Doppler Effect
The Doppler effect was named after Christian Johann Doppler. This Austrian physicist 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.
The response of sound waves to moving bodies is illustrated in the example of the sounding of the locomotive whistle of a moving train. When the train blows its whistle while it is at rest in the station, stationary listeners who are either ahead of the engine or behind it will hear the same pitch made by the whistle, but as the train advances, those who are ahead will hear the sound of the whistle at a higher pitch. Listeners behind the train, as it pulls further away from them, hear the pitch of the whistle begin to fall.
The faster the train moves the greater will be the effect of the rising and falling of 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, Christop Hendrick 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.
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 their movements. 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, however, 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.
In 1929, Hubble and Milton Humason established a mathematical relationship that enables 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 along with the corollary idea of the big bang, that is, the instant creation of the universe from a compressed state of matter, owes much of its existence to Hubble's work, which in turn is an important development of the Doppler effect in light waves. While some recent research challenges the red shift phenomenon for galaxies, most astronomers continue to accept Hubble's findings.
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 that 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 ultrasonography, echocardiography, and radiology, all of which employ ultrasonic waves.
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