thick-nesses of a given colormay be requiredto give the bestresults.Another modification is to project on a white screen a largebright spectrum(L-106), in the variouscolored regions of whichare held pieces of colored paper, flowers, etc. The effects arestriking, andwith a little practice the component colors of suchthings as leaves may be quickly identified. The strong red inmany green plants is a surprise to most persons.L-88. Combination of Colors by Addition. Arrangement ismade to illuminate a white screen with several colors of lightsimultaneously. The sources should be capable of motionso asto cause the different colorsto overlap or notas desired.A simple arrangementconsists in mountingelectric lamps andcolor filters at the corners of an equilateral triangle having awhite surface Fig.309). The filtersgivetheprimarycolors, red,green, andblue, and the lamps and filters are chosen so that thecombination of all three colors at the center of the triangularscreen produces white, while any possible combination of theprimaries will be found at some point on the screen. If a rod ismounted at the center of the screen, the colors of the threeshadows radiating romthe centerwill be the complementsof the


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406 LIGHT [L-89primary colors. With two rods set up so that their shadowscross, the third primary color will show at the point of intersec-tion. When a riangular pyramid s attachedat the centerof thescreen so thatone edgefaces each of the lamps, each surfacewillshow a mixture of two primary colors.L-89. Apparatus s commerciallyavailable for attachment to aprojectionanternso astoproduce threeindependent ightbeams,which may be colored with filters. The three spots of colored

FIG.3O9.Color pyramid.lightsoformedona screenmaybe madeto overlap inanydesiredmannerto show combinations of two or three colors. A simplesubstitute for this apparatusis the following1 (Fig. 310). ThreeWrattenfilters, Nos. 19, 47, and61, are cut in theshapeof sectorsof a circle with anangleof 120 at the center of each. Theyarelaid on a sheet of clear glass so that they fit accuratelyat thecenter, cemented around the outside edges, and covered withanother sheet of glass, which is then bound to the first like aianternslide. If suchfilters arenotavailable, coloredcellophane1HEILEMANN,J. J., Am.Phys. Teacher, 4, 211, 1936.


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L-90J COLOR AND RADIATION 407may be usedtwo thicknesses each of red andof dark blue andtwo or three of green. Thelightfromaprojection lantern passesthrough a 1-in, hole H cut in a screen held in the slide holder.The condenser C of the lantern is arranged so as to bring theimageof the filament ustbeyondtheprojection ens Pwhen heimage of the holeison the screen. The color-filter slide is heldinfront ofthisprojection lens, exactlyatthe locationof the image

FIG. 310.Apparatusfor combining colorsbyaddition.of the source, therebycoloringthespot oflight on the screen in amannerdepending uponthe ratios of the illuminatedareas of thethree filters. To show how the light is divided among separatecolors, a piece of plate glass may be inserted in the beam at anangleof 45 to its axis. The portion of the light thus removedfrom the beam is reflected by a plane mirror M parallel to theplate glass through a second projection lens, which focuses an

FIG. 311.Method of combining colorsbysubtraction.image of thefilteron thescreen close to the spot ofcoloredlight,i.e., the imageof the hole.L-90. Combination of Colors by Subtraction. Instead ofadding colors as in L-89, they may be combined by subtraction.A strongbeamof lightfrom a slitS (Fig.311) is arranged o thatpart ofitpasses throughaprismand is spreadout in a spectrumon a screen and part goes past the edge of the prism to anotherscreen. A color filter is inserted in the beamnear the slit. The

C

Toscreen

/c.reglassslide

filter


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408 LIGHT [L-91transmittedcolor and its components interms of the parts ofthespectrum ransmitted are both seen. Ifnowdifferent filters thathave beenexamined separately are put into the beam together,the resultsareevident. By holding them near theslit and partlyoverlapping, the separate and combined effects can be seen onboth screens.L-91. Color Due to Absorption. Light from a lantern isreflected to the ceiling by pieces of red, green, and blue glass.The color of the reflected lightis nearlythe same inall cases, butthe light that goes through the glass to a screen is decidedlycolored by absorption. By using a plane mirror, the reflectedlight maybe thrown onthe screen beside the transmitted light.L-92. Complementary Colors. White light from an arc isfocused on a slit (Fig. 312) andpasses through the lens L2 and* prismP oformaspectrumon the plane mirror M.This mirror reflects thelight through a largelensL3,whichformsanenlargedimage of the face of theprismonthe screen. Thisimage will be white if thelens catches all the lightA B from the prism reflectedFIG. 3l2.Optical arrangement for demon. by he mirror M. But fastratingcomplementary colors, smallstrip of mirror M2isinserted in the spectrum near Mand tipped slightly, the imageformedby the lightreflected fromM2will reachadifferent place

on the screen, say B, and will be colored; and since part cf thelightis thus removed from theoriginalimageat A, thisimagewillalso be colored. The colors of these two images are comp1e.mentary since they add to produce white, as can be shown byremoving M2 or by making it parallel to M. The mirror M2should be thin, as otherwise light will be lost on account ofobstruction by its edges.A similar result may be obtainedby putting the lensL3 inthedispersed beam from the prismP so as to catch all the lightfromit, with Mjustbeyond the lens. Thelightisthen reflectedbyMback throughL3, which forms animageofP on a screen. M2 is


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L-94] COLOR AND RADIATION 409insertedin front of Mas before. In this case, however, in ordertosecure enough dispersion, the lens and mirrormayhaveto besofarfrom theprismthat the imageproducedonthescreen issmall.In either case, the lensL3 must be largeenough to receive all thelightdispersedbytheprism,since otherwise awhite imagecannotbe formed.L-93. Color Disks. A disk maybemadeorpurchased havingcolored sectors of such proportions that when the disk is spunrapidlytheeyeperceivesagrayishwhite instead of the individualcolors. Twodisks of solid color, with one radial slotin each, maybe slipped through one another and fastened to the same rotor.When the rotor turns rapidly, the effect is a color whose hue,brightness, andsaturationdependon thecombinations used. Byrotating one disk with respectto the other, the colored sectionsmay be varied.An interesting "rainbowtop"is now on the market, in whichfour smalldisks eachbearing 120 sectors ofred, yellow,andblueareslowlyturnedwhile the topspins, so that continual changes ofcolor occur.

L-94. Complementary Colors Produced Subjectively. Adisk of heavy cardboard or beaverboard 20 in. in diameter isClara' DIck(8/ack/ed // Redj Bri/n'

Fio. 313.The red light appearsgreen when thedisk rotates counter-clockwise.painted dull black on one half and glossy white on the other(Fig. 313). Anopening, half in the white and half inthe black,is cut at one edge of the disk. The piece removed is cut in twoand glued to the back of the disk, near the edges of the opening,to balance t. The disk is mounted on a rotator, either hand ormotor driven, so that it faces the class. A red light is placedbehind it so as to be seen through the opening for an instantduring each revolution of the disk, and the front is brightlyilluminated. Two desk lamps, each with a photoflood lampwhose direct light is properlyshielded from the class, are satis-factory. The observerwatches or the redlightasthedisk turns.


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410 LIGHT [L-95If the direction of rotation is such that immediately afte

suton partv light

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