March 2007 F I G U R E 1

Size: px

Start display at page:

Download "March 2007 F I G U R E 1"

Hector Hutchinson
6 years ago
Views:

1 March 2007 Tips and techniques for creative teaching A 50-Cent Analytical Spectroscope During a recent unit on astronomy, my students asked, How do we know all that we know about the stars, since they are so far away and no one from Earth has ever visited them? A fair question, to be sure. While astronomers believe they have decent estimates of the mass, temperature, size, and composition nearby stars, it can boggle the student mind to think of how this type of distant science is even possible. The activity described here will not fully answer the question, but it should give students an insight into this process. In keeping with the Benchmark, Understanding the Nature of Scientific Inquiry, students can use a 50-cent analytical spectroscope to gather, analyze, and interpret scientific data (Kendall and Marzano 1996). Students can construct a simple, inexpensive instrument that demonstrates some key differences among light sources in the classroom an analytical spectroscope. With the spectroscope, students can observe homemade stars, graph results, and draw appropriate conclusions. The materials are basic: an empty paper towel tube, black construction paper, rubber bands, a hobby knife, masking tape, and sheets of diffraction grating. The only material not likely found around the house is a sheet of diffraction grating one sheet can make dozens of spectroscopes, and it is available from many science supply stores for about $15. Setting the stage Before students begin building the spectroscope, a primer of stars is probably in order (Fraknoi 1995). Students seem naturally fascinated with the sky, and a little preparation on the teacher s part can go a long way. For example, a slide show on stars, a good introductory video, or a discussion of the Hertzsprung- Russell Diagram can serve to show students that classifying stars is an interesting activity that dates from the early days of keen-eyed ancient shepherds to modern astronomers use of such technological marvels as the Hubble Space Telescope. Building the spectroscope To build an analytical spectroscope, each student should receive an empty paper towel tube, which will serve as the body of the instrument. Each student should also have access to scissors, two rubber bands, one half-sheet of black construction paper, masking tape, a hobby knife, F I G U R E 1 Cut out all straight lines from the circle, but don t cut out the circle itself. Tape the diffraction grating over the rectangle, like a window. The end-piece is made of black construction paper, and measures about 8 cm on a side. and a piece of mm diffraction grating. As for the hobby knife, I have found it useful to hold a clinic before the activity with strict safety guidelines discussed and posted. [Safety note: Students must wear safety goggles. It is a good idea to inventory all sharp items distributed for student use, to be sure they all are properly returned when the activity is completed.] To build an analytical spectroscope, students should follow these instructions: Cut the endpiece mounting for the diffraction grating out of black construction paper. A black square measuring about 8 cm on each side is suggested (Figure 1). This will become the eyepiece for the spectroscope. End-piece Mounting for Diffraction Grating (eyepiece end of spectroscope; not actual size). 58

2 F I G U R E 2 Partially Constructed Spectroscope (this end points toward light source and does not rotate; not actual size). Place the end of the paper towel roll on top of the center of the black construction paper square in a perpendicular fashion and trace its circle on the paper. Draw 8 lines (rays) on the black paper, extending from the circle out to the edge of the paper. Cut a small rectangular window in the middle of the traced circle; it should be about half the size of a postage stamp. Carefully tape a slightly larger piece of the diffraction grating over the small window around its edges. You should be able to see through the window you have made. Cut each of the rays as they extend from the circumference of the circle all the way out to the edge of the paper. Do not cut the circumference itself. As you face the taped diffraction grating, carefully bend each of the cut flaps in toward you, with each flap crease along the circumference of the circle. As they bend, they will overlap each other, creating a sort of baffle, or light-shield. Take the whole endpiece assembly and place it on one end of the paper towel tube, like a cap. Secure it gently with a rubber band or two along the outside of the black paper. The endpiece should rotate about the tube and should be snug, not loose. The main light entering the tube now should be from the open, opposite end. The opposite end of the spectroscope will contain the nanometer scale and also a slit for light to enter. This end is made The eyepiece: A diffraction grating is mounted on rotating black cardboard secured with a rubber band. F I G U R E 3 Empty paper towel tube The endpiece: A nanometer scale made using Microsoft Paint, which points toward the light source. The slit is exactly 21 mm from the edge of the nano-scale. Nanometer-scale End of a Spectroscope (this end is pointed toward the light source; actual size). Slit is exactly 21 mm from the black edge of the nanometer scale, is parallel to it, and should be roughly the same height as the scale itself. Light will enter here, and appear as color bands along the scale. March

3 from white copy paper, and embedded within it is the black nanometer scale, shown in Figure 3 (p. 59) in actual size. This endpiece will eventually be securely taped in place and will not move about the tube like the eyepiece. This rectangle, which can be photocopied, measures cm. Take the open end of the whole tube and gently squeeze it into a noticeable oval, which remains oval-shaped after you stop applying pressure. Place the cm white rectangle from the previous step, flat on the table. Within the oval outline, carefully cut a slit for light to enter using the hobby knife. A piece of corrugated scrap cardboard placed underneath the white rectangle may be helpful in obtaining a clean, crisp cut. The width of the slit should be around 1 2 mm, and its height is about the same as that of the black nanometer scale (make a photocopy of the true-to-size Figure 3). [Author note: The scale was created in Microsoft Paint.] Lay the Figure 3 (p. 59) rectangle on the table, face up, and position the open, ovalshaped end of the tube on top Call or for a FREE catalog! 60

