To observe and analyze types of spectra using a diffraction grating.
Ask students to describe the colors of a rainbow. Identify the rainbow as a spectrum and ask students if all spectra look alike. Ask students to predict if the colors will always be in the same order and if they think the same colors will appear in every spectrum.
Explain that spectra can be made with water, glass prisms, and even feathers. (see Space Based Astronomy, Unit 2, page 28 ff.)
Materials
Procedure Distribute the activity sheet and the diffraction grating(s). Have students work in groups. Read over the description of a diffraction grating with students. Students can examine the grating to see the little lines, using a hand lens or microscope if available.
Turn on the bright clear bulb. Show students how to observe the bulb using the diffraction grating(s). They may have to rotate the grating to see a spectrum. Turn off other lights in the room. Students can work in teams with one observing while the other colors in the spectrum on the activity sheet. When students have finished recording their data, switch to the low intensity bulb. It's best if you have two lamps available, and rapidly compare the two. Tell students that they are looking for differences in the spectra. Give students time to record answers. Ask them to predict how a flashlight's spectrum will look. After predictions are made, darken the room and distribute flashlights. (Students should discover that brighter lights have more blue light while fainter sources have more of their spectrum in the red.)
With the fluorescent tube and street lights, students should be looking for emission lines (see the Glossary in Space Based Astronomy) rather than a shift in spectrum colors. The fluorescent tube and the blue-white street light both use mercury vapor as their glowing gas. Emission lines of mercury vapor in the green and violet should be visible in the spectrum.
If you have gas spectral tubes and a high voltage display unit, this is an opportunity to use them. By observing spectral tubes, students will see different gases emit different wavelengths of light. If you're a middle or elementary school teacher, and don' have this equipment, perhaps you can borrow it from a high school.
Interdisciplinary Connection
Modern civilization exploits the characteristics of almost the
entire electromagnetic spectrum. As a Science-Technology-Society
activity, ask students to identify how the different wavelengths
of the electromagnetic spectrum are being used in their community.
Research can be expanded to identify the advantages of different
wavelengths for different applications. Brainstorm novel applications
to meet presently unmet needs.
KAO Connection
Astronomers can determine the composition of distant stars and gas clouds by analyzing their spectra--both in visible wavelengths and beyond. Many molecules emit radiation at infrared wavelengths. The same water vapor that absorbs infrared radiation in earth's atmosphere, for instance, can also be detected by its infrared signature in a comet. Spectra also allow astronomers to take the temperatures of stars. Blue-white stars are the hottest while red stars are the coolest.
Glass prisms and water droplets create rainbows by bending different wavelengths of light by different amounts. Rainbows can also be made using a diffraction grating. A diffraction grating has between 15,000 and 30,000 tiny lines etched into every inch of the plastic film. When light passes through the diffraction film, it forms little light sources between each line. These light sources interfere with each other. Different wavelengths combine in slightly different directions producing a rainbow spectrum.
Different sources of light produce different spectra. Using a diffraction grating, observe the spectrum of each type of light and then draw the spectrum with colored pencils. Show differences you find.
| Light Source | Red | Orange | Yellow | Green | Blue | Indigo | Violet |
| 150-200 watt bulb | ......... | ......... | ......... | ......... | ......... | ......... | ......... |
| 20-50 watt bulb | ......... | ......... | ......... | ......... | ......... | ......... | ......... |
| Flashlight | ......... | ......... | ......... | ......... | ......... | ......... | ......... |
| Fluorescent tube | ......... | ......... | ......... | ......... | ......... | ......... | ......... |
| Blue-white street light at night | ......... | ......... | ......... | ......... | ......... | ......... | ......... |
| Yellow street light at night | ......... | ......... | ......... | ......... | ......... | ......... | ......... |
Compare the flashlight with the two light bulbs. How do the spectra change as the light source becomes brighter and hotter?
The fluorescent tube and street lights contain glowing gases. The coating around these lights spreads out the spectrum, but you can still see bright lines. These bands are the characteristic fingerprint of the gases which make the spectra. Mercury vapor produces bright green and purple lines. Sodium vapor produces bright yellow lines. Which lights contain mercury vapor?
KAO Corner:
The spectrum of a red star has more light at the red end of the
spectrum. Is a red star hotter or cooler than a blue star? Is
its surface brightness greater or less? The spiral arms of our
Milky Way Galaxy are filled with clouds of glowing gas. Predict
how the spectrum from one of these glowing clouds might look.
What could astronomers learn from these spectra?
