Doppler Effect2

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Doppler Effect - Real-life applications

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S OUND C OMPRESSION AND THE D OPPLERE FFECT

As stated in the introduction, one can observe the Doppler effect in a number of settings.

If a person is standing by the side of a road and a car approaches at a significant rate of

speed, the frequency of the sound waves grows until the car passes the observer, then

the frequency suddenly drops. But Doppler, of course, never heard the sound of an

automobile, or the siren of a motorized ambulance or fire truck.

In his day, the horse-drawn carriage still constituted the principal means of

transportation for short distances, and such vehicles did not attain the speeds necessary

for the Doppler effect to become noticeable. Only one mode of transportation in the

mid-nineteenth century made it possible to observe and record the effect: a steam-

powered locomotive. Therefore, let us consider the Doppler effect as Doppler himself

didin terms of a train passing through a s tation.

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THE SOUND OF A TRAIN WHISTLE.

When a train is sitting in a station prior to leaving, it blows its whistle, but listeners

standing nearby notice nothing unusual. There is no differenceexcept perhaps in

degree of intensitybetween the sound heard by someone on the platform, and the

sound of the train as heard by someone standing behind the caboose. This is because a

stationary train is at the center of the sound waves it produces, which radiate in

concentric circles (like a bulls-eye) around it.

As the train begins to move, however, it is no longer at the center of the sound waves

emanating from it. Instead, the circle of waves is moving forward, along with the train

itself, and, thus, the locomotive compresses waves toward the front. If someone is

standing further ahead along the track, that person hears the compressed sound waves.

Due to their compression, these have a much higher frequency than the waves produced

by a stationary train.


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At the same time, someone standing behind the traina listener on the platform at the

station, watching the train recede into the distancehears the sound waves that

emanate from behind the train. It is the same train making the same sound, but because

the train has compressed the sound waves in front of it, the waves behind it are spread

out, producing a sound of much lower frequency. Thus, the sound of the train, as

perceived by two different listeners, varies with frame of reference.

THE SONIC BOOM: A RELATED EFFECT.

Some people today have had the experience of hearing a jet fly high overhead, producing

a shock wave known as a sonic boom. A sonic boom, needless to say, is certainly not

something of which Doppler would have had any knowledge, nor is it an illustration of

the Doppler effect, per se. But it is an example of sound compression, and, therefore, it

deserves attention here.

The speed of sound, unlike the speed of light, is dependant on the medium through

which it travels. Hence, there is no such thing as a fixed "speed of sound"; rather, there

is only a speed at which sound waves are transmitted through a given type of material.

Its speed through a gas, such as air, is proportional to the square root of the pressure

divided by the density. This, in turn, means that the higher the altitude, the slower the

speed of sound: for the altitudes at which jets fly, it is about 660 MPH (1,622 km/h).

As a jet moves through the air, it too produces sound waves which compress toward the

front, and widen toward the rear. Since sound waves themselves are really just

fluctuations in pressure, this means that the faster a jet goes, the greater the pressure of

the sound waves bunched up in front of it. Jet pilots speak of "breaking the sound

barrier," which is more than just a figure of speech. As the craft approaches the speed of

sound, the pilot becomes aware of a wall of high pressure to the front of the plane, and

as a result of this high-pressure wall, the jet experiences enormous turbulence.

The speed of sound is referred to as Mach 1, and at a speed of between Mach 1.2 and

Mach 1.4, even stranger things begin to happen. Now the jet is moving faster than the

sound waves emanating from it, and, therefore, an observer on the ground sees the jet

move by well before hearing the sound. Of course, this would happen to some extent


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anyway, since light travels so much faster than sound; but the difference between the

arrival time of the light waves and the sound waves is even more noticeable in this

situation.

Meanwhile, up in the air, every protruding surface of the aircraft experiences intense

pressure: in particular, sound waves tend to become highly compressed along the

aircraft's nose and tail. Eventually these compressed sound waves build up, resulting in

a shock wave. Down on the ground, the shock wave manifests as a "sonic boom"or

rather, two sonic boomsone from the nose of the craft, and one from the tail. People in

the aircraft do not hear the boom, but the shock waves produced by the compressed

sound can cause sudden changes in pressure, density, and temperature that can pose

dangers to the operation of the airplane. To overcome this problem, designers of

supersonic aircraft have developed planes with wings that are swept back, so they fit

within the cone of pressure.

D OPPLERRADAR AND O THERS ENSING T ECHNOLOGY

The Doppler effect has a number of applications relating to the sensing of movement.

