Craig Thorn
Downers Grove Community High school Downers Grove, Illinois
An Inexpensive Littrow Spectrograph
The optical spectrograph is an instrument which provides data fundamental to both physics and chemistry and, in fact, finds application to all branches of science. Because of its versatile and fundamental nature, one can learn a great deal of physics and chemistry by performing simple spectroscopic experiments. This is especially true in the case of a high school student since such a person is just being introduced to the concepts of physics and the spectrograph provides much fundamental information about the nature of light and the properties of atoms and molecules. Indeed, the mere construction of such an instrument is a valuable education in the nature of light. Fortunately, a spectrograph of sufficient quality (i.e., dispersion and resolving power) need not be expensive if one is willing to construct it from readily available parts, as this article describes.
forty-dollars includigg light sources, has a piate factor of 2 A/mm a t 3200 A and 22 A/mm a t 5800 A. Figure 1 shows the plate factor for the spectrograph as a function of wavelength: first, as calculated from measurements made on various spectra ("observed" curve) and second, as calculated from values of the
Spectrograph Optics 60' flint glass prism Achromatic lens, 1.041 m focal length First surface mirror 90' reflecting prism Condensing lenses, 2-3-in. focal . length, Z L / ~ i ndiam
Edmund No. 30,143 ($8.25) Edmund No. 30,202 ($5.00) ($0.30) Edmund No. 535 Edmund No. 3223 ($1.25)
Spectrograph
The final design decided upon for the spectrograph was a Littrow mounting of a 60' flint glass prism since this mounting offered a mximum dispersion a t a minimum cost. The optical parts arelisted in the table. The resulting instrument, which cost approximately 'Address for reprint requests. Grove, Illinois.
4622 Douglas Rd., Downers
Figure 1 . graph.
Theoretical and observed plate factor for the Littrow ipectro
index of refraction for the glass prism, assuming the prism to be set a t minimum deviation ("theoretical" curve). The difference between the two curves is attributable primarily to the fact that in a Littrow mounting it is difficult to set the prism exactly at EDITOR'SNOTE: The instrument here described represents the culmination of a project initially undertaken in 1959 for the Illinois State Science Fair, Spectral studies using the instrument were entered in subsequent years, each time earning the author outstanding awards, including a regional award in the FSA program and a national award by the American Nuclear Society. The author continues his preparation for a career in science at Princeton where he is a freshman finding time to do research on spectra of cyanide complexes.
Volume 41, Number 4, April 1964 / 209
minimum deviation because a t this setting the entrant and exit beams will traverse the same path and cannot be separated to record the spectrum. The theoretical resolviug power was calculated to he 10,000 as compared to the measured value of about 2000. The various parts of the instrument are constructed primarily of plywood. The case (Fig. 2), which is the main item to be built, consists of a trapezoidal bottom, two ends 8 X 8 in. and 20 X 8 in. (the larger is built from four smaller pieces to make a slot for the film holder), and a side 83/4 X 48 in., all of which are made from 3/4-i~1.plywood. A baffle, 4 in. high, is placed across the middle of the case to reduce stray reflections. The cover for the case consists of a trapezoidal piece, identical with the bottom, and a side, both made from '/n-in. plywood. This two-piece cover is fastened to the case with 24 wood screws spaced around the edge. Plastic weatherstripping is fitted to the case to seal the joint with the cover, and in addition the inside of the instrument is painted flat black. For mounting the slit, a sliding diaphragm is attached to the side of the case, centered 9 in. from the end. The outer sleeve of the diaphragm is made from 3/4-in. plywood, and an inner sliding sleeve is made from '/n-in. plywood. A front of 3/4-in. plywood is attached to the end of the inner box, and the slit is mounted on this board. Provision for sliding is included since positioning the slit exactly a t the focal point of the lens can best be done hy trial after the instrument is assembled.
Figure 2.
Spectrograph with r o v e r removed.
The slit itself is constructed from single-edged razor blades with the stiff backs removed. One blade is fixed and the other is moved by a 4 4 0 screw and a spring, which prevents slack motion. The blade moves in a track formed by two razor blades (one a t the top and another a t the bottom), and the assembly is held in place by a l/n-in. plywood frame (Fig. 5 ) . To record the spectra, and 18-in. strip of film is fastened bet,meen two glass plates ill an adjustable film holder whicl~has a wooden trough into which the film and glass plates just fit. The trough is mounted on two s c r c ~ s('/4-in.-20), pivoted in a plywood back which
clamps onto the end of the spectrograph ease (Fig. 3). To prevent any lateral motion, the trough moves on two tracks made from aluminnm angle. With this construction four spectra may be recorded on the same strip of film by blocking off all but the center in. of the slit and moving the film trough up or down with the screws to record each spectrum.
Figure 3.
Film holder.
