You can make a spectroscope to allow you to separate the colors of light. White light is a mixture of colors of light, and not all light that appears white is the same. Chemists make fine distinctions between different colors because the colors of light can tell a lot about what something is made of. The composition of the sun and the atmospheres of other planets are studied by measuring colors of light produced, reflected, or absorbed. You might be surprised at what you see when you look at different light sources.
There are many experiments you can do, and ways you can modify the basic spectroscope design given here. I am always struck by the beauty of the spectrum I see, but then maybe I'm easily amused.
|What does the spectroscope do?||How to make a spectroscope||Further information|
You will need an old CD-ROM or music CD that you don't mind destroying, some dark heavy paper or light cardboard (dark construction paper works well), scissors, and glue and/or tape. A razor blade or sharp knife is useful, but not needed. You will also need the files you can download from this site.Simplest version:
Click here to get the pattern and instructions in PDF format for two simple spectroscopes per sheet, printed with text instructions and shapes to cut out. You can print this directly onto construction paper, or print onto ordinary paper and then copy onto something else. If you are making them for younger kids, you can cut the slit yourself with a sharp knife before handing them out for the kids to cut out and glue or tape together. If you use scissors to cut the slit, it is best to cut it oversized and then narrow it with parallel strips of opaque tape.Standard version:
Click here to get the PDF file to make three spectroscopes per sheet. This design is slightly different from that above. It has a smaller eye opening, which minimizes stray light. The text instructions are not included on the same page, so you should copy them from below for this version, or click here to get a more detailed set of text instructions as a PDF file. Visual instructions follow this section.
If you can't yet read PDF files, you can click here to download the Acrobat Reader software for free from Adobe.
1. Cut Out
2. Crease on
3. Add CD Wedge
4. Fold Up
& Blacken Tip
The spectroscope will separate light into its component colors by diffraction, deflecting the longer wavelength (red) light more than the shorter wavelength (blue/violet) light. Each wavelength has a different color, so you see a rainbow. With the spectroscope you can see which wavelengths are present, and how bright each is. There is a lot to be learned from the precise wavelength of light emitted or absorbed by an object, and each wavelength of light has a particular color, so scientists tend to consider color to be equivalent to wavelength. However, because of the way your eyes work, not each color you see corresponds to a single wavelength of light. Mixtures of light of various colors can appear to your eye to be the same as light of a wavelength that does not even appear in the mixture! This is very different from the way your ears work: if you hear an A and a C played together on a piano, you would not mistake the sound for a B. The difference in pitch is analogous to the difference in color: both are wavelength changes. Yet red and green light seen together appear to be the color between them in the spectrum. (What color is that?) With a spectroscope you will see that white light from various sources can be dramatically different. What do you see when you look at fluorescent lights, incandescent lights, street lights, gymnasium lights, or other lights? Colors that look the same can be very different. The light from your TV or computer screen has a very limited set of component wavelengths, yet it can show you very many colors.
Your spectroscope will enable you to see these differences. Look at light that has passed through colored transparent objects, or reflected off of colored paper, for example. As you think about what you observe, keep in mind that mixing light is different from mixing paint. The reason for this is that paint provides a color by soaking up, or absorbing, certain colors, and reflecting what's left of the light. The color you see is provided by light that was present in the original white light, that has not been removed by absorption by the paint. If you mix two colors of paint, the colors of light absorbed by each pigment will be removed from the reflected light, you see what is left. This type of mixing of colors is called subtractive. Printers use black ink (that absorbs all colors of light), and a set of 3 subtractive primary colors of ink (cyan, magenta, yellow) to mix to print colors. Additive mixing of colors results when colored beams of light are combined, and is less familiar to most people than subtractive mixing. People who handle theater lighting must be aware of the very different effects of mixing light compared to paint. The additive primaries are red, green, and blue. If you magnify a white region of your computer monitor display, you may be able to see that it is made up of tiny glowing spots of red, green, and blue! If you mix red and green light in the right proportion, the result will be yellow light. If you look at yellow objects with your spectroscope, you can tell whether they give off only yellow light, or a combination of red and green.
