Doppler effect





Doppler Effect 3030
Photo by: Uladzimir Bakunovich

The Doppler effect is an effect observed in light and sound waves as they move toward or away from an observer. One simple example of the Doppler effect is the sound of an automobile horn. Picture a person standing on a street corner. A car approaches, blowing its horn. As the car continues moving toward the person, the pitch of the horn appears to increase; its sound goes higher and higher. As the car passes the observer, however, the effect is reversed. The pitch of the car horn becomes lower and lower.

Explanation

All waves can be defined by two related properties: their wavelength and frequency. Wavelength is the distance between two adjacent (next to each other) and identical parts of the wave, such as between two wave crests (peaks). Frequency is the number of wave crests that pass a given point per second. For reference, the wavelength of visible light is about 400 to 700 nanometers (billionths of a meter), and its frequency is about 4.3 to 7.5 × 10 14 hertz (cycles per second). The wavelength of sound waves is about 0.017 to 17 meters, and their frequency is about 20 to 20,000 hertz.

The car horn effect described above was first explained around 1842 by Austrian physicist Johann Christian Doppler (1803–1853). To describe his theory, Doppler used a diagram like the one shown in the accompanying figure of the Doppler effect. As a train approaches a railroad station, it sounds its whistle. The sound waves coming from the train travel outward in all directions. A person riding in the train would hear nothing unusual, just the steady pitch of the whistle's sound. But a person at the train station would hear something very different. As the train moves forward, the sound waves from its whistle move with it. The train is chasing or crowding the sound waves in front of it. An observer at the train station hears more waves per second than someone on the train. More waves per second means a higher frequency and, thus, a higher pitch.

An observer behind the train has just the opposite experience. Sound waves following the train spread out more easily. The second observer detects fewer waves per second, a lower frequency, and, therefore, a lower-pitched sound.

Words to Know

Hubble's law: The law that shows how the redshift of a galaxy can be used to determine its distance from Earth.

Redshift: The lengthening of the frequency of light waves as they travel away from an observer; most commonly used to describe movement of stars away from Earth.

It follows from this explanation that the sound heard by an observer depends on the speed with which the train is traveling. The faster the train is moving in the above example, the more its sound waves are bunched together or spread out—thus, the higher or the lower the pitch observed.

Doppler effect in light waves

Doppler predicted that the effect in sound waves would also occur with light waves. That argument makes sense since sound and light are both transmitted by waves. But Doppler had no way to test his prediction experimentally. Doppler effects in light were not actually observed, in fact, until the late 1860s.

In sound, the Doppler effect is observed as a difference in the pitch of a sound. In light, differences in frequency appear as differences in color. For example, red light has a frequency of about 5 × 10 14 hertz; green light, a frequency of about 6 × 10 14 hertz; and blue light, a frequency of about 7 × 10 14 hertz.

Suppose that a scientist looks at a lamp that produces a very pure green light. Then imagine that the lamp begins to move rapidly away from the observer. The Doppler effect states that the frequency of the light will decrease. Instead of appearing to be a pure green color, it will tend more toward the red end of the spectrum. The faster the lamp moves away from the observer, the more it will appear to be first yellow, then orange, then red. At very high speeds, the light coming from the lamp will no longer look green at all, but will have become red.

Applications

The green lamp example described above has been used to great advantage by astronomers when observing stars. The light of a star as seen

The Doppler effect. (Reproduced by permission of The Gale Group.)
The Doppler effect. (Reproduced by permission of
The Gale Group
.)

from Earth is always slightly different from its true color because all stars are in motion. When astronomers observe stars in our own Milky Way galaxy, for example, they find that the color of some stars is shifted toward the blue, while the color shift in other stars is toward the red. Blueshift stars are moving toward Earth, and redshift stars are moving away from Earth.

In 1923, American astronomer Edwin Hubble (1889–1953) made an interesting discovery. He found that all stars outside our own galaxy exhibit redshifts of light. That is, all stars outside our galaxy must be moving away from Earth. Furthermore, the farther away the stars are, the more their redshift and, thus, the faster they are moving away from us.

Hubble's discovery is one of the most important in all of modern astronomy. It tells us that the universe as a whole is expanding. Like dots on the surface of a balloon that's being blown up, galaxies throughout the universe are racing away from each other. One conclusion to be drawn from this discovery is that—at some time in the past—all galaxies must have been closer together at the center of the universe. Ever since that time, those galaxies have been moving away from each other. This conclusion is the basis for the currently popular theory about the creation of the universe, the big bang theory.

