A First Course in Physics · Ch. XXI, §§ 580–586 · Chapter 21: Color Phenomena
68. Spectra: what light tells us about the stars
Goal: Your student can tell a continuous spectrum from a bright-line spectrum, explain spectrum analysis and the dark Fraunhofer lines in sunlight, and explain how the Doppler effect reveals the motion of stars.
You'll need
- The book, open to these pages
- An old CD or DVD (a homemade spectroscope), a cardboard tube or box, tape, and two razor-thin edges of card to make a slit
- Different lights to look at: an old-style incandescent bulb, an LED bulb, a fluorescent tube, a streetlight (from a distance)
- A notebook and pencil
What's in the book
Hot solids and liquids give continuous spectra; glowing gases give bright lines (sodium, lithium, calcium, strontium); spectrum analysis and new elements; absorption spectra; the solar spectrum and Fraunhofer lines; sodium and iron in the sun; helium found first in the sun; Doppler's principle for light and the speeds of stars. Five questions follow.
The lesson, step by step
Hook question
SayHow can anyone know what the sun is made of without going there?
Read together
Read §§ 580–586. The key rule:
every gas or vapor will absorb exactly those wave lengths which it itself is capable of emitting when incandescent.
Try it: a CD spectroscope
Make a narrow slit in one end of a box or tube and tape a piece of CD (or hold the CD at an angle) at the other end; look at the slit's rainbow reflected in the CD. Point it at an incandescent bulb: a smooth, continuous rainbow (§ 580). Point it at a fluorescent tube: separate bright lines (mostly from mercury vapor). Compare an LED bulb and a yellow sodium streetlight if you have one. Never point it at the Sun.
Watch only: flame tests
Watch only: don't try this at home. The book's flame tests (§ 581) and burning sodium (§ 585) need a laboratory burner and reactive chemicals (sodium metal can burst into flame on contact with water). Look up a video of a teacher's "flame test" demonstration: each salt colors the flame differently (sodium yellow, lithium and strontium red, copper green), which is how fireworks get their colors.
Explain the idea
Each element gives its own set of lines, like a fingerprint (§ 582). Cooler gas in front of a hot source takes out exactly those lines, leaving dark gaps: the Fraunhofer lines in sunlight (§ 585). That is how sodium, iron and other elements were found in the sun, and how helium was discovered there before it was found on earth. If a star moves toward us its lines shift toward violet; away, toward red (§ 586).
What we know now: the shift of light toward the red from distant galaxies showed in the 1920s that the universe is expanding. Spectra also show the sun is mostly hydrogen and helium.
Say it back
Your student explains why the dark line in the yellow of the sun's spectrum shows there is sodium in the sun.
Questions
Work questions 1–5 on book page 460.
Afterwards
Use your spectroscope on every kind of light you can find this week and sketch what each one shows.
Tip
Inexpensive diffraction-grating glasses or cards, sold for science classes, work even better than a CD. Never point any spectroscope at the Sun.
Answers to the book's problems
1. There is a dark line in the solar spectrum exactly where sodium's bright yellow line falls, and sodium vapor darkens that same line. 2. The same as sunlight's, with the same dark lines, since moonlight is reflected sunlight. 3. Hold the mixture in a colorless Bunsen flame before the slit, look through the prism, and see whether calcium's green and red lines appear. 4. The molecules of a gas are free and vibrate at their own natural rates, like tuning forks; the crowded, jostling particles of a solid vibrate at all rates. 5. A wide slit, or a blurred image, makes the colored images of the slit overlap and cover the dark lines.
Worked out for this site from the scan. If a number in your copy differs, trust the book.