Figure 4.
Arc tamp and hood.
The three optical holders-one for the 90° reflecting prism, orre for the lens and 60' prism, and one for the mirror-can be seen in Figure 2. The 90' prism mount is a length of 3/4-in. round dowel which is fastened vertically in front of a hole in tlie side of the case near the diaphragm. This mounting holds the prism in a fixed plane but allo~vsit to be rotated. The lens is held in a hole in an "Ln-shaped plywood piece and the prism is mounted on a block behind the lens on the same board as the lens. The mount is attached to the floor of the case so that the holder can be moved forward and backward to fix the distance from the lens to film a t 41 in., the focal length of the lens. The prism holder allows the prism to he rotated and tilted to place the edge of the prism parallel to the slit and light beam. Finally, the mirror is mounted on a small board which is fastened to a 3/.,-in. dowel along the axis of the dowel. This dowel pivots n.it,li its axis parallel to the bottom of the case, allox~ingthe spect,rum to be shifted np and dovn in the film plane. The pivoted dowel is in turn mounted on a small board which can he rotated on an axis perpendicular to tlie floor of the case, making it possible to shift the spectrum from side to side in the plane of the film.
The arc lamp is made froln two racks and pinions with electrode clamps which move in aluminum angle tracks (Fig. 4). For the power supply, a set of resistances is placed in series with the arc, and switches are provided to vary the resistance. The resistances are cone heaters and dryer heating elements, giving resistances ranging from 1.7 to 7.2 ohms. In addition to the arc, an electric spark source was used to produce some spectra. The power source for the spark was a 12,000 volt transformer (used for neon signs) with a capacitance (about 0.001 pf) placed acroes the output, and an inductance wired in series with the spark gap. Also a small fluorescent lamp was mounted to produce a reference spectrum of mercury (as seen in Figs. 7 and 8). Additional light sources such as a 100 watt projection lamp for recording absorption spectra and discharge tubes were available for special purposes.
When using the densitometer-comparator to measure distances between spectral lines, the operator rotates the screw moving the film holder to scan across the spectral line. When the deusitometer shows a maximum line intensity, the distance reading is taken. The direction from which the spectral line is approached in scanning is reversed several times, and the various values obtained are averaged to give a best value. To allow for changes in film sensitivity and lamp output with wavelength in recording absorption spectra, a reference exposure of the lamp and cell filled with pure solvent in addition to the desired absorption spectrum is recorded. Then the densities of the two spectra are measured a t given intervals, and a ratio of the absorption spectra density to the reference density is used as a measure of the optical density.
Figure 7. Hydrogen rpectro (lower three exporurprj with mercury refernce spectrum (topl.
Figure 8. Lithium rpectro (lawer three exporurerl with mercury reference v e r t r u m (fopl.
Figure 5.
Slit and facuring diaphragm.
Figure 9.
Figure 6.
Denritometer and comparator.
Densitometer
To make full use of the recorded spectra, a densitometer-comparator (Fig. 6) was constructed to measure spectral lines to within +0.002 in. The optical system of the comparator consists basically of a 300 W, 35mm projector with a special film holder. The projector optics were Edmnnd No. 70,320 ($11.50). The holder is made from angle aluminum and is fastened to each end of the blade of a machinist's square ruled with 100 divisions per inch. The holder is moved by a '1, in.-20 screw to scan the spectrum. The entire optical system is mounted on a hinged platform so that the four spectra on each film can be brought into position in front of the photocell, which measures the optical density of the photographic image. This photocell, an IP40 tube (Allied Radio, $3.60), is monnted about 2 ft from the film, and its output is amplified by a simple push-pull amplifier using a 12AU7A tube.
Absorption 3pectrum of chlonrphyll b.
This equipment has been used to record the spectra of 58 different elements, typical of which are the spectra of hydrogen and lithium (Figs. 7 and 8). Using the densitometer, the absorption spectra of several plant pigments were recorded, (Fig. 9, for example). Measurements made on a hydrogen spectra gave a value of 109,680 cm-' for Rydberg's Constant, and calculation of the ionization potential for lithium y~eldeda value of 5.39 ev which compares favorably with the accepted value of 5.390 e ~ The . ~ spectra of CX and C? were analyzed and the values obtained for Do,using a linear BirgeSponer extrapolation, were 11.0 ev for CN and 7.5 ev for C?. Acknowledgment
I express appreciation to Argonne National Laboratory for supplying samples of about thirty compounds which were used to produce line spectra of the vasious elements. Also I thank Mr. Henry C. &ass, my high school physics teacher, for his encouragement and suggestions throughout this project. Moom, CAARWTTEE., "Atomic Energy Levela," Vol. I, U. S. Government Printing Office, Washington, D. C., 1949, p. 8. Volume 41, Number 4, April 1964
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