The reflecting power of a surface is defined as the ratio of the intensity of the light reflected at perpendicular incidence to the intensity of the incident light. There are two ways of determining the reflecting power of a metallic surface. One is by a direct, or photometric (dioptric) method. The other is by an indirect (katoptric) method

Sieg, L. P.

English

University of Northern Iowa

Proceedings of the Iowa Academy of Science Volume 23 Annual Issue Article 30 1916 On the Variation in the Reflecting Power of Isolated Crystals of Selenium and of Tellurium with a Variation in the Azimuth of the Incident Plane Polarized Light L. P. Sieg State University of Iowa Copyright ©1916 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Sieg, L. P. (1916) "On the Variation in the Reflecting Power of Isolated Crystals of Selenium and of Tellurium with a Variation in the Azimuth of the Incident Plane Polarized Light," Proceedings of the Iowa Academy of Science, 23(1), 179-186. Available at: https://scholarworks.uni.edu/pias/vol23/iss1/30 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. REFLECTING POWER OF CRYSTALS 179ON THE VARIATION IN THE REFLECTING POWER OPISOLATED CRYSTALS OP SELENIUM AND OFTELLURIUM WITH A VARIATION IN THEAZIMUTH OF THE INCIDENT PLANEPOLARIZED LIGHT.L. P. SIEG.The reflecting power of a surface is defined as the ratio ofthe intensity of the light reflected at perpendicular incidenceto the intensity of the incident light. There are two ways ofdetermining the reflecting power of a metallic surface. One isby a direct, or photometric (dioptric) method. The other ishy an indirect (katoptric) method. In the latter an analysisis made of the elliptically polarized light coming from the surface in question, upon which plane polarized light is incident.This analysis yields what are called the optical constants ofthe substance. These constants are the index of refraction, theabsorption index, and the reflecting power. Both methods havebeen used repeatedly and the concordance of the results givesus such faith in the indirect method, that, in view of the factthat the absorption index and index of refraction of the metalconstituting the surface are much more easily determined bythe second method, we generally employ this indirect method.Nevertheless it is always desirable when possible to check theresults by some direct experimental attack.Most of the work done on metallic surfaces has been done,not only by the indirect method, but also on rather largepolished surfaces of the metals in question. In view of thefact that metals are essentially crystalline, it becomes at oncenn open question as to whether the optical constants determinedfrom large polished surfaces represent the real facts. For unlessthe crystals constituting the metal belong to the cubic system,one should certainly expect a set of optical constants depending on the orientation of the crystalline axis. In this view,then, the constants ordinarily determined can represent only,except in the case of metals of the cubic system, certain meanI'alues of the actual constants.1Sieg: On the Variation in the Reflecting Power of Isolated Crystals ofPublished by UNI ScholarWorks, 1916180 IOWA ACADEMY OF SCIENCEWith this view in mind, some two years ago, the writer setone of his students, Mr. C. H. Skinner, at work to determinethe optical constants by a katoptric method of an isolated crystal of selenium. Although selenium is not regarded as ametal, it has, nevertheless, optically speaking, many qualitiesof a metal. The crystals are optically very dense and offerbrilliant reflecting surfaces. Skinner succeeded in provingdefinitely that there were distinct differences in the opticalconstants, depending upon whether the long axis of the hexagonal crystal was vertical or horizontal. This work is notyet published in full, but an abstract has recently appeared1.Some years ago Drude2 and Miiller3 worked on isolated crystalsof antimony sulphide, and proved that from one cleavage plane,(0 1 0) two distinct sets of optical constants were obtained.Other work has been done, notably by G. Horn4, on the absorption of certain crystals, but the writer is not aware that anywork on elemental metallic crystals with the exception of adetermination of the absorption of bismuth and antimony bythe latter author, has been done. The reason for this is notfar to seek, for usually the crystals of metals are microscopic insize. The difficulty in working with such small surfaces bythe