SEPTEMBER. 1953
JIM G. MALIK, JOHN L. SPEIRS, and M A X T. ROGERS Michigan State College, East Lansing, Michigan
Tm
interferometer is the most precise instrumei~t available for the measurement of refractive indexes of gases, liquids, and solutions. The ordinary commercial instruments can detect a change in refractive index of =t1 X for liquids and * 4 X lo-* for gases. There are, a t the present time, at least two companies' which will supply a Rayleigh type interferometer (1) within a period of 6 to 15 months. The construction of such an instrument was undertaken a t this laboratory. The actual construction was not complicated and it was realized that other chemists and chemistry teachers could make valuable use of such an instrument.
or solution placed in one of the divided interfering beams will change the optical path of that beam and cause the fringes to be displaced appreciably. USES
The interferometer then actually detects any change in the optical path. This has been adapted by ingenious methods to measure lengths (standard meter) (6, '7), doublets and fine structure .of spectral lines (8, Q), and conce~~tration (10, 11) or index of refraction of gases, liquids, or solutions (12, 13'). The instrument should find many uses in different laboratories. I t is an excellent example of the interference of light for a GENERAL THEORY physics or optics laboratory. In a quantitative analAn interferometer, in principal, is an instrument ysis (10, 14) laboratory, once calibrated for KC1 sowhich divides a collimated beam of light from a single lutions as an example, the instrument could be used in source into two parts by means of two adjacent slits. reporting concentrations to 10.00015 g./ml. I t can These travel different parallel paths and recombine to be used as a precise physical tool or in an instrnmental form vertical straight interference fringes or bands. course (15). Its greatest use to date has been in biologIf the optical path of one of these divided interfering ical studies and analyses (16-29). There are about 170 entries in Chemical Abstracts beams is changed, then there is a sideways shift of the whole band system. This shift is observed in a tele- concerning the interferometer and its uses. Unforscope. The magnitude of this shift is found in terms of tunately, only about 15 percent of these are in readily the number of fringes displaced from the original po- available journals. To give an idea of the extended apsition and can be calibrated (2-5,54, '74) in terms of the plications for which the instrument has been used, a respective change in refractive index. Any gas, liquid, partial list is given here. The analyses of the following have been carried out: ' Hilger and Watts, Ltd.; U. S. agent is Jarrell-Ash Co., 165 smoke (241, nonaqueous solutions (57), flue gas (251, Newbury St., Boston 16, Mass. Bdrd Associates, Inc., 33 beer (26, W), milk (28),soils (%8), colloids (4,20,26,28), University Rd., Cambridge 38, Mass.
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JOURNAL OF CHEMICAL EDUCATION
SIDE VIEW
TOP V I E N light
Single slit
Double slit
Liqi~iaoell
Devistion
Cylindrical lens
SOUFDe
-. river and well water (28, 29), mine air (25, 80-82), content of COz in lungs (88, 541, C 0 2 tension in venous blood (pregnancy test) (55, 86), gas mixtures, and gas and water purity (29, 87-42). I t has been used as an interferential galvanometer (4S), to measure lengths (6, 7, 4 4 , coefficients of thermal expansion of metals (45, 45), percentages of peptone in serums in biological work (467, in serological studies (47, 48), and in immunological studies (@). It has heen used as a precise pressure gage (50-52), to measure wave lengths (8,58), in fermentation rate studies (54), and general kinetic rate studies (5547). These are in addition to its use in measuring refractive indexes (5842). DESCRIPTION OF INSTRUMENT
without a filter produces colored double slit interference fringes that are beautiful. An absorption or interference filter3 is used just before the slit to isolate the mercury green line. Next, an inexpensive l'/*-in. condensing lens with a 6-in. focal length serves well to focus the light on a very narrow slit. The slit may be purchased4 or satisfactorily made by drawing a razor blade with a straight edge across a silver mirrored surface. Several attempts a t making such a slit should be tried and examined with the microscope to find the finest and smoothest-edged slit. It cannot be too narrow or fine. A good surface can be inexpensively made by depositing silver on microscope slides. A 2-in. achromatic lens of 7-in. foci1 length collimates the light from the narrow vertical slit which then passes through a vertical double slit aperture. It is desired here to keep the aperture size small and the distance between apertures small, as this will give wider fringes. The distance between apertures is determined by the wall thickness of the cell. Each slit aperture was approximately 0.5 mm. These mere cut in a thin piece of aluminum using a milling tool with a 45-degree angle. The result is two parallel vertical beams from a single source. When the two beams are brought back together they interfere with one another and fringes are produced. The cells for either gases or liquids may be used. The gas cells were made of square aluminum t ~ b i n g . ~
A description (68) of the train of the interferometer will introduce the various parts and their function (see Figure 1). This will also indicate the starting place and materials required to construct an interferometer as well as its operation. All the parts herein described were made of aluminum, which is inexpensive, light, nontarnishable, and easily machined. These were placed on a two-meter optical bench consisting of two long parallel rods and three or four supports. If such a bench is available with sliding supports for lenses, a good share of the project is finished. For actual measurements, a monochromatic light source is needed. The mercury lamp is recommended because of its exceptional brightness and the shape of 'An interference filter was obtained from Fish-Schumsn its source. A sodium lamp is traditionally used though it is not as ~atisfactorv.~The mercurv light source Corp., 70 Partmsnd Rd., Xew Rochelle, N. Y. However, the great majority of refractive indexes haw been determined with the sodium light source. a
' Bausch and Lomb Co., Rochester, N. Y. ' The Aluminum Company of America (Aleoia) hss many sizes
and shapes mailrtble.
