Name ____________________________________________ Date ___________ Section_______
ACTIVITY 8
Light and Spectra
Learning Goals
In this activity, you will explore the properties of continuous and line emission. These properties are
fundamental to our understanding of temperatures, compositions, luminosities, and velocities of
astronomical objects. By working through this activity, you will be able to:
1. Explain how the temperature of an incandescent lightbulb affects the intensity and colors of the
observed spectrum.
2. Compare the observed continuous spectrum to a series of Planck (blackbody) curves.
3. Examine emission spectra of five elements, noting the patterns and intensities of the lines.
4. Identify three “unknown” elements.
5. State the significance of each element having its own unique spectral signature of emission lines.
Step 1—Incandescent Bulbs and the Continuous Spectrum
The first image (shown by your instructor or displayed by other means) contains actual spectra
from an incandescent bulb, starting at its brightest at the top and ending with its dimmest at the
bottom. These are “continuous” spectra because the colors run smoothly across the wavelengths.
Use colored pencils—or work with shading with regular pencils—to reproduce the brightest spectrum in Figure 8.1a and the dimmest in Figure 8.1b. If you use a pencil, label each wavelength
with the color that appears in the spectrum at that wavelength.
(a)
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(b)
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FIGURE 8.1
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ACTIVITY 8
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Light and Spectra
1. Based on your experience, is a dim bulb hotter or cooler than a bright bulb? ____________
2. How does the temperature affect the bulb’s intensity?
3. How does the temperature of the bulb affect the colors?
4. There are obviously colors missing in the spectrum when the bulb is at its dimmest compared
to the spectrum of the bulb when it is at its brightest. Which colors are missing?
5. What do your answers to questions 2, 3, and 4 imply about the intensity of the light and the
colors observed when viewing a hot versus a cool incandescent bulb?
Step 2—Relating the Results to Planck (Blackbody) Curves
Figure 8.2 shows a series of Planck curves that represent the intensity of light produced for a
range of wavelengths and for four different temperatures. These curves were generated mathematically and conform to the usual practice of having short wavelengths on the left and long wavelengths on the right for the x-axis. This is opposite to what is displayed in the continuous and
emission spectra being examined and reproduced here.
Planck curves for temperatures 3500–2000 K
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3500 K
A measure of the intensity
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3000 K
5
UV
FIGURE 8.2
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ACTIVITY 8
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Light and Spectra
6. Do these theoretical curves support the summary you provided in question 5? Explain your
answer, bringing in the intensity of the wavelengths within the visible part of the spectrum
(300–700 nm).
7. The peaks in intensity for these curves occur in which region of the spectrum? Can we see
light at these wavelengths?
The melting point for tungsten, the element for the filament of a bulb, is about 3700 K, which
means that the filament probably does not get much hotter than about 3500 K. Examine
Figure 8.3 that gives an expanded view of the curve for 2000 K.
Planck curves for temperatures 2000 K
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A measure of the intensity
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FIGURE 8.3
8. Assume that the temperature of the filament in the bulb at its dimmest was around 2000 K
and explain how we were still able to see some red in the spectrum. If we could see at infrared
wavelengths, would the bulb appear brighter or dimmer?
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ACTIVITY 8
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Light and Spectra
9. There are many stars that have surface temperatures in the 3500–2000 K range. Would they
appear bright or dim to us? What kind of telescope would astronomers use in order to view
these stars at their brightest wavelengths?
10. Where would be the best location for that telescope?
Step 3—Emission Spectra
The second series of images (shown by your instructor or displayed by other means) to review contains the true-color emission spectra of five elements. In Figure 8.4, use colored or regular pencils
to reproduce these spectra, placing the lines at the correct wavelengths. Indicate the brightness of
the line by making wider lines on your paper for brighter lines in the spectrum. Be careful to note
at what wavelength each color appears and the intensity of each line.
11. Compare the spectra that you sketched. Did any of the elements have the same emission
spectrum? Comment on both the similarities you observed in these spectra and also the differences.
Step 4—Identifying “Unknown” Elements
The final series of images (shown by your instructor or displayed by other means) to examine
involve “unknown” elements shown in actual color, intensity, and spacing of the lines. Consider
the emission spectra you sketched in Figure 8.4 to be the laboratory reference spectra and the
unknowns to be from a newly discovered star. The pattern of the lines, colors, and spacing are
important here since we may not know the wavelengths in advance.
12. Based on your comparisons between the known and unknown elements, what elements are
present in this star?
13. From your results, summarize how spectra can be used to find the composition of a gas.
Include in your summary the significance of each element having a unique spectrum.
ACTIVITY 8
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Light and Spectra
Element names:
a. __________________________
b. __________________________
c. __________________________
d. __________________________
e. __________________________
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FIGURE 8.4