Gas emission tubes are widely used laboratory and demonstration apparatus because the line source nature of the tube produces line spectra when viewed directly through a diffraction grating. These are long and distinctly visible images of the tube, located to either side of the zeroth order at an angle determined by the wavelength. A continuous spectrum with each distinct color being a vertical line is produced by an incandescent bulb having a vertical line filament. P. J. Ouseph first contrasted spectra from thin white and black lines in an interesting activity reported in this journal in 1989. For more detail about why line spectra are useful, see Appendix I at TPT Online. I am reporting here a simple and useful method for presenting the spectral distribution of diverse sources and filters, or the reflected light from colored samples (inks, color swatches, etc.), that has the same visual impact and didactic benefits of the line sources noted above. This technique was devised as part of inquiry curriculum for the Light and Color class at Bellevue College.

1.
P. J.
Ouseph
, “
Spectra of white and black lines
,”
Phys. Teach.
27
,
458
459
(
Sept.
1989
). The activity described compares viewing a white line on black paper and black line on white paper through a prism and noting the apparent reversal of the spectral order. The demonstration is used to get students to think more clearly about how the spectra are formed. See Appendix I at TPT Online, https://doi.org/10.1119/1.5080573 .
2.
David
Sitar
, “
Imaging emission spectra with handheld and cellphone cameras
,”
Phys. Teach.
50
,
524
525
(
Dec.
2012
). Discusses the performance of a digital camera and two cellphone cameras for capturing spectra.
3.
This is distinct from the inherent spectral resolution of the grating. David Halliday, Robert Resnick, and
Kenneth S.
Krane
, “Dispersion and Resolving Power,” in
Physics
, 5th ed. (
Wiley
,
New York
,
2002
), Chap. 43-3, pp.
986
987
, provides an excellent discussion of resolution issues related to the use of the grating itself.
4.
Donald F.
Collins
, “
Video spectroscopy—Emission, absorption, and flash
,”
Phys. Teach.
38
,
561
562
(
Dec.
2000
). Collins created a similar comparison with an adjacent continuous spectrum by vertically aligning his Geissler tube with a slit in the window shade through which direct sunlight entered. Similarly, the authors in Ref. 12 vertically aligned their glow sticks for effective spectral comparisons.
5.
Hugh D.
Young
and
Roger A.
Freedman
, “
Continuous Spectra
,” in
Sears and Zemansky’s University Physics
, 13th ed. (
Pearson
,
London
,
2011
), Chap. 39-5, pp.
1310
1314
.
6.
James S.
Walker
, “Blackbody Radiation and Plank’s Hypothesis of Quantized Energy,” in
Physics
, 5th ed. (
Pearson Addison-Wesley
,
2016
), Chap. 30-1, pp.
1052
1053
.
7.
Readers can view the appendixes at TPT Online, https://doi.org/10.1119/1.5080573 under the Supplemental tab.
8.
PASCO Color Mixer (OS-8496)
, PASCO scientific, Roseville, CA, https://www.pasco.com/prodCompare/color-mixer/index.cfm.
9.
The Photograph in Fig. 11 does not display the colors reliably. The response curve of the RGB detectors in the camera near this wavelength have only a small overlap. They quickly transition from displaying green to displaying Red as the actual (yellow) wavelengths transitions across this boundary. The appearance for students viewing through a grating is a nearly monochromatic color matching the LED.
10.
David R.
Falk
,
Dieter R.
Brill
, and
David G.
Stork
,
Seeing the Light: Optics in Nature, Photography, Color, Vision, and Holography
(
Wiley
,
New York
,
1985
), Chap. 9-4, pp.
241
247
.
11.
Jennifer
Birriel
and
Ignacio
Birriel
, “
Glow sticks: Spectra and color mixing
,”
Phys. Teach.
52
,
400
525
(
Oct.
2014
). Similar simultaneous comparisons of spectra can be seen here.
12.
The author has been obtaining these as a passenger on Alaska Airline flights. He is pursuing the original source in communication with the airline.
13.
Students could decide that there is no red or green in the source by direct observation of the LED through the glasses, but we discourage direct observation of the UV sources (and encourage students to view all UV experiments for short periods only).
14.
Aidan
Byrne
. “
Compact disk spectroscopes revisited!
,”
Phys. Teach.
41
,
144
(
March
2003
).

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