ordinary methods led the writer to suggest this task ofobtaining a special method to another of his students, L. D."Weld. The latter has succeeded admirably in this problem,and a preliminary report of the work is presented to the Academy of Science at this meeting.Aside from the importance of crystals in yielding usthe true constants of various substances, there is goodreason for believing that the crystalline surface should be better adapted for the purpose than any artificially formed polished surface. If this is correct, then even the cubic crystalsshould be re-examined by this method. The one reliable traitof all crystals seems to be their maintaining of constant angles,and even if there seem to be striations on some of the surfaces,it may be that these irregularities would not seriously affectthe optical constants. Then too where it is possible to oOiaiua fresh surface by cleavage, one should expect the most relia»C. H. Skinner, Phys. Rev. 7, 1916. 285.•P. Drude, Ann d. Phys., 34, 1888, 489.'E C. Miiller, N. Yahrb. f. Miner. Bell. Bd.. 17. 1903, 187.•G. Horn, N. Yahrb. I. Miner. Bell. Bd., 12, 1899, 269.2Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 30https://scholarworks.uni.edu/pias/vol23/iss1/30REFLECTING POWER OF CRYSTALS 181ble results. Drude5 and Muller" both found that a fresh surfaceof antimony sulphide deteriorated rapidly after coming intocontact with the air.The writer thought it to 'be highly desirable to determinethe reflecting power of certain of these crystals by a directmethod. If these results should show definitely a difference inthe reflecting power with difference in azimuth of the incidentplane polarized light, then there would be a larger amount ofreliability to be placed in the results that are being obtainedin this laboratory by the more exact, 'but indirect methods.The plan of the experiments was very simple, but the execution has been rather difficult. The first arrangement was anapparatus by which plane polarized light from a monochroma-tor, varying in wave length throughout the visible spectrum,could be thrown upon a linear thermopile, and secondly reflected upon this thermopile by reflection at nearly normal incidence from the crystalline surface. The ratio of the seconddeflection of the connected galvanometer to the first would giveat once the reflecting power. By rotating the crystal in theplane of its surface through 90°, and repeating the above experiment, one could get the reflecting power from this azimuth.The results were, however, unsatisfactory because not enoughenergy could be obtained from the small crystalline areas togive reliable deflections of the galvanometer.A second attempt was made in which a crystal of selenium,connected in a Wheatstone's bridge, was substituted for thethermopile. The effect of light on the selenium is to decreaseits resistance, and it was thought in view of the fact that aselenium receiver is much more sensitive in the visible spectrumthan the thermopile that definite results could be obtained.However, the crystal employed, although more sensitive thanthe thermopile, proved unfortunately to be more erratic andunsteady in its action. While perseverance would no doubthave led to a more satisfactory crystal receiver, it was decided for the time. being to abandon this plan of attack andto use a spectrophotometric method. The apparatus was arranged in several different ways which need not be gone intohere, before what proved to be a satisfactory method was found.The plan is shown in figure 15. Light from a Nernst glower, 0'P. Drude, loc. clt.«E. C. MUller, loc. clt.3Sieg: On the Variation in the Reflecting Power of Isolated Crystals ofPublished by UNI ScholarWorks, 1916182 IOWA ACADEMY OP SCIENCE(later a nitrogen filled tungsten lamp was employed) placedin a light-tight box is focused by a lens, L5 at the points Nland H. This division of the light was accomplished by meansof a mirror Mr partly intercepting the light. Nl is a Nicolprism which can be set at any desired angle. Passing throughN1 the light meets the lens, L2 and by it would be brought to afocus at A were it not for the crystal plate at X. This latterreturns the light through the totally reflecting prism, P and itFlO. 