doppler effect & its applications

8/3/2019 Doppler Effect & Its Applications

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DOPPLER EFFECT IN

SOUND


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DOPPLER EFFECT

TheDoppler effect isnamed Austrian physicist Christian Doppler who

proposed it in 1842, is the change in frequency of a

wave for an observer moving relative to the source of thewave. It is commonly heard when a vehicle soundinga siren or horn approaches, passes, and recedes from anobserver. The received frequency is higher (compared to

the emitted frequency) during the approach, it isidentical at the instant of passing by, and it is lowerduring the recession.


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he relative increase in frequency can be explained as

follows. When the source of the waves is moving towardthe observer, each successive wave crest is emitted froma position closer to the observer than the previous wave.

Therefore each wave takes slightly less time to reach the

observer than the previous wave. Therefore the timebetween the arrival of successive wave crests is reduced,causing an increase in the frequency. While they aretraveling, the distance between successive wave fronts isreduced; so the waves "bunch together".


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Conversely, if the source of waves is moving awayfrom the observer, each wave is emitted from aposition farther from the observer than theprevious wave, so the arrival time betweensuccessive waves is increased, reducing thefrequency.


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Change of wavelength caused by motion of the source


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For waves that propagate in a medium, suchas sound waves, the velocity of the observer and of

the source are relative to the medium in whichthe waves are transmitted. The total Dopplereffect may therefore result from motion of thesource, motion of the observer, or motion of themedium. Each of these effects is analyzedseparately. For waves which do not require amedium, such as light or gravity in general

relativity, only the relative difference in velocitybetween the observer and the source needs to beconsidered.


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HOW IT CAME INTO EXISTANCE

Doppler first proposed the effect in 1842 . Thehypothesis was tested for sound waves by Buys Ballot in1845. He confirmed that the sound's pitch was higher

than the emitted frequency when the sound sourceapproached him, and lower than the emitted frequencywhen the sound source receded fromhim. Hippolyte discovered independently the same

phenomenon on electromagnetic waves in 1848 InBritain, John Scott Russell made an experimentalstudy of the Doppler effect (1848).


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COMMON MISCONCEPTION

Craig Bohren pointed out in 1991 that some physicstextbooks erroneously state that the observed

frequencyincreasesas the object approaches an observer

and then decreases only as the object passes theobserver. In most cases, the observed frequency of anapproaching object declines monotonically from a valueabove the emitted frequency, through a value equal to

the emitted frequency when the object is closest to theobserver, and to values increasingly below the emitted

frequency as the object recedes from the observer.


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Bohren proposed that this common misconceptionmight occur because the intensity of the sound increasesas an object approaches an observer and decreases onceit passes and recedes from the observer and that this

change in intensity is misperceived as a change infrequency. Higher sound pressure levels make for asmall decrease in perceived pitch in low frequencysounds, and for a small increase in perceived pitch for

high frequency sounds


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APPLICATIONS OF DOPPLER

EFFECT


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The siren on a passing emergency vehicle will start outhigher than its stationary pitch, slide down as it

passes, and continue lower than its stationary pitch asit recedes from the observer. n other words, if thesiren approached the observer directly, the pitch wouldremain constant (asvs, r is only the radial

component) until the vehicle hit him, and thenimmediately jump to a new lower pitch.

1) IN SIRENS


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Because the vehicle passes by the observer, the radial

velocity does not remain constant, but instead varies asa function of the angle between his line of sight and thesiren's velocity

wherevs is the velocity of the object (source of waves)with respect to the medium, and is the angle between

the object's forward velocity and the line of sight fromthe object to the observer.

cos.sr

vv


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The Doppler effect for electromagneticwaves such as light is of great usein astronomy and results in either a so-called

redshift or blue shift. It has been used tomeasure the speed at which stars and galaxiesare approaching or receding from us, that is,the radial velocity. This is used to detect if anapparently single star is, in reality, aclose binary and even to measure the rotationalspeed of stars and galaxies.

2) ASTRONOMY


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The use of the Doppler effect for lightin astronomy depends on our knowledge thatthe spectra of stars are not continuous. Theyexhibit absorption lines at well defined frequencies thatare correlated with the energies required toexcite electrons in various elements from one level toanother. The Doppler effect is recognizable in the factthat the absorption lines are not always at the

frequencies that are obtained from the spectrum of astationary light source. Since blue light has a higher

frequency than red light, the spectral lines of an

approaching astronomical light source exhibit a blueshift and those of a receding astronomical light sourceexhibit a redshift.


