Doppler Effect

Doppler Effect

Doppler effect in sound waves

Doppler effects in light waves

Other uses of the Doppler effect

Resources

The Doppler effect is named after Austrian physicist Johann Christian Doppler (1803–1853). Doppler observed and explained the changes in pitch and frequency of sound and light waves, as well as all other types of waves, caused by the motion of moving bodies. The general rule of the Doppler effect is that the wave frequencies of moving bodies rise as they travel toward an observer and fall as they recede from the point of observation.

While Doppler, in 1842, demonstrated the phenomenon named after him in the area of sound waves, in the same year he also predicted that light waves could be shown to exhibit the same response to the movement of bodies similar to those of sound waves.

Doppler effect in sound waves

The response of sound waves to moving bodies is illustrated by the locomotive whistle of a moving train as heard by observers standing near the track. When the train blows its whistle while it is at rest, stationary listeners who are either ahead of the engine or behind it will hear the same pitch made by the whistle, but if the train is moving, then as it advances, those who are ahead of it will hear a higher pitch and those behind will hear a lower pitch.

The faster the train moves the greater will be the effect on the pitch. Also, if the train remains at rest but the listeners either move toward the sounding train whistle or away from it, the effect will be the same. Those who move toward the train will hear a higher pitch, while those who travel away from the train will hear a lower pitch.

When the train is at rest it is the center of the sound waves it generates in circles around itself. As it moves forward, it ceases to be the center of the sound

waves it produces. The sound waves move in the same direction of the train’s motion. The train is chasing or crowding its waves up front, compressing them, so that the listener in front of the direction of its movement hears more waves per second, thus producing the effect of a higher frequency. The listener standing behind the train hears a lower pitch because the waves have spread out behind the forward motion of the train. Thus, there are fewer waves per second. The listener is now hearing a lower frequency than is actually being produced by the whistle.

In 1845, the Doppler effect received further confirmation in an elaborate experiment devised by a Dutch meteorologist, Christopher Heinrich Buys-Ballot. He placed a band of trumpet players on an open railroad flatcar and had it ride by listeners with perfect pitch who recorded their impressions of the notes produced by the whistle. Their written recordings of the pitches clearly demonstrated the Doppler wave effect.

Doppler effects in light waves

The Doppler effect in light waves can be observed by the spectral analysis of light emitted by luminous objects. The light from a stationary distant object whose chemical composition is known is refracted at a specific band of light on a spectroscope. That band is known as its index of refraction. If the light, instead, appears at another frequency band in the spectroscope, it can be inferred from the Doppler effect that the body is in motion. When the light appears at a higher frequency band, then the body is no longer stationary but moving toward the observer. The Doppler effected light wave is displaced toward the higher frequency band, which is the blue end of the spectroscope. If the known body’s light waves appear at a lower frequency band of the spectroscope, towards the red end, then the body is now in motion away from the observer.

With the use of the spectroscope, astronomers have been able to deduce the chemical composition of the stars. The Doppler effect enables them to determine the movements of stars and galaxies throughout the Universe. In our own galaxy, all stars will be shifted either to the blue or red end because of a slight Doppler effect, indicating either a small movement toward or away from Earth. In 1923, Edwin Hubble, an American astronomer, found that the light from all the galaxies outside our own were shifted so much toward the red as to suggest that they were all speeding away from our own at very great velocities. At the same time he saw that the recession of galaxies nearer to us was much less than those further away.

KEY TERMS

Hubble’s law— The law that states a galaxy’s redshift is directly proportional to its distance from Earth.

Index of refraction— The ratio of the speed of light as it travels through any two media.

Redshift— The lengthening of the frequency of light waves as they travel away from an observer caused by the Doppler effect.

Spectroscope— An instrument for forming and examining light as it is refracted through an optical spectrum.

In 1929, Hubble and Milton Humason established a mathematical relationship that enabled astronomers to determine the distance of galaxies by determining the amount of the galaxy’s red shifts. This mathematical relationship is known as Hubble’s law or Hubble’s constant. Hubble’s law shows that the greater the velocity of recession, the further away from Earth the galaxy is.

The concept of the expanding universe and of the Big Bang, that is, the sudden expansion of the universe from an originally compressed state, owes much of its existence to Hubble’s work, which in turn is an important development of the Doppler effect in light waves. Recent research based on the Doppler shifts of distant galaxies has confirmed not only that the Universe is expanding, but that its expansion is accelerating.

Other uses of the Doppler effect

In addition to its uses in science, the Doppler effect has many practical applications. In maritime navigation, radio waves are bounced off orbiting satellites to measure shifts, which indicate changes in location. In highway traffic speeding detection, radar employs the Doppler effect to determine automobile speeds. There are also a number of medical applications of the Doppler effect found in various forms of ultrasonography, which employ ultrasonic waves (sound waves too high too hear, generally in the megaherz range) to produce images of the body’s interiors. Doppler radar is used in meteorology and military systems.

See also Wave motion.

Resources

BOOKS

Berg, Richard E., and David G. Stork. Physics of Sound. 3rd ed. Upper Saddle River, NJ: Prentice Hall, 2004.

Doviak, Richard and Zrnic, Dusan S. Doppler Radar and Weather Observations. New York: Dover, 2006.

OTHER

National Center for Supercomputing Applications. “The Doppler Effect.” <http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html> (accessed October 25, 2006).

Jordan P. Richman