4 of it. The tube and rectangle are now perpendicular. Note that each numeral on the scale represents that number times 100 nanometers, so the 7 for example, represents 700 nm. Take the tube away and carefully cut the rays starting from the periphery of the white rectangle, in toward the oval, but do not cut the oval. Like the eyepiece, each of the small flaps created by the rays will be folded and creased in toward you. After all creases are made and the slit is cut, the nanometer-end scale is ready to be installed. Reposition the open end of the tube onto the end-piece and flaps you just created, being careful not to disturb the delicate slit and nanometer scale. Gently but firmly tape the end-piece into position; unlike the other end of the tube, it will not move after being installed. The whole tube should now be light safe except for the light entering through the slit you F I G U R E 4 Atomic Spectroscopy. have made on one end, and the window on the other. To test their spectroscopes, students need a fluorescent light, such as those typically used as classroom overhead lights. Students should look through the eyepiece and, pointing the nanometer scale end toward the fluorescent light, see colored bands of light. Students turn the eyepiece until these rainbow bands line up horizontally along the nanometer scale. There should be a noticeable discreet bright-green line almost exactly halfway between the 5 and the 6. Since each numeral on the scale actually represents that number multiplied by 100 nanometers, the bright-green line is actually showing the signature of a trace of mercury used in the manufacture of the fluorescent lights. The mercury is actually showing up at a wavelength of 546 nm (Vogt 2001). Students should take note of the rest of the colors the widths of each color band and where each falls along the scale. The blue hues have shorter wavelengths; the reds are longer. What s happening? Visible light is only a small part of the radiation from the Sun and stars, but it is the only part that the human eye can detect. Visible light March

5 is a very narrow band of radiation ranging from roughly 400 to 700 nm in wavelength. The diffraction grating in the student-made spectroscope breaks up the white light entering through the slit; the nanometer scale shows the approximate wavelengths of the colored light in billionths of a meter. A classroom tool The spectroscope (Figure 4, p. 61) can be used to examine the spectral signature of the fluorescent lights found in most classrooms. As students peer into their spectroscopes, they will notice, first and foremost, the rainbowlike patterns that appear. The colors will seem to be superimposed on the nanometer scale, a reference to compare the widths, intensities, and possible bright lines or dropouts that may be present for a given light source. Students can draw graphs of what they see with colored pencils. Other lights can be examined and graphed, for example, incandescent light bulbs, krypton flashlights, and candles. Each source of light will have a unique spectrum falling along the nanometer scale. Some aspects will be the same for each light source, but some will not. Observant students will notice the subtle differences that appear in their spectroscopes for each light source displayed. [Safety note: Students should be cautioned to never look directly at the Sun.] I usually set up four stars in a central location in the classroom and have students gather in a semicircular pattern with their spectroscopes, graph paper, and colored pencils. A small fluorescent lamp, an incandescent bulb, a krypton flashlight, and a candle are easy sources of different types of light that will yield different views inside the spectroscopes. If teachers have access to the physics lab, they can also use a transformer and light up discreet tubes of glowing gas, such as nitrogen, oxygen, and hydrogen. Each will look different inside the darkness of the spectroscope. The spectroscope described here is fairly easy to build and costs about 50 cents. Students may be interested in knowing that much more expensive and accurate spectroscopes are found in observatories. These spectroscopes are attached to sophisticated telescopes, and serve as aids in studying starlight coming to us from many light years across empty space. Since the invention of the telescope in the 17th century, astronomers have used visible starlight to learn much about the temperature, movement, distribution, and chemical makeup of the stars. The stars, planets, meteors, and moons are fun to observe and inspiring to study. This simple activity may cause students to pause, look up at the stars, and perhaps wonder (Sagan 1995). And such wonder, said Albert Einstein, is the first true source of all knowledge. John Frassinelli (jfrass895@att.net) is a science teacher at Christian Brothers High School in Memphis, Tennessee. References Fraknoi, A., ed The universe at your fingertips: An astronomy activity and resource notebook. San Francisco: The Astronomical Society of the Pacific. Kendall, J., and R. Marzano Content knowledge: A compendium of standards and benchmarks for K 12 education. Aurora, CO: Mid-continent Regional Educational Laboratory. Sagan, C The demon-haunted world: Science as a candle in the darkness. New York: Random House. Vogt, G.L Space-based astronomy. Washington, DC: U.S. Government Printing Office. 62

Using the spectrometer

Using the spectrometer MATERIALS LIST Investigation 13.1 Stars and Spectroscopy 4 Spectrometer (also known as a spectroscope) 4 Colored pencils 4 Incandescent light source ChAPTER 13 The Universe How can we use a spectrometer