15.is finally focused on the lower half of the slit of the monochro-jnator PSE. The other portion of the original beam of lightpasses through L1 to the mirror 3/,, thence through the lens Lx,the Nicols N,, and AT3, the latter with an attached circular scaleand vernier, and lastly is focused by means of the lens Lt uponthe upper half of the slit of the monochromator. These twobeams pass through the instrument and form on the emergentslit two images of the front slit, one above the other, and formedby light coming along the two separate paths. The wave lengthfor which one wishes to make an examination is obtained bysetting the graduated screw S. The instrument had of coursebeen calibrated. These two images are viewed through theeyepiece E. The procedure is to set S for a wave length atone end of the visible spectrum, then to rotate the Nicol, Ns,until the two images are matched in brightness. . By proper4Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 30https://scholarworks.uni.edu/pias/vol23/iss1/30REFLECTING POWER OF CRYSTALS 183adjustment of the mirror .¥,, the light coming by the longerpath can always he made, with the Nicols parallel, more intensethan that by the shorter path, so that it is always the one tobe reduced in intensity for an intensity match.Having run through the spectrum with the crystal X setsay with its long axis agreeing with the electric vector fromthe Nicol Nl} the latter is now revolved through 90°, and theoperations are repeated. Then to make certain that this changedazimuth of the polarized light does not cause changes in intensity due to the monochromator as well as due to a difference in the reflecting power of the crystal, two more settingsare employed. The crystal is rotated through 90°, and theelectric vector from N^ is first vertical, and secondly horizontal.From -these four sets of readings one is in a position to calculatethe reflecting power of the crystal with two different azimuthsof plane polarized light. The present work was not designedas an attack on the absolute reflecting power of the surfaces,but merely to obtain the differences due to these two differentazimuths of .the incident light. In order to get absolute resultsone must know the area illuminated, and then substitute forX a substance of the same area and of known reflecting power—silver for instance. This was done in the present case by usingblack glass, and only carefully enough to make certain thatthe light reflected from the silver-backed glass mirror M2 represented nearly constant reflecting power through the spectrum.This agrees with published results7.In the near future it is planned to obtain the absolute reflecting power of these and of other crystals. Not the greatestconfidence can be placed in the variation of the reflecting powerthrough the spectrum, that is as to whether there are any slightmaxima, or minima, for the uncertainties in matching brightness make absolute reliability in this regard only to be obtained through extremely long and careful experimentation.However, the fact of the variation in magnitude of the reflecting power with the azimuth of the incident plane polarized light stands out clearly, and this was the prime object ofthe research up to the present point.The first crystal tried was a hexagonal crystal of selenium*made by sublimation in an atmosphere of hydrogen. The curve'Wood, Physical Optics, Ed. 2, p. 466."The writer Is much indebted to Dr. F C. Brown for the crystal ofselenium, and to Mr. W. E. Tisdale for the crystal of tellurium, mentionedlater. 5Sieg: On the Variation in the Reflecting Power of Isolated Crystals ofPublished by UNI ScholarWorks, 1916184 IOWA ACADEMY OF SCIENCEshown in figure 16 represents typical results of these experiments. The method of using four settings was not adopted atthe time these first experiments were performed, so althoughthe differences in the reflecting power are genuine, there areundoubtedly corrections that should be made. It will be noticed'that the reflecting power is practically constant throughout thespectrum. This is in a way substantiated by direct observa.tion, for the light reflected from these brilliant surfaces is freefrom any observable trace of color. It will be noted that the.45reflecting power is greatest when the electric vector is alongthe long axis of the crystal. This is in agreement with whatwe should expect, because it is along this axis that the elasticityis greatest. The intensities of the reflected light are taken asproportional to the square of the cosine of the angle throughwhich the Nicol N~ is turned from its position parallel to theNicol AT,. The direct results here given corroborate the resultsobtained by the indirect method, and so no doubt add to theconfidence in the latter.The