Physics
Doppler Effect
The Doppler Effect refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. This effect is commonly observed with sound waves, where the pitch of a sound appears to change as the source or observer moves. In astronomy, it is used to determine the motion of celestial bodies based on the shift in their spectral lines.
Written by Perlego with AI-assistance
Related key terms
1 of 5
11 Key excerpts on "Doppler Effect"
- Christopher Lavers(Author)
- 2017(Publication Date)
- Thomas Reed(Publisher)
The Doppler Effect applies both to acoustic waves, such as sound in air and sonar waves in water, and to electromagnetic waves. For example, when the source of radar waves moves towards an observer, successive wave crests will be emitted from a position a little closer to the observer than the one before. Hence, each wave takes slightly less time to reach the observer than those previously. The time between the arrival of successive wave crests at the observer is thus reduced, resulting in an increase in the recorded frequency. While they are travelling, the distance between successive wave fronts is reduced so the waves ‘bunch together’. In a similar way, if the source of waves moves away from the observer, each successive wave will be emitted from a position slightly further away from the observer than the previous one (figure 8.3), so the arrival time between successive waves is thus increased, reducing the recorded frequency. The distance between successive wave fronts is then increased, so the waves ‘spread out’. 126 Physics Wave Concepts 126 For waves that propagate in a medium, such as sound waves travelling in air or sonar waves travelling underwater, the velocity of an observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler Effect may thus result from motion of the source, or motion of the observer, or relative motion of the medium itself. As long as there is relative motion taking place, a Doppler shift is observed. For waves that do not require a medium, such as light or other electromagnetic waves such as radar, only the relative difference in velocity between the observer and the source should be considered. 8.5 Relative velocity Consider a source of waves travelling at velocity Vs in the direction indicated in figure 8.4. A target will return Doppler shifted echoes if travelling at velocity Vt in the direction shown.
eBook - PDFSatellite Geodesy
Foundations, Methods, and Applications
- Günter Seeber(Author)
- 2008(Publication Date)
- De Gruyter(Publisher)
6 Doppler Techniques The Doppler Effect or Doppler shift , named after the Austrian physicist Christian Doppler (1803–1853), denotes the difference between the frequency of the radiation received at a point and the frequency of the radiation at its source, when observer and source are moving with respect to each other (e.g. NGS, 1986). The effect is well known from the fact that an acoustic signal, emitted by a vehicle passing the observer with high speed, shifts suddenly from a higher to a lower tone. When the vehicle is approaching, the observer receives a higher frequency compared to a non-moving source; and with increasing range a lower frequency is observed (Fig. 6.1). It is evident that the frequency shift depends on the relative velocity between source and observer. T λ 1 λ 2 T Figure 6.1. Doppler Effect Observation techniques based on the Doppler principle are widely used in science and technology for the determination of velocities. In astronomy the radial veloc-ity of distant cosmic objects can be derived from the Doppler shift of spectral lines (e.g. red shift of galaxies). In navigation velocity over the ground can be obtained by using the Doppler Effect ( Doppler log ). In space technology and satellite geodesy Doppler observations played an important role from the beginning, because most of the early satellites transmitted on a stable frequency. Velocity changes, and hence orbital elements, could be derived from the measured frequency shift with rather simple equip-ment. Using these techniques the orbital elements of the very first artificial satellites were determined and published without any other information (Priester, Hergenhahn, 1958). The frequency shift caused by the Doppler Effect can be described as a function of time (Fig. 6.2), and yields a characteristic curve. The Doppler curve of an artificial satellite lies in between curves (2) and (3), depending on the satellite’s range and altitude.
