Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the 2" x 4" piece of C-Spectra^® diffraction grating into 18 equal pieces, each approximately 1.5-cm square.
2. Cut the black construction paper into 4½" x 2" rectangles.

Safety Precautions

The materials used in this activity are considered safe. Caution students against looking directly at the sun, even through the spectroscope. Students should follow all normal classroom safety guidelines.

Disposal

Leftover C-Spectra^® can be stored in a plastic resealable bag for future use.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs. This laboratory activity can reasonably be completed in one 45- to 50-minute class period. The prelaboratory assignment may be completed before coming to lab. Students may have time to draw a few sample spectra from light sources in the classroom during the activity, and more may be assigned as homework.
• Approximately 16–20 inches of electrical tape will be needed per spectroscope. This may be measured and cut in advance.
• It may be desirable for each student to make a spectroscope. Empty paper towel tubes, while not as sturdy, can also be used. Additional C-Spectra gratings are available from Flinn Scientific (Catalog No. AP1714).
• C-Spectra diffraction gratings are washable and can be cleaned with a soft cloth and mild detergent. Take care as fingerprints and scratches will reduce the effectiveness of the grating.
• The transparency of the C-Spectra makes it difficult to locate when put down. To reduce the number of “lost” squares, keep the pieces in one location and have students obtain the grating only when ready to tape it in place. This also minimizes handling of the C-Spectra.
• Students will not be able to see the Fraunhofer lines when viewing the sun’s spectrum with their spectroscopes due to the limited resolution of the diffraction grating. The Sun’s continuous spectrum can be seen by aiming the spectroscope in the general direction of the Sun, but students should never aim the spectroscope directly at the Sun.

Teacher Tips

• See a free video at flinnsci.com, keyword AP7161vid.
• Students can look at various light sources around the school—fluorescent, sunlight (be sure they do not look directly at the Sun) or other sources that might be in the lab such as incandescent lights, black lights or ultraviolet lamps (students should not look directly at the ultraviolet light). Spectrum tubes will produce the best line emission spectra, however, they are expensive. If your lab does not have any, a high school physics teacher might be willing to loan some tubes with the necessary power supply. “Neon” novelty lamps (available at many party stores and discount stores) are an inexpensive alternative source of line emission spectra.
• Have students use the spectroscope to look at different light sources in their neighborhood. Evening works best. Suggested light sources include fluorescent and incandescent lights, street lights, novelty lamps, and “neon-type” signs. Ask students to record the type, location and color of each light source viewed with the unaided eye, then draw and label each spectrum they see in the Data Table with colored pencils. Discuss the “homework” spectra in class the next day. If students are sharing tubes, give them extra time to complete the assignment.
• Students interested in computer graphics may be able to design and print a simulated spectrum with the computer color palette.
• Students can also look at various solid-colored objects to see which colors are absorbed by the object are which are reflected. Looking at light that has passed through different colored filters is another option.
• Visit the following website at http://jersey.uoregon.edu/vlab/elements/Elements.html (accessed February 2007) to view line emission and absorption spectra of the elements.
• To further explore absorption and emission spectra this spectroscope could be used with the following Flinn Scientific Student Laboratory Kits: AP5607—Flame Test Kit (middle school level) and AP8823—Absorption Spectroscopy Kit (advanced level) and also AP1716—Flame Test/Emission Spectroscopy Demonstration Kit (advanced level).

Answers to Prelab Questions

1. Which color of visible light has the longest wavelength? Which has the shortest?

   Red has the longest wavelength and violet has the shortest.

2. Briefly describe the three types of spectra that can be viewed with a spectroscope.

   The three types of spectra are: continuous, which show all wavelengths of visible light; line emission, which are brightly colored bands at specific wavelengths unique to each element; and absorption, which are continuous spectra with gaps or black lines formed when white light passes through a cooler gas.

3. What precautions are recommended when handling the C-Spectra^® diffraction grating?

   Hold the diffraction grating by the edges as fingerprints and scratches will reduce the effectiveness of the grating.