next crystal employed was one of tellurium, formed inhydrogen. It is .planned in this laboratory to make a carefulstudy of the optical constants of this element, but up to thepresent time this work is not completed. It was thought, however, important to try these preliminary experiments on thiscrystal for future reference in the more extended work. Theparticular crystal used was a very long acicular hexagonal cry6Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 30https://scholarworks.uni.edu/pias/vol23/iss1/30REFLECTING POWER OF CRYSTALS 185stal, and was made by sublimation in an atmosphere of hydrogen by Mr. Tisdale of this laboratory. The electric oven inwhich this was done is described in this volume of the Proceedings, see page 209. The surface of the crystal was about0.5 mm by 7.0 mm, and was one of the six similar sides of thehexagonal crystal. In these last experiments correction wasmade for the possibility of a variation of the intensity of thelight passing through the monochromator, and also for a variation in the area of the crystal illuminated when it was rotatedthrough a right angle. The corrections were made as follows.Let r!=the intensity of the reflected light when the electricvector lies along the crystal's long axis.r2=the intensity of the reflected light when the electric vectoris perpendicular to the crystal's long axis.at=the reduction factor for the intensity due to the passagethrough the monochromator when the electric vector is vertical.a,=the reduction factor when the vector is horizontal.fc^the fraction of the crystal illuminated when the crystalis vertical.62r=the fraction illuminated when horizontal.Then r^aj}^, for example, will be the intensity as measuredby experiment (square of cosine of the angle of rotation of Ntfor an intensity match) of the light reflected from the surfacewhen both the vector and the crystal's long axis are vertical.The other combinations are readily interpreted.Let r1o161=mSolving these we obtaini i ir,/rz=(7n,p/n<7) ; a^/at=(m(/ /np) ;b,/bf=(mn/pq)In the following table are listed the values of the square ofthe cosine of the angle of rotation of the Nicol, N3, for the different wave lengths, and for the four positions stated above.It will be seen that the ratio of the two reflecting powers is,as in the case of selenium, practically a constant throughoutthe spectrum, the one with the electric vector along the long7Sieg: On the Variation in the Reflecting Power of Isolated Crystals ofPublished by UNI ScholarWorks, 1916186 IOWA ACADEMY OF SCIENCEaxis being greater than the one at right angles to this in theratio of about 1.7 to 1. the values of &i/b2 should come out aconstant, and the variation in the table is merely an indicationof the experimental errors. The ratio aja., would not necessarily be a constant, although it so proves to be within the errorsof the experiment.TELLURIUM.WAVELENGTH(MICRA)m P « n a,/a. &,/62 r,/r2.66 .48 .53 .36 .25 1.14 .78 1.68.64 .48 .57 .38 .25 1.13 .74 1.70.62 .50 .55 .35 .27 1.06 .84 1.71.60 .50 .57 .38 .25 1.15 .76 1.73.58 .50 .59 .40 31 1.05 .81 1.55.56 .55 .59 .36 .30 1.06 .88 1.74.54 .52 .59 .38 .27 •1.11 .79 1.73.52 .53 .57 .38 .27 1.14 .81 1.72.60 .55 .57 .38 .28 1.15 .84 1.72.48 .55 .53 .38 .28 1.19 .88 1.66Conclusion. —The importance of a study of isolated crystalshas been pointed out and preliminary direct results on the reflecting power of a selenium and of a tellurium crystal havebeen obtained. These results have supported the original contention, and are, the writer believes, the first direct results published for these two elements to show that they have differentreflecting powers. Heretofore these reflecting powers have beendetermined from large polished mirrors of these elements, andof course only one value for each element for each wave lengthhas been determined. Further work along this line is to bedone in the immediate future.PHYSICS LABORATORY,STATE UNIVERSITY OF IOWA.8Proceedings of the Iowa Academy of Science, Vol. 23 [1916], No. 1, Art. 30https://scholarworks.uni.edu/pias/vol23/iss1/30

On the Variation in the Reflecting Power of Isolated Crystals of Selenium and of Tellurium with a Variation in the Azimuth of the Incident Plane Polarized Light

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On the Variation in the Reflecting Power of Isolated Crystals of Selenium and of Tellurium with a Variation in the Azimuth of the Incident Plane Polarized Light

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