SEPTEMBER, 1953
439
Intnrferornetsr (Gas Cells Disconnected,
Two pieces, 100 cm. long, are boxed together, the ends are situated one in each beam and inclined to t h beams ~ milled flat, and optical flats cemented to the ends with a t a 45-degree angle. One is fixed, the other attached an optical sealing v a x or Glyptal. The cells should through a gear redurtion system to a dial indirator. A then be tested for leaks.8 Optically flat glass is very large rotation of the dial indicator mill result in a small inexpensively obtained from war surplus stock' hut (5' or so) rotation of the flat plate. The rotation of the should be test,ed to make sure it is flat to a quarter of a glass flat effectively increases or decreases the amount of wave length.s A modification is to have four pieces of glass in the light path, and therefore t,he optical path, the aluminum, 50 cm. long, boxed together. The cells causing the fringes to shift sideways. Since a solution should have inlets and outlets to introduce gases for will cause a dis~ontinuousshift, the compensating plates measurement and to enable t,he cells to be evacuated. can be used to return the fringe syst,em t o normal and The liquid cells may be 1 mm. to 10 cm. long (see thus either the number of fringes may be counted or o divided by use made of the dial indicator. If the dial indicator Figure 2). These have t ~ compartments a center piece with flat glass cemented t o either end. mere calibrated for a particular solut,ion by plotting The standard liquid can be pnt in one side and the un- the various known concentrations versus dial readings, known in the ot,her.
I.1.u.e
2.
Liquid C.11(2 X 2 X 2 cm.)
The compensating plates (64) consist of two optical flats cut from the same ~ i e c of e glass if ~ossihle. These 'Fill each cell with silver nitrate ~olutianand allow to stand several hours in a KC1 solution. ' Edmund Salva~eCo., 101 E. Gloucester Pike, Barrington, N .I *.
For procedure to trst for flatness see, for example, ref. (67).
~ n vied of interferometer s h o v i n g id lndicatar
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JOURNAL OF CHEMICAL EDUCATION
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:inner -rp-.
h r q... n
change per fringe is known, rapid determinations mav h e made, unless the un-
- - -- - - - -\ plate
counting the large number of fringes is tedious or imFigure 3 There should he as little difference as possiTho dermtion prism makes the upwr and lower fields adjacent i n the weniece hle in refractive indexes of the concentration of an unlcnown solution could readily the standard and unknown. To give an idea of the a o curacy ohtainahle, a movement of one whole fringe could he found using the graph and the dial reading. Next is an achromatic collimating lens, similar to correspond to roughly +0.0000004 in refractive-index the first one, set in reverse which focuses t,he light on a change. Compensating plates, hovever, will lead to better cylindrical lens. This is cylindrical for great magnifying power in the horizontal direction only and may be results, as up to '/so of a fringe can he estimated readily satisfactorily made from a stirring rod. When the sys- by an experienced operator, corresponding roughly t o *0.000000009 in refractive-index change. If a gas is tem is in alignment, the fringes are seen in it. slowly entering an evacuated cell, the number of fringes OPERATION passing hy may he counted in the eyepiece; but if a The most difficult part of the construct,ion is that of liquid or solution is placed in onc of the beams, a dismaking a.nd adjusting the compensating plates. In rontinuous shift t a k ~ splace. Compensating plates work with gases, t,hese can actnallv he eliminated, hut must therefore be used to find the original position of I hr iringrc nnd h6we thv I I I I I I I I W ~ i ~ , i t ~ < diq~I:w(l. w .\I1 t l w f~.ingcsx i t h < < J ( I I I I ~ I112111>11t1wntrl h c k :t11