The Doppler effect has many other practical applications. Weather observers can bounce radar waves off storm clouds. By studying the frequency of the waves that return, they can determine the direction and speed with which the clouds are moving. Similarly, traffic police use radar guns to determine the speed of vehicles. The faster a car or truck is traveling, the greater the change in the frequency of the radar waves it reflects.

Sound waves are used for underwater observations. A submarine sends out sound waves that are reflected off other underwater objects, such as another submarine or a school of fish. The frequency of the reflected sound tells the direction and speed of the other object.

[ See also Radar ; Redshift ; Sonar ; Wave motion ]



User Contributions:

Nicole Shaw
Report this comment as inappropriate
Mar 15, 2007 @ 12:12 pm
I enjoy the web site it help me with my project in Phsycial Science. Thanks alot.
Paul
Report this comment as inappropriate
Mar 7, 2008 @ 9:21 pm
Clearest explanation of Doppler in a way I can understand I've read for a long time. Thank you!
Herb Paynter
Report this comment as inappropriate
Apr 21, 2008 @ 10:10 am
I am writing a book for a major publisher and am referring to the Doppler effect. I have searched for an existing artwork of the famed Doppler train illustration. I have tried to locate the Gale Group to seek their permission to use this artwork (hopefully at print resolution). Can you help me find this group?

Thank you,
Herb
Ray
Report this comment as inappropriate
Oct 6, 2008 @ 9:21 pm
this website help me with a project that was due the next day, thanks.
Isabella
Report this comment as inappropriate
Oct 15, 2008 @ 3:03 am
I have a hopeless year 10 Astronomy teacher who was unable to teach my about the Doppler Effect and this site has made it all clear! I finally understand the Doppler Effect fully and its relationship to astronomy
Thank you!
Ahmar Qasim
Report this comment as inappropriate
Dec 12, 2008 @ 8:08 am
Excellent explanation of Hubble! This why we make our traffic lights green instead of red!!

Thanks,

Ahmar Qasim
susan
Report this comment as inappropriate
Jan 5, 2009 @ 10:22 pm
I was watching a t.v program and a person from the show was dressed as the dobler effect, i did'nt know what it was so l looked it up and found this information. It is very interesting as you are given other info also. Thank you
dammy
Report this comment as inappropriate
Nov 12, 2009 @ 4:04 am
Thats gret!the dobbler explanation came out clear which made me to understand better thanks alot.
Report this comment as inappropriate
Jul 10, 2010 @ 10:10 am
This is such a great site!
The info has really helped, and it tells a lot!

really helped me with my physics project!
Report this comment as inappropriate
Feb 28, 2011 @ 2:02 am
This is the clearest explanation ever read.
Thanks for the good work.
Report this comment as inappropriate
Mar 31, 2011 @ 3:03 am
how this law is useable for doppler's log. how does the doppler log work. Provide me all theory of doppler log. Does it concern with the water current. What is STW.
Chris
Report this comment as inappropriate
May 28, 2011 @ 1:01 am
Thank You,
This amazing web-site helped me study for my Final Exams that are very and this is really helpful. I'm surely going to recommend this to other people that have problems with this!!!

Best Regards,

Chris Jameseiontio (jahme-seiho-ntio)
Report this comment as inappropriate
Jul 13, 2011 @ 7:07 am
Is there any laboratory experiment which certifies that redshift is a Doppler efect? Because the proponent of the Electric Universe theory, especially the astronomer Alton Harp, have shown the opposite
yenyen
Report this comment as inappropriate
Jan 28, 2012 @ 11:23 pm
this is sOoO AWESOME!!!

this help me a lot!!!

THANK YOU!!!XD
Report this comment as inappropriate
Jan 31, 2012 @ 6:18 pm
THIS IS REALLY NICE .IT EXPLAINS THE CONCEPT VERY DEEPLY .GREAT SITE!!!
Keshar Bhandari
Report this comment as inappropriate
Sep 15, 2013 @ 11:11 am
I found this website verry useful. First time I'd visited this website was 2 years ago. And still I am visiting this site. I gain a lot of things about science and technology from this site. I would like to give thanks for this site.

Comment about this article, ask questions, or add new information about this topic:

CAPTCHA


Doppler Effect forum