Sample Data

Answers to Questions

1. Which light sources appear to produce a continuous spectrum?

   Sunlight (see Safety Precautions) and incandescent lightbulbs show a continuous spectrum.

2. Do all of the colors of light in the continuous spectra have equal width? Describe any differences.

   The red, green and blue bands appear wider than the yellow, orange and violet—although it is sometimes hard to tell where one color ends and another begins.

3. Which light sources appear to produce line emission spectra? (Some may show bright emission lines superimposed on a continuous spectrum.)

   Fluorescent, black light, “neon” lights and street lights all show emission lines.

References

Special thanks to David A. Katz, retired, Wilmington, DE, for providing the idea for this activity to Flinn Scientific.

Schwabacher, Alan. “What does the Spectroscope do?” http://uwm.edu/~awschwab/specweb.htm#diffract (accessed December 2006)

“Spectral Lines,” Physics 2000.

Introduction

A spectroscope is a device for forming and observing the color spectrum of visible light. A spectrum is produced when light from any source is bent or dispersed. Does every type of light show the same spectrum? Find out by using common household materials and a holographic diffraction grating to build a simple, working spectroscope.

Concepts

• Diffraction of light
• Continuous spectra
• Absorption spectra
• Emission spectra

Background

The spectroscope uses a diffraction grating to separate light into its component colors. A diffraction grating is a transparent material with parallel lines etched into it. When white light strikes the grooves on the diffraction grating film, the light is separated, or diffracted, into its component wavelengths known as a spectrum (plural—spectra). A similar effect can be seen when light strikes the grooves on a compact disc (CD). Each color in the spectrum has a specific wavelength. Red has a longer wavelength and will diffract more than the shorter blue/violet wavelengths. This is why the colors of the rainbow always appear in the same order—red, orange, yellow, green, blue, indigo, and violet, known by the acronym ROY G BIV (see Figure 1).

Depending on the source of the light, different types of spectra can be seen using a spectroscope. White light that is emitted when an object is heated will produce a rainbow-like continuous spectrum. In other words, all wavelengths of visible light are emitted. Incandescent lightbulbs viewed through a spectroscope will show a continuous spectrum.

In the late 1850s, two scientists, Gustav Kirchhoff (1824–1887) and Robert Bunsen (1811–1899), placed various substances in a flame, allowed the light from the flame to pass through a prism and viewed the resulting spectra. They found that each element produced a unique spectrum that was different from any other element. It is now known that when an atom absorbs energy, the atom’s electrons will “jump” to a higher energy level. This process is sometimes called “exciting” the electrons. As the excited electrons release the extra energy and return to their normal state, electromagnetic radiation is emitted at specific wavelengths. Although most of this radiation cannot be seen, some of the radiation is within the range of visible light (see Figure 1). The colors of light emitted after excitation and relaxation of an element’s electrons are unique to that element. The spectrum produced from exciting one element contains only specific wavelengths which are seen as brightly-colored bands. This spectrum is called a line emission spectrum (see Figure 2). The lines are due to the different excited electrons returning to their normal states. “Neon-type” signs viewed through a spectroscope will show a line emission spectrum. Prior to Kirchhoff’s and Bunsen’s experiment, William Wollaston (1766–1828) noticed several dark lines in the sun’s spectrum. He assumed they were boundary lines between the wavelengths of color. In 1814, Joseph von Fraunhofer (1787–1826), a German optician, studied and recorded more than 600 of these dark lines, which are now called Fraunhofer lines. What these scientists were seeing were actually absorption lines. Kirchhoff was the first to explain the cause of absorption lines. Absorption spectra result when a source of white light travels through a cooler gas. As radiation from the light source travels through the gas, some of it interacts with the atoms in the gas and excites electrons in the gas. For example, light from the sun travels through the solar atmosphere, a relatively cooler gas. What happens when an excited electron returns to its normal state? Electromagnetic radiation is emitted, and radiation at that particular wavelength is absorbed by the gas, thus resulting in “gaps,” or dark lines, in the continuous spectrum (see Figure 3). So even though sunlight produces a continuous spectrum, when the solar spectrum is viewed with a quality spectroscope, an absorption spectrum is actually seen. By analyzing the pattern of dark lines in the solar spectrum and comparing it to absorption spectra of other elements, scientists have detected over 60 different elements in the sun’s atmosphere. The dark lines in an absorption spectrum are at the same wavelengths as the bright lines in the emission spectrum of the same element (see Figure 4). A continuous spectrum results from superimposing an element’s line emission spectrum over its absorption spectrum.