For a chart showing spectra of common light sources to compare with your observations, see Jacobs, Stephen F. "Challenges of Everyday Spectra" J. Chem. Educ. 1997 74 1070. The chart can be found here.
For a basic introduction to the science of color, see chapters 4 "Color", and 5 "Light Waves" in: Asimov, I. "Understanding Physics: Light, Magnetism, and Electricity", Walker & Co., New York, 1966
For a more rigorous treatment at the college level, see chapters 44 "Diffraction" and 45 "Gratings and Spectra" in: Halliday, D.; Resnick, R. "Physics Part II" Wiley, New York, 1960
For a non-technical introduction to the way light and diffraction are described by the more modern and complete theory of quantum electrodynamics, see: R. P. Feynman "QED The Strange Theory of Light and Matter", Princeton Univ. Press, Princeton, NJ, 1985
For a comprehensive treatment of these topics at a high level, see: Hecht, E.; Zajac, A."Optics" Addison-Wesley, Reading, MA., 1974
Other CD-ROM spectroscopes:
Brouwer, H. "Line spectra using a CD disc" J. Chem. Educ. 1992, 69, 829.
http://littleshop.physics.colostate.edu/CD_Spectroscope.html "CD Spectroscope"; Little Shop of Physics, Colorado State University's Hands-On Science Outreach Program, 1997.
Wakabayashi, F.; Hamada, K.; Sone, K. "CD-ROM Spectroscope: A Simple and Inexpensive Tool for Classroom Demonstrations on Chemical Spectroscopy" J. Chem. Educ. 1998, 75, 1569; A simplified version: "CD Light: An Introduction to Spectroscopy" J. Chem. Educ. 1998, 75, 1568A.
To make spectroscopes, copy these figures onto opaque paper or lightweight cardboard and cut them out. You can either print the PDF file directly onto dark paper, or print onto white paper and photocopy onto dark paper. Dark construction paper works well, or any paper if you photocopy black onto the reverse. It is more important that the paper be opaque than that it be dark. Cut on the solid lines, including the small slit, but don't cut on dotted lines. Cut the slit carefully with straight smooth edges, so as to let through some light, about 0.5 mm wide. You can cut it wider, and then form a narrow slit by the gap between two pieces of opaque tape. Crease on the dotted lines along a ruler or other straightedge, so it can fold to make a little box. Now choose a data or music CD that you don't want as a CD, such as those you get unsolicited in the mail, and cut it into wedges using a pair of stout scissors. You can get about 16 useful wedges out of one CD to use as diffraction gratings. If the CD tends to crack as it is cut, you can prevent that by cutting while it is under warm (not hot) water. Attach a wedge of CD where indicated by the dotted outline, on the side of the paper that will become the inside (usually the unprinted side). Make sure the iridescent shiny side of the CD wedge is exposed, but it's convenient to tape the narrow point to hold it on, and to cover the mirror-like bit near the tip. Crease on the dotted lines along a straightedge, and fold to make a little box with the CD piece inside on the bottom. Glue or tape edges closed (a to a, b to b, etc.) so that they don't leak light, but do not cover the slit. Rubber cement can be applied to all the flaps and to all the lettered regions the flaps will cover. After the cement dries, the spectroscope can be neatly folded into shape in alphabetical order. Unlabelled flaps need not be glued. It's convenient to tape at x less thoroughly or not at all so the back can be opened to look at or readjust the diffraction grating (CD piece). You're finished!
Now point the slit at a light, and look through the hole at the CD. Note the arrows on the sides of the spectroscope that show the directions to the light source, and to look. Try looking at an incandescent light bulb, and then at a fluorescent light bulb. Are all fluorescent lights the same? Try street lights and other light sources. Look at light reflected off of colored paper, or shining through transparent colored plastic, glass, or juice. How does white light from your computer monitor compare to white paper? Does the paper look the same under different lights? Can you tell why colors look ok under some street lights and not others, despite the similar appearance of the lights themselves? (The lights look similar to your eye, but not to the spectroscope.) What happens if you widen the spectroscope slit?Alan Schwabacher
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Revised January 8, 2002