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3) TEMPRATUREMEASUREMENTS

Another use of the Doppler effect, which is foundmostly in plasma physics and astronomy, is theestimation of the temperature of a gas which is emittinga spectral line. Due to the thermal motion of theemitters, the light emitted by each particle can beslightly red- or blue-shifted, and the net effect is abroadening of the line. This line shape is calleda Doppler profile and the width of the line is

proportional to the square root of the temperature of theemitting species, allowing a spectral line (with the widthdominated by the Doppler broadening) to be used toinfer the temperature.


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4) RADAR SYSTEMS

The Doppler effect is used in some typesof radar, to measure the velocity of detectedobjects. A radar beam is fired at a moving

target , as it approaches or recedes from theradar source. Each successive radar wave has totravel farther to reach the car, before being

reflected and re-detected near the source. Aseach wave has to move farther, the gap betweeneach wave increases, increasing the wavelength.


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In some situations, the radar beam is fired at themoving car as it approaches, in which case eachsuccessive wave travels a lesser distance,decreasing the wavelength. In either situation,calculations from the Doppler effect accuratelydetermine the car's velocity


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5) MEDICAL SECTOR

An echocardiogram can, within certain limits, produceaccurate assessment of the direction of blood flow andthe velocity of blood and cardiac tissue at any arbitrary

point using the Doppler effect. One of the limitations isthat the ultrasound beam should be as parallel to theblood flow as possible. Velocity measurements allowassessment of cardiac valve areas and function, anyabnormal communications between the left and rightside of the heart, any leaking of blood through the valves

and calculation of the cardiac output. Contrast-enhanced ultrasound using gas-filled microbubblecontrast media can be used to improve velocity or other

flow-related medical measurements.


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6) FLOW MEASUREMENT

Instruments such as the laser Doppler velocimeter(LDV), and acoustic Doppler velocimeter (ADV)have been developed to measure velocities in a fluid flow.

The LDV emits a light beam and the ADV emits anultrasonic acoustic burst, and measure the Dopplershift in wavelengths of reflections from particles movingwith the flow. The actual flow is computed as a function

of the water velocity and face. This technique allowsnon-intrusive flow measurements, at high precision andhigh frequency.


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BATS NAVIGATION(example of doppler effect)

To help them find their prey in the dark, most batspecies have developed a remarkable navigation systemcalledecholocation. To understand how echolocationworks, imagine an "echo canyon." If you stand on theedge of a canyon and shout "hello," you'll hear yourown voice coming back to you an instant later. Batsmake sounds the same way we do, by moving air past

their vibrating vocal chords. Some bats emit the soundsfrom their mouth, which they hold open as they fly.


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The bat can tell if an insect is to the right orleft by comparing when the sound reaches its

right ear to when the sound reaches its left ear:If the sound of the echo reaches the right earbefore it reaches the left ear, the insect isobviously to the right. The bat's ears have acomplex collection of folds that help it determinean insect's vertical position. Echoes coming frombelow will hit the folds of the outer ear at a

different point than sounds coming from above,and so will sound different when they reach thebat's inner ear.


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Other animals that use echolocation


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SUPERSONIC WAVES & SHOCKWAVES

Supersonic speed is a rate of travel of an object that islarger than the speed of sound (Mach 1). For objectstraveling in dry air of a temperature of 20 C (68 F)

this speed is approximately 343 m/s, 1,125 ft/s,768 mph or 1,236 km/h. Speeds greater than five timesthe speed of sound (Mach 5) are often referred toas hypersonic. Flight during which only some parts of

the air around an object, such as the ends of rotorblades, reach supersonic speeds are called transonic


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Sounds are traveling vibrations in the form of pressurewaves in an elastic medium. In gases, sound travelslongitudinally at different speeds, mostly depending on

the molecular mass and temperature of the gas,and pressure has little effect. In water at roomtemperature supersonic speed can be considered as anyspeed greater than 1,440 m/s (4,724 ft/s). In solids,

sound waves can be polarized longitudinally ortransversely and have even higher velocities.


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SHOCK WAVES

Ashock wave (also calledshock front or simply"shock") is a type of propagating disturbance. Like anordinary wave, it carries energy and can propagatethrough a medium (solid, liquid, gas or plasma) or insome cases in the absence of a material medium,through a field such as the electromagnetic field. Shockwaves are characterized by an abrupt, nearlydiscontinuous change in the characteristics of themedium. Across a shock there is always an extremely

rapid rise in pressure, temperature and density of theflow. In supersonic flows, expansion is achieved throughan expansion fan. A shock wave travels through mostmedia at a higher speed than an ordinary wave.


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THANK YOU


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SUBMITTED BY :-

Nandini Sreekumarclass : XI

B

Roll.no: 03

doppler effect & its applications

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