R. K. Edwards,' W. W. Brandt, and Audrey 1. companion Illinois Institute of Technology
Chicago
A Simple and Inexpensive Student Spectroscope
W a v e mechanics and the orbital theory of atoms and molecules are assuming an increasingly important role in the content of high school and frcshman college chemistry courses. I t is clear t,hat this trend will continue, since wave merhanics permits unification of large areas of chemistry which hitherto had to he treated as cdections of isolated facts. There is a grt-at nced for laboratory experiments pertinent to atomic and moleonlar structure which can be done by the individual ~u~dergraduate or high school student. Only a few such experiments have heen described in t,he literatwe, and these are chiefly atomic-molecular model-huilding esercisrs. In this paprr the design of a spectroscope constructable by students in the laboratory is discussed, along with some modifications proposed by students and esperiments for which it may be used. These have been tested over a two-year period by more than 1000 students. Kormally the students are presented with the very simple design (Model 1) shown in Figure 1 and are encouraged to apply their ingenuity toward making a more sophisticated version. A sturdy box, preferably smaller than a standard cigar box, a t least one inch in in. wide) a t depth, is fit,ted with a slot (1in. deep and each of two facing ends. Slot A is narrowed to form the entrance slit of the spect,roscope by fastening to it t,wo razor blades so that the distance hetween the sharp edges is considerably less than 1 mm. If the box used is longer than 8 in., it is advantageous to construct a second parallel slit in a cardboard partition in the center of the hox (C). Slot B is fitted with a piece of flat diffraction grating? in. by 1 in.) cut so that the lines 1 Present address: Chemical Engineering Division, Argonne National Laborator,", Argonne, Illinois. Inexpensive replica grating may be obtained from Edmunds Scientific Co. More than 1000 spectroscopes have been fitted with grating casting less than $10.00.
on the grating are parallel to the slit formed at A . Proper alignment is critical to the success of the experiment. The box is then inverted open-face down and fastened to a board or sturdy piece of cardboard so that the cardboard protrudes a few inches past the exit slot. A protractor is mounted on this hoard with its origin near the exit slot. A piece of wire, bent a t opposite planar right angles and inserted at the origin of the protractor, serves as a movahle sighting device for the diffracted light beams. When a nichrome wire, bent to hold a portion of solid KaCI (or, alternatively, a piece of asbestos soaked in saturated KaC1 solution) is held in a Bunsen flame near the slit, the undiffractrd incident beam may be
Spectroscope model No. 1. A, Entrance slit mnd light source; B, diffraction grating; ond C,semnd (optional) slit.
Figure 1.
located with the sighting wire as an intense yellow line linear with the slit system. The reading on the protractor, near 90°, should be considered the zero point of measurement. To either side of this, a weaker yellow line will he observed, corresponding to the first order diffraction line. For the grating suggested this line occurs at approximately 20° from the zero point. A measurement should be taken on each side of the incident heam. and the two values of 0, the diffractionangle
Volume 39, Number 3, Morch
1962
/
147
relative to the zero point, averaged. With the known wavelength of the sodium light (5892 A) and the relationship governing diffnction through a grating
With either spectroscope, the mercury blue and green lines are visible and measurable in the light emitted by an ordinary fluorescent lamp. Students found comparison of this spectrum with the continuum of a tungsten lamp interesting, and those more enterprising visited and measured Chicago store-front neon lights. Occasionally, with well-aligned spectroscopes, secondorder diffraction lines and doublets are observed. Basically, the spectroscopes may be used strictly as instruments of qualitative analysis or as tools with which electron behavior may be studied. Since our emphasis was on the jumping elcctron, students were asked to fit their observed lithium measurement to a quantum jump in Lif using the Bohr formula,
nh = d sin 0
the distance d between lines on the grating may be calculated for the observed first-order (n = l ) diffraction angle 6'. Knowledge of d calibrates the spectroscope, and the wavelengths of emission lines of other elements can then be measured. Some typical lines measured by students are given in Table 1. Table 1. Some Representative Measurements Obtained with Spectroscope Model 1
Element
Observed value
Literature valuea
Lithium Calcium Barium Strontium
6780 A 6400 5590 A 5890 A
6708 A 6203 A 5535 A 6060 A
1/h = RZa[(l/nl)'
- (lln~)'],
[R = 109,737 ~ m - ~ ]
varying the effective nuclear charge 2, and guessing the quantum numbers n, and n2 until theory agreed
"Flame Spectra" in "Handhook of Chemistry and Physics," 40th ed., Chemical Rubber Publishing Company, 1958-59, p. 2745.
Student refinements to the model included a rotatable frame for the grating (making alignment easier), addition of a collimating lens to the optical system, light-proofing, intricate sighting devices, and baffles to cut out the incident beam after calibration. A second version of the spectroscope,~Model2), is illustrated in Figure 2. The entrance slit A is located at an angle of approximately 20 degrees with the dotted line CB. Opposite the diffraction grating B there is a 2-in. rectangular hole a t C, over which is pasted a small piece of graph paper, preferably with lightly-ruled millimeter lines and heavily-ruled centimeter lines. If the instrument is properly aligned the image of the slit will appear superimposed on the graph paper scale, as the light source illuminates both scale and slit. For calibration of the scale one may use the purple (4047 A) and the green (5461 A) lines of a standard mercury arc or known lines of two salts ignited in a Bunsen flame. For a path length CB of 7 in., a normal calibration is approximately 150 A per mm of scale. Some typical results obtained by linear interpolation
Figure 2. Spectroscope model No. 2 lrchematic). A. Entrance slit and lighl murce; B, diffraction grating; and C, graph-poper r d e .
with experiment. Advanced students were sent to standard reference works4 to search for accepted explanations of their observed lines in terms of changes in spdf orbital configurations. As additional research-oriented experiments, with Model 2 and a low pressure Hz lamp, students may attempt to locate the 6563 A and 4861 A lines of the Balmer series and compute the Rydberg constant R. With Model 1, students may substitute for the diffraction grating a properly cleaved crystal of NaBrOs. grown according to the recipe of Holden and Singer.' This adaptation has the twofold advantage of introducing the student to a prism spectroscope and to the many other fascinating crystal experiments described by Holden and Singer. The authors gratefully acknowledge the helpful suggestions of Professor T. J. Neubert and others of the staff of the Department of Chemistry.
Table 2. Some Representative Measurements Obtained with Spectroscope Model 2
Element
Observed value
Literature value"
Mercury Mercury Sodium
4346 A 5749 24 5853 A
4358 8 5770 A 5893
"Wavelengths for Spectroscope Calibration," in "Handbook of Chemistry and Physics," 40th ed., Chemical Rubber Publishing Company, 1958-59, p. 2745.
of the scale are given in Table 2. Measurements with Model 2 obviously are more accurate than those with Model 1.
'MOORE,C., "Atomic Energy Levels," NBS Circular 467, Vol. I, 1949; WHITE,H. E., "Introduction to Atomic Spectra," McGrt~w-HillBook Co., Inc., New York, 1934. 6 HOLDEN, A., AND SINGER,P., "Crystals and Crystal Growing," Doubleday-Anchor, New York, 1960.
Based an personal communication with Mr. A. Hine.
+ 148
/
Journal o f Chemical Education
+
+
