5.6 The doppler effect  (Page 4/35)

The training of astronomers includes much work on learning to decode light (and other electromagnetic radiation). A skillful “decoder” can learn the temperature of a star, what elements are in it, and even its speed in a direction toward us or away from us. That’s really an impressive amount of information for stars that are light-years away.

Key concepts and summary

If an atom is moving toward us when an electron changes orbits and produces a spectral line, we see that line shifted slightly toward the blue of its normal wavelength in a spectrum. If the atom is moving away, we see the line shifted toward the red. This shift is known as the Doppler effect and can be used to measure the radial velocities of distant objects.

For further exploration

Articles

Augensen, H.&Woodbury, J. “The Electromagnetic Spectrum.” Astronomy (June 1982): 6.

Darling, D. “Spectral Visions: The Long Wavelengths.” Astronomy (August 1984): 16; “The Short Wavelengths.” Astronomy (September 1984): 14.

Gingerich, O. “Unlocking the Chemical Secrets of the Cosmos.” Sky&Telescope (July 1981): 13.

Stencil, R. et al. “Astronomical Spectroscopy.” Astronomy (June 1978): 6.

Websites

Doppler Effect: http://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect. A shaking bug and the Doppler Effect explained.

Electromagnetic Spectrum: http://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html. An introduction to the electromagnetic spectrum from NASA’s Imagine the Universe ; note that you can click the “Advanced” button near the top and get a more detailed discussion.

Rainbows: How They Form and How to See Them: http://www.livescience.com/30235-rainbows-formation-explainer.html. By meteorologist and amateur astronomer Joe Rao.

Videos

Doppler Effect: http://www.esa.int/spaceinvideos/Videos/2014/07/Doppler_effect_-_classroom_demonstration_video_VP05. ESA video with Doppler ball demonstration and Doppler effect and satellites (4:48).

How a Prism Works to Make Rainbow Colors: https://www.youtube.com/watch?v=JGqsi_LDUn0. Short video on how a prism bends light to make a rainbow of colors (2:44).

Tour of the Electromagnetic Spectrum: https://www.youtube.com/watch?v=HPcAWNlVl-8. NASA Mission Science video tour of the bands of the electromagnetic spectrum (eight short videos).

Introductions to quantum mechanics

Ford, Kenneth. The Quantum World. 2004. A well-written recent introduction by a physicist/educator.

Gribbin, John. In Search of Schroedinger’s Cat. 1984. Clear, very basic introduction to the fundamental ideas of quantum mechanics, by a British physicist and science writer.

Rae, Alastair. Quantum Physics: A Beginner’s Guide. 2005. Widely praised introduction by a British physicist.

Collaborative group activities

1. Have your group make a list of all the electromagnetic wave technology you use during a typical day.
2. How many applications of the Doppler effect can your group think of in everyday life? For example, why would the highway patrol find it useful?
3. Have members of your group go home and “read” the face of your radio set and then compare notes. If you do not have a radio, research “broadcast radio frequencies” to find answers to the following questions. What do all the words and symbols mean? What frequencies can your radio tune to? What is the frequency of your favorite radio station? What is its wavelength?
4. If your instructor were to give you a spectrometer, what kind of spectra does your group think you would see from each of the following: (1) a household lightbulb, (2) the Sun, (3) the “neon lights of Broadway,” (4) an ordinary household flashlight, and (5) a streetlight on a busy shopping street?
5. Suppose astronomers want to send a message to an alien civilization that is living on a planet with an atmosphere very similar to that of Earth’s. This message must travel through space, make it through the other planet’s atmosphere, and be noticeable to the residents of that planet. Have your group discuss what band of the electromagnetic spectrum might be best for this message and why. (Some people, including noted physicist Stephen Hawking, have warned scientists not to send such messages and reveal the presence of our civilization to a possible hostile cosmos. Do you agree with this concern?)