Experiment Overview

The purpose of this experiment is to build a working spectroscope. The spectroscope will be used to view and compare spectra of various light sources.

Materials

Prelab Questions

Read through the lab and answer the following questions on a separate sheet of paper.

1. Which color of visible light has the longest wavelength? Which has the shortest?
2. Briefly describe the three types of spectra that can be viewed with a spectroscope.
3. What precautions are recommended when handling the C-Spectra^® diffraction grating?

Safety Precautions

The materials used in this activity are considered safe. Do not look directly at the sun, even through the spectroscope. Please follow all normal classroom safety guidelines.

Procedure

1. Using the end of the cardboard tube as a guide, use a pencil to trace two circles having the same diameter as the tube onto black construction paper.
2. Carefully cut out each circle, making sure the diameter of each circle is no smaller than the diameter of tube. Each circle must completely cover the open ends of the tube.
3. Using the hole punch, make a hole in the center of one of the circles.
4. Obtain a 1.5-cm square piece of C-Spectra^®, holding it by only the edges. Caution: Fingerprints and scratches will reduce the effectiveness of the grating.
5. Take the circle with the punched round hole and, holding the square of C-Spectra by the edges, cover the hole with the C-Spectra. Secure with small pieces of cellophane tape. Do not place any tape over the part of the C-Spectra that will be visible through the hole (see Figure 5).
   {12580_Procedure_Figure_5}
6. Use the electrical tape to secure the circle with the C-Spectra facing inward to one end of the cardboard tube. Use enough tape so that light will enter the tube only through the hole, not around the edges.
7. Fold the other black paper circle in half and cut a 1-cm slit in the middle of the half circle, starting at the fold (see Figure 6). Make an identical 1-cm slit 1 mm from the first one, and then cut the resulting small strip from the circle.
   {12580_Procedure_Figure_6}
8. Unfold the circle. There should be a slit approximately 2 cm x 1 mm in the middle of the circle. The edges should be clean, not frayed.
9. Press pieces of electrical tape firmly around the edges of the circle with the slit, but do not fasten to the tube yet. Use enough tape to completely cover the edge of the circle (see Figure 7).
   {12580_Procedure_Figure_7}
10. Hold the circle with the slit over the open end of the tube and look through the hole in the other end at any light source. Rotate just the tube until a clear spectrum of the light is visible on both sides of the slit and the spectrum is as wide as possible. Tape the circle to the end of the tube in this position (see Figure 8).
    {12580_Procedure_Figure_8}
11. View various light sources provided by the teacher by looking through the end of the tube containing the C-Spectra and aiming the slit at the light. Note differences in bands of colors, the width and intensity of the bands and any dark lines.
12. (Optional) Take the spectroscope home and look at various light sources in the house and neighborhood. (Evening works best.) Suggested light sources include fluorescent and incandescent lights, novelty lamps, street lights and “neon-type” signs. Note: To view a spectrum from sunlight, the spectroscope should be aimed near the Sun but never directly at the Sun. Fraunhofer lines will not be visible due to the limited resolution of the diffraction grating.
13. Using colored pencils, sketch each spectrum in the data table. Only draw a spectrum from one side of the slit, not both. For each spectrum observed, record the location and source of the light, and the color of light to the unaided eye (without the spectroscope).

Student Worksheet PDF

12580_Student1.pdf