Spectrum Analysis: Breaking it all down

Spectrum Analysis: Breaking it all down

Spectrum Analysis

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Spectrum Analysis: Breaking it all down

The constituent colors in a beam of light are revealed when the light is dispersed by a raindrop or passed through optical instruments known as prisms and diffraction gratings. In each case, an array of colors, or spectrum, is observed.Much of what we know about the makeup of matter has been gained through spectroscopy, the study of spectra. Our current understanding of atomic structure can be traced back to Johann Balmer's analysis of the spectrum of hydrogen. On a larger scale, just about everything we know about objects in the far reaches of the universe comes from examining light. Since each element emits a unique collection of colors, spectra, like fingerprints, may be used to determine the makeup and physical condition of stars.The colors contained in light from a particular source are revealed when the light is scattered from an optical instrument called a diffraction grating. Unlike a prism, which relies on refraction to disperse different wavelengths of light, a diffraction grating employs interference to separate spectral colors. A grating consists of a very large number of equally spaced parallel lines etched on the surface of either glass, plastic or metal.

While most people are familiar with the beautiful array of colors reflected from the surface of a compact disc, they may not realize that, like a diffraction grating, a compact disc is covered with thousands of tightly-spaced, nearly parallel lines. These lines are sections of a spiraling line engraved on the disc's surface. This data spiral contains the information-bearing pits that are burned on the disc during the recording process.The availability and low cost of recordable compact discs make it possible for every student in your class to make and use their own spectroscope. In this edition of CoolStuff we provide instructions for making three types of CD-R spectroscopes. The inexpensive devices are easy to construct and will allow students to study the spectra of light sources both in and outside the classroom.

Each type of spectroscope has unique advantages. The reflection spectroscope allows students to observe both emission and absorption spectra. Using this device they can observe the portions of the white light spectrum absorbed by a variety of substances, including chlorophyll. They may be surprised to learn that a leaf, the universal embodiment of greenness, rejects green light and absorbs light from the other portions of the visible spectrum.

Using the entire surface of a clear CD-R disc, the simple transmission spectroscope described below has great light gathering power. It may be used to study the spectra of sources of low intensity such as LEDs and the moon.

The CD-R tube spectroscope resembles many commercially available devices in both appearance and performance. The dramatic spectra produced with this spectroscope are seen throughout this newsletter.

Any one of the three spectroscopes may be used in school or at home. When used in your laboratory, we suggest that you set up as many light sources for observation as possible. These sources may include incandescent bulbs, fluorescent lamps, neon indicators, LEDs, computer monitors…the list goes on. We have included a data sheet that students may use to record their observations. If you have students examine light sources outside the classroom, you may find that they return with spectra of sources such as sodium and mercury streetlights, neon signs, halogen headlights, and electric range elements.

Constructing and Using Compact Disks as Diffraction Gratings: Three Approaches

1. Reflection CD Spectroscope

A CD reflection spectroscope consists of a CD mounted diagonally at the end of a shoebox. The figure below shows how the device is constructed. The CD may be held in place with either a small piece of tape or by inserting it into a slit cut in the bottom of the shoebox. A second narrow slit, approximately 5-cm long and 5-mm wide, is cut in the shoe box lid. Situated over the CD and parallel to the width of the shoebox, this aperture admits the light to be analyzed. At the opposite end of the CD a viewing window, roughly the size of a postage stamp, is cut in the shoebox’s end panel. Trial and error is used to obtain the optimal spectral display when positioning the CD.

Unlike the other activities in this issue of CoolStuff, any CD can be used to construct the Reflection CD Spectroscope.

 Reflection CD Spectroscope

Putting a plastic Petri dish over the slit on the top of the shoebox permits the examination of absorption spectra. When placed in the Petri dish, virtually any colored solution will produce dark stripes on a continuous incandescent spectrum. Solutions of chlorophyll, cobalt chloride, and potassium permanganate are readily available and produce good results.

Students should never look at the Sun with these devices!

The plan for CD Spectroscope is based on a design that appeared in an article by F. Wakabayashi, K. Hamada and K. Sone, J.Chem.Educ. 75, 1569 (1998)

2. Simple Transmission Spectroscope

While observing the reflected spectra from a CD is very revealing, a somewhat more convenient way of performing spectral analysis is with a recordable CD (CD-R). A CD-R without a label is used as a transmission grating. This type of grating permits direct viewing of light sources.

When CD-Rs are purchased in packages of 30, 50, or 100, one or two clear blank discs may be included as a form of protective packing material. While these discs don’t have labels, they do have spiral data tracks and hence may be used as diffraction gratings.

Actual photo taken using these instructions.

Actual photo taken using these instructions.

A clear CD-R may be used as a grating simply by holding it about 20 cm from the eye and centering the light source to be analyzed in the hole of the CD. A particular advantage of this type of spectroscope is that it may be used to study sources of low intensity such as an LED or the moon.

If clear CD-Rs with data tracks are not available, you will have to remove the laminated label from an ordinary CD-R. To do this, position the disc with the label side up. Using packing tape or duck tape and a quick pull of one or two pieces of the wide tape placed across the label will result in quick removal of the label.

Simple Transmission Spectroscope

3. CD-R Tube Spectroscope

To cut the smaller discs that will be used as spectroscope gratings, you will need a clear CD-R disc with a data track, a cardboard mailing tube or paper towel roll, a pencil or pen and a pair of tin snips or heavy duty scissors. Repeat this process to obtain additional gratings. You should be able to obtain at least four gratings from a single CD-R.

Place one end of the mailing tube on the CD-R so that the edge of the tube touches the outer edge of the disc (see figure below). Use a marking pen to trace around the tube so as to leave a circular mark on the surface of the disc.

Use the tin snips (or a large pair of good scissors) to cut around the circular mark on the CD-R.

CD-R Tube Spectroscope Figure 1.
CD-R Tube Spectroscope Figure 2.

Now cover one end of the mailing tube with an opaque material such as construction paper or aluminum foil. A narrow slit, approximately 0.5 mm wide and 2 cm long should be cut in this end cap to permit light into the tube.

The circular grating is attached to the other end of the tube using household cement or hot glue. Before attaching the grating, make sure that the grating’s grooves are parallel to the slit.

CD-R Tube Spectroscope Figure 3.
CD-R Tube Spectroscope Figure 4.

Why not have your students make it, take it and then teach it? Have your students make a piece of apparatus that illustrates a concept for each of your major science units. Then have them take it home and teach parents and siblings about what they are studying in class.

Making a piece of apparatus or creating a demonstration gives students a sense of ownership. Students always take pride in what they make themselves. Those who are not mathematically adept often shine when allowed to apply their creativity to produce something tangible.

As every teacher knows, you never really understand something until you are asked to teach it. Encouraging students to explain phenomena to others gives them an opportunity to make a concept their own.

CD-R Tube Spectroscope Figure 5.

The CD spectroscope has proven to be one of the most engaging devices I've had my students construct. If you are interested in increasing student understanding while getting families involved with science, the CD spectroscope is for you!

January 01, 2006 Collin Wassilak

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