For instance, physicians and medical technicians apply it to measure the rate and

direction of blood flow in a patient's body, along with ultra-sound. As blood moves

through an artery, its top speed is 0.89 MPH (0.4 m/s)not very fast, yet fast enough,given the small area in which movement is taking place, for the Doppler effect to be

observed. A beam of ultrasound is pointed toward an artery, and the reflected waves

exhibit a shift in frequency, because the blood cells are acting as moving sources of

sound wavesjust like the trains Doppler observed.

Not all applications of the Doppler effect fall under the heading of "technology": some

can be found in nature. Bats use the Doppler effect to hunt for prey. As a bat flies, it

navigates by emitting whistles and listening for the echoes. When it is chasing downfood, its brain detects a change in pitch between the emitted whistle, and the echo it

receives. This tells the bat the speed of its quarry, and the bat adjusts its own speed

accordingly.

DOPPLER RADAR.


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Police officers may not enjoy the comparisongiven the public's general impression of

bats as evil, blood-thirsty creaturesbut in using radar as a basis to check for speeding

violations, the police are applying a principle similar to that used by bats. Doppler radar,

which uses the Doppler effect to calculate the speed of moving objects, is a form of

technology used not only by law-enforcement officers, but also by meteorologists.

The change in frequency experienced as a result of the Doppler effect is exactly twice the

ratio between the velocity of the target (for instance, a speeding car) and the speed with

which the radar pulse is directed toward the target. From this formula, it is possible to

determine the velocity of the target when the frequency change and speed of radar

propagation are known. The police officer's Doppler radar performs these calculations;

then all the officer has to do is pull over the speeder and write a ticket.

Meteorologists use Doppler radar to track the movement of storm systems. By detecting

the direction and velocity of raindrops or hail, for instance, Doppler radar can be used to

determine the motion of winds and, thus, to predict weather patterns that will follow in

the next minutes or hours. But Doppler radar can do more than simply detect a storm in

progress: Doppler technology also aids meteorologists by interpreting wind direction, as

an indicator of coming storms.

T HE D OPPLERE FFECT IN L IGHT WAVES

So far the Doppler effect has been discussed purely in terms of sound waves; but

Doppler himself maintained that it could be applied to light waves as well, and

experimentation conducted in 1901 proved him correct. This was far from an obvious

point, since light is quite different from sound.

Not only does light travel much, much faster186,000 mi (299,339 km) a secondbut

unlike sound, light does not need to travel through a medium. Whereas sound cannot be

transmitted in outer space, light is transmitted by radiation, a form of energy transfer

that can be directed as easily through a vacuum as through matter.

The Doppler effect in light can be demonstrated by using a device called a spectroscope,

which measures the spectral lines from an object of known chemical composition. These


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spectral lines are produced either by the absorption or emission of specific frequencies

of light by electrons in the source material. If the light waves appear at the blue, or high-

frequency end of the visible light spectrum, this means that the object is moving toward

the observer. If, on the other hand, the light waves appear at the red, or low-frequency

end of the spectrum, the object is moving away.

HUBBLE AND THE RED SHIFT.

In 1923, American astronomer Edwin Hubble (1889-1953) observed that the light waves

from distant galaxies were shifted so much to the red end of the light spectrum that they

must be moving away from the Milky Way, the galaxy in which Earth is located, at a

high rate. At the same time, nearer galaxies experienced much less of a red shift, as this

phenomenon came to be known, meaning that they were moving away at relatively

slower speeds.

Six years later, Hubble and another astronomer, Milton Humason, developed a

mathematical formula whereby astronomers could determine the distance to another

galaxy by measuring that galaxy's red shifts. The formula came to be known as Hubble's

constant, and it established the relationship between red shift and the velocity at which

a galaxy or object was receding from Earth. From Hubble's work, it became clear that

the universe was expanding, and research by a number of physicists and astronomersled to the development of the "big bang" theorythe idea that the universe emerged

almost instantaneously, in some sort of explosion, from a compressed state of matter.

WHERE TO LEARN MORE

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.

Bryant-Mole, Karen. Sound and Light. Crystal Lake, IL: Rigby Interactive Library, 1997.

Challoner, Jack. Sound and Light. New York: Kingfisher, 2001.

Dispenzio, Michael A. Awesome Experiments in Light and Sound. Illustrated by

Catherine Leary. New York: Sterling Publishing Company, 1999.


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"The Doppler Effect."The Physics Classroom (Web

site). (April

29, 2001).

Maton, Anthea. Exploring Physical Science. Upper Saddle River, N.J.: Prentice Hall,

1997.

Russell, David A. "The Doppler Effect and Sonic Booms"Kettering University (Web

site). (April 29,

2001).

Snedden, Robert. Light and Sound. Des Plaines, IL: Heinemann Library, 1999.

"Sound WaveDoppler Effect"(Web

site). (April 29, 2001).

"Wave MotionDoppler Effect"(Web

site). (April 29, 2001).

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