John R. Amend Montana State Univers~ty Bozeman, 59715
Colored Slides as Emission Spectra Unknowns
M u c h of our understanding of the structure of atoms has been derived from study of the emission spectra of simple atoms and development of theoretical models which attempt to explain these spectral observations. The beginning general chemistry student is usually introduced to several of these models (Balmer, Rydberg, and Bohr). Hen-ever, the expense of moderate numbers of calibrated spectroscopes and their associated equipment has limited student experience with emission spectra and, therefore, his conceptual understanding of the phenomenon upon which the theoretical atomic models are based. This paper will discuss the use of 35-mm colored slides as inexpensive, convenient, and accurate emission spectraunknowns for use with large groups of students. We have successfully used inexpensive diffraction grating spectroscopes1 for qualitative introduction of band and line spectra and to illustrate the unique nature of the emission spectra of several different gaseous and flame test elements. This introduction involves transformer-operated discharge tubes set up by the laboratory instructor, incandescent lamps, and student flame tests with sodium and lithium. Following this introduction to the field of spectroscopy, each student is issued a colored slide of the emission spectrum of an unknown element. These slides also carry mercury reference spectra, and a mercury lamp (mineral light) is available in the laboratory for qualitative identification of the mercury lines. Slides calibration chart is const,ructed using the mercury lines as known reference points, and the wavelengt,h of the unknown lines is determined. The unknown is t,hen identified by referring to a chart of the emission spectra of the several possible elements. Accuracy is quite good, as will be illust,rated later in reference to student measurement of some of the hydrogen lines. Pre~aralionof Slides differentBoth methods of preparing theWeemissionused spectra slides. methods involve simultaneous photography of both the unknown and reference mercury spectra. The simpler method produces mercury lines superimposed on the unknown
spectrum, while a slightly more complex set-up produces a reference spectrum located above or below the unknown spectrum, but in wavelength alignment with it. Suitable mercury reference sources are a "mineral light" with the uv filter removed, or a mercury discharge tube. We have used both gaseous discharge tubes and Osram lamps as unknown spectra sources. Unknown elements used successfully with our freshmen include Ne, Zn, He, Na, Cs, and H. Hydrogen slides have also been used to check a simple studentconstructed energy level model for the hydrogen atom. Data for this measurement collected by 246 first-quarter freshmen is presented later in this paper. Slides were taken with a 35-mm single lens reflex camera. A sheet of replica diffraction grating (Edmund Scientific) was taped over the lens, and the source was viewed through a razor blnde slit assembly mounted on a large baffle board. Proper orientation of the grating is necessary to produce a horizontal spectrum. Although good slides may be prepared using a standard 50 or 58-mm focal length lens, a 135-mm telephoto lens will spread the visible spectrum over most of the slide, resulting in higher accuracy of wavelength determination. The exposure time for a given lens-source-film combination must be determined experimentally by running a series of test exposures in the range 1/2-10 sec. We have found an exposure of 1 sec a t f/2.8 with Ektachrome color film to be satisfactory for most of our sources. Figure 1 shows the experimental configuration for photography of both unknown and reference spectra. The mercury lamp was masked in this experiment to produce shorter lines than the unknown, thus permitting identification of the reference lines when the slide is projected. If one chooses this method, it is wise also to prepare some slides showing only the mercury spectrum. Students can use the mercury reference slide to help sort out reference and unknown lines. Two superimposed spectra photographed by this method are shown in Figure 2. Figure 3 presents the experimental configuration used to separate reference and unknown spectra while Simple cardboard-tube spectroscopes are avdable for about from Damon Educational, 240 Highland Avenue, Needham Heights, Mass. 02154. 1
256
Results of 246 Student Measurements of Hydrogen Spectral Lines Line Red Blue Violet Violet
272
True A Value
(A)
Mast Common Value (k) Mean (A) (A)
6562.85 4861.33 4340.47 4101.74
/ Journal of Chemical Education
6560 (521246) 4860 (841246) 4340 (541246) 4100 (103/246)
6548.5 4851.85 4336.07 4096.13
Standard Deviation (A) 45.98 15.80 2.5.15 31.57
Mean Error (A) -14.35 -9.48 -4.40 -5.61
% Error 0.22 0.20 0.10 0.14
Figure 1.
ConRgurotionfor photography of ruperimpo9ed spectra. Figure 3.
ConRguration for photography of reparote reference and urn-
known spectra.
Figure 2. Hydrogen spectrum photographed a t &own in Figure I. Notice thot the mercury liner ore shorter thon the unknown (hydrogen1 lines, and thus identifiable.
Figure 4. Helium spectrum phologrophed os shown in Figure 3. The mercury reference spectrum i s below the helium spectrum in the photograph. The ultraviolet response ir limited by the tronrmisrion chorocter-
maintaining wavelength alignment. The reference source is placed slightly lower than the unknown source, and a small mirror placed a t an angle as shown will present an image of the reference source to the slit. Careful alignment of this mirror is essential for a bright reference spectrum. This method, although more difficult to set up, produces slides that are somewhat easier to use than those described earlier (Fig. 4). The accuracy students are able to achieve in measurement of the projected spectral lines is quite good. The table summarizes data for the four visible region
ittic of the camera lens. ond the for left m e r c y line i s ot 3663 A. The Other mercury liner ore red helium line on the far right is ut 6678
5
shown ot4046.4077.4358.5460,
and 5790 A.
hydrogen lines collected by 246 first-quarter general chemistry students. This data was taken the second week of the quarter, and thus reflects variation in student skill in graphing as well as the inherent accuracy of t.he method. The author will be happy to supply interested readers with copies of the student experiment.
Volume 48, Number 4, April 1971
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