Inventory Control A Simple Diffraction Grating Spectroscope: Its Construction and Uses AdoH Cortel and Luls Fernhdez Centre de Documentad6 i Experimentacid de Batxlllerat. Conclli de Trent 160. Barcelona 08020. Spain The cunstruction ofa diffraction grating spectroscope has already been reported in several articles of THISJOLIHNALI. We describe here a procedure to fit a linear scale to this spectroscope, allowing the measurement of the wavelength of spectral lines without increasing the cost of the assembly, and reducing the measurement errors in the 10-nm range. Also described are some annlications of this modified snec.. troscope for secondary school laboratory experiments. With a shoebox (or anv similar box) about 30 cm lone. cut shown in Fig&e 1. two rectangular openings (A and Insert a piece of diffraction eratine (500-600 l i n e ~ / m minto )~ a slide frame (it is advisahie to cave all slide frames to he used made out of cardboard), and attach it to the A opening with adhesive tape. A slide frame fit with a cardboard strip will be used as a slit of variable width at B. At this point, the spectroscope can be used for the qualitative observation of spectra of gases (fluorescent lamps, commercial discharge tubes, sodium lamps), metallic salts, and absorption spectra (Fraunhofer lines of sunlight). Attach another shoebox without cover to the first one as shown in Figure 2. A rectangular openingot'about + cm wide should hecut lenmhwiseat the hottum ofthis h~)x.Twuslide frames (C and ~ i w i lhelp l to hold a strip of tracing paper P of the same width as a slide. A mirror is fit to the common wall of the two boxes in such a way that the plane of the mirror exactly forms a right angle with the slit (and the lines of the diffraction grating). If you look with your left eye a t the spectrum of a fluorescent light through the diffraction grating and, simultaneously, a t the tracing paper with the other eye, the two images will be easily superimposed by means of the mirror. You can achieve more easily the same result by adding a piece of opaque paper or cardboard to the back of the tracine-. oaner. . s&king~itintothe shoebox, to reach the same illumination in both eyes. Mark the spectral lines of mercurs (r,iolet 136 nm, green 546 nm, and yellow 578 nm) on the t k n g paper. If we consider the equation giving the angles in which constructive interference is observed in the diffraction grating (sin y = Xld for first-order spectrum), in small angles, the apparent height of the spectral lines will he proportional to the wavelength; therefore, the wavelength scale will be very approximatelv linear. The heieht of the soectral lines mea&ed from thk slit, allows sin ;to he calcuiated for eachline, and the distance d between two consecutive lines in the diffraction grating. ~
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edited by DENNIS SIEVERS Central Community High School Route 50 West Breese. IL 62230
Remove the strip of tracing paper from its settings. Using the 436-, 546-, and 578-nm lines as references, a linear scale can be constructed with adivision per 20 nm, from 400 to720 nm. Fit the strip of tracing paper with the scale into the settings (the slide frames C and Dl, and shift it so that, by performing the operation descrihed above. the reference lines coincide exactly with the spectral lines of a fluorescent light. After finding the correct position, reattach the s t r i of ~ tracing paper with adhesive tape. The spectroscope is now ready to measure the wavelengths of any spectrum being observed. The wavelength measurement of the most prominent Fraunhofer's lines with this device gives the values of 600, 520, and 490 nm (in good agreement with the accepted values of 589, 527, and 486 nm for the lines D. E. and F. he observed wavelengths from a' sodium respectivel;). lamp (600 nm) and a hydrogen . - discharee tube (670.490. and 440nm) justify the assignment of the'iines and F in the sunlight to sodium and hydrogen, respectively. Thus, the experiment allows interesting discussions about the relationship between light emission and absorption. The students may check that the values (656,486, and 434 nm) for the wavelengths obtained after Balmer's equation, for n = 3, 4, and 5, nearly agree with those measured in the spectrumof hydrogen. In contrast, the wavelengths a t 440,470,490,500,
Figure 1. Spectroscopefor !he qualitative observation ol specba: (A) Diffraction grating mounted into a slide frame; ( 0 )variable cardboard strip slit in a
Slide frame.
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Driscoll, J. A. J. Chem. Educ. 1974, 51, 97. Hughes, E. J. Chem. Educ. 1984, 61,908. We have used a piece of diffraction grating (approx. 525 lineslmm) cut from a Rosco Prism Filter PPL-7503. ROSCO 36 Bush Avenue. Pon Chester. NY 10573 348
Journal of Chemical Education
Figure 2. Finished spectroscope to measure !he wavelengm of spectral lines: (C) and (Dl slide kames; (M) mirror; and (P) hacing paper.
590, 680, and 720 nm measured in the spectrum of helium (accepted values 447,471,492,501,587,668, and 706 nm) do not show the regularity observed for hydrogen; hence, the difficulties to apply Bohr's postulates t o multi-electronic atoms become apparent. Some low-pressure sodium lamps allow an interesting exercise of gas identification. Those used in street lights (for instance, Sylvania SLP or General Electric SOX/H) show, just after switching on, many spectral lines that progressively are masked by the strong sodium emission. At first, a violet band a t 430-450 nm, two green lines a t 540 nm, and many lines from 580 to 700 nm are easily observed. The green lines and the lines from 580 to 700 nm can he assigned to neon, by comparison with the spectrum of a discharge tube of the element. The violet band a t 430-450 nm can be attributed to argon, by comparison with the corresponding spectrum. In sharp contrast, from smallest low-pressure so-
dium lamps like those used a t school laboratories (for instance, Philivs 931223) only the argon hand a t 430-450 nm can be obselved, associated with t h e intense line due to sodium. The spectroscope may also he used for flame analysis of metallic chlorides. The wavelength measurement of the most characteristic lines in the spectra gives for Li 680 nm, Na 600 nm, Ca 640 and 570 nm, Sr 620 and 650-700 nm, Cu bands a t 550-500 and 440-420 nm. These characteristic lines allow analysis of mixtures such as: Cu+Ca, Li+Sr, Sr+Ca, Cu+Sr, Na+Sr, Li+Ca+Sr, etc. Desdte the low resolution reached with this svectroscove, its lowcost and the theoretical basis inwlved initscondtrktion and use make the device a valuable and helpful tool fnr elementary quantitative spectroscopic measurements in secondary school teaching.
Volume 63 Number 4
April 1986
349
