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).
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).
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
high-pressure 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
red-filtered 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.
Literature Cited
1. Several more complicated methods of introducing atomic
spectra have been described, including Cortel, A.; Fernandez, L.
J. Chem.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.
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