In the Classroom edited by
cost-effective teacher
Hal Harris University of Missouri–St. Louis St. Louis, MO 63121
Challenges of Everyday Spectra
W
Stephen F. Jacobs Optical Sciences Center, University of Arizona, Tucson, AZ 85721
Teachers usually introduce light and its spectrum to students using standard sources such as hydrogen, spectroscopes (with or without a slit and scale), and a wall chart for spectrum identification (1). The stunning beauty of spectra is immediately appreciated, but where do we go from there? (2). Why not challenge your students to go out and investigate the spectra of the night lights around town? What is needed is a small spectrum chart that identifies the various spectra and a diffraction grating through which to view them (see Fig. 1). For distant lights, the simple diffraction grating is more convenient than a spectroscope with a slit, since the distant lights serve as their own slit. Some things your students can investigate are: •
How many different spectra can be found?
•
How is the fluorescent lamp spectrum related to that of the incandescent lamp and low-pressure Hg spectrum?
•
Why is there a black line in the high-pressure Na spectrum where there is a bright yellow line in the low-pressure Na spectrum?
•
Why is there a bright red line in the spectrum of highpressure Hg when it is absent from the low-pressure Hg spectrum?
•
Which lamps cause most light pollution, and why?
•
Is there a difference between new automobile tail lights that are red LEDs vs. the old ones that are redfiltered incandescents?
•
Watch the night lights wake up! Why do yellow Na lamps start out red and then change color?
The above investigations are discussed in a booklet that is available with the chart and grating from most educational supply catalogs or the author. In addition, advanced experiments are described, showing how to observe the sun’s Fraunhofer absorption spectrum with just a grating and how to predict and measure the grating’s resolving power. You may have qualms about getting the grating and charts back for reuse if you entrust them to your students for night experiments. Nevertheless, the payoff is tremendous. The experiment of exploring real-world lights can really excite your students’ curiosity and stimulate a lot of interest in your subject.
Figure 1. Common night lamp spectra. a: Incandescent. b: Fluorescent. c: Hg (low pressure). d: Hg (high pressure). e: Metal halide. f: Na (high pressure). g: Na (low pressure). h: Neon sign (red). i: Neon sign (other colors).A color version of this figure appear on page 1023.
Literature Cited 1. Several more complicated methods of introducing atomic spectra have been described, including Cortel, A.; Fernandez, L. J. Chem.
W This article appears on JCE Online at http://jchemed. chem.wisc.edu/.
1070
Educ. 1986, 63, 348; Dammgen, U.; Keune, H. J. Chem. Educ. 1985, 62, 155; Hughes, E., Jr.; Arnold, G. J. Chem. Educ. 1984, 61, 908; Orna, M. V. J. Chem. Educ. 1981, 58, 965. 2. With some effort and a wavelength scale you could make contact with the atomic structure of hydrogen. Five lines of the hydrogen Balmer series are visible. See, for instance, Zeilik, M. Astronomy, 7th ed.; Wiley: New York, 1994; p 95.
Journal of Chemical Education • Vol. 74 No. 9 September 1997