eBook - PDFTransesophageal Echocardiography Multimedia Manual
A Perioperative Transdisciplinary Approach
- André Denault, Pierre Couture, Annette Vegas, Jean Buithieu, Jean-Claude Tardif, André Y. Denault, André Y. Denault, Pierre Couture, Annette Vegas, Jean Buithieu, Jean-Claude Tardif(Authors)
- 2016(Publication Date)
- CRC Press(Publisher)
2 Basic Principles of Doppler Ultrasound Pierre-Guy Chassot University of Lausanne, Lausanne, Switzerland Claude Tousignant St. Michael’s Hospital, University of Toronto, Toronto, Ontario, Canada Doppler Effect The Doppler Effect is a well-known phenomenon: the sound of a train whistle has a higher pitch when the train is traveling toward the listener than when mov- ing away, though the emitting frequency remains the same. In 1842, the Austrian physicist Johann-Christian Doppler, studying the direction of movement of stars, mathematically described this frequency shift of recorded waves when a luminous or acoustic source, in relative motion, is compared with the stationary observer. The speed of traveling waves (c), such as light, sound, or ultrasound, is constant through a deter- mined medium and depends on the characteristics of this medium. When the emitting source is in motion, regardless of its speed, the sound waves are “compressed” in front of the transmitter, which is “catching up” with the transmitted waves it has produced (Fig. 2.1A). When the source moves slightly toward the stationary receiver before emitting the next wave, the two wave peaks are closer together when they reach the receiver (observer), where the wave- length (l) is shortened and the frequency ( f ) is increased (1). This happens because the product of l and f is a constant wave propagation of speed c: c ¼ f l (2:1) If the source moves away, the opposite occurs: l increases and f decreases.
eBook - ePub- Crispian Oates(Author)
- 2023(Publication Date)
- Wiley(Publisher)
8 Principles of Doppler UltrasoundTHE Doppler Effect
The Doppler Effect is the change in the detected frequency of a sine wave when there is relative motion between the transmitter and the receiver. This will occur for all wave phenomenon. It is the change in pitch you hear when a motor bike races past you. The pitch is higher as it comes towards you and drops to a lower pitch as it moves away.NOTE
For historical reasons and as it is the name of a person, Christian Doppler, who first described the effect, ‘Doppler’ is always spelt with a capital ‘D’.To see why there is this change in received frequency, consider the example of a boat rowing towards a source of waves on a lake (Figure 8.1 ). It will intercept those waves more rapidly than if the boat was stationary. If it were being rowed away from the source, the boat would intercept the waves at a lower frequency. If the boat was stationary, it would bob up and down at the same rate as the waves were being generated.In this case, the source of waves is stationary and the boat is a moving receiver.When a fire engine passes you at speed, with its sirens sounding, you hear a change in the pitch or frequency of the sound. The frequency is higher as it comes towards you and lower as it moves away. In this case, the fire engine is a moving transmitter and you are a stationary receiver.Illustration of a boat moving at velocity ‘v’ encountering wind‐blown waves moving at velocity ‘c’.FIGURE 8.1It doesn’t matter which is moving and which is stationary; it is the relative motion between the transmitter and receiver that causes the Doppler Effect.In the case of clinical ultrasound, the Doppler Effect will occur when there is relative motion between an echo target and the transducer, e.g. moving blood or cardiac wall. We can use this phenomenon to non‐invasively detect movement and measure velocities of targets within the body.
eBook - PDFThe Analysis of Starlight
Two Centuries of Astronomical Spectroscopy
- John B. Hearnshaw(Author)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
6 • The Doppler Effect 6.1 E A R L Y H I S T O R Y O F T H E D O P P L E R E F F E C T An important chapter in the history of astronomical spec- troscopy opened on 25 May 1842. On this day Christian Doppler (1803–53) (Fig. 6.1), the professor of mathematics at the University of Prague (then part of Austria), delivered a lecture to the Royal Bohemian Scientific Society entitled ‘Concerning the coloured light of double stars and of some other heavenly bodies’ [1]. By analogy both with sound and waves in the sea, Doppler maintained that light waves undergo a change in frequency of oscillation, and hence of colour, either when the luminous source or the observer is in motion relative to the aether (whose existence was at that time supposed necessary for the transport of light waves). He gave formulae for the frequency change ν when either the source or observer were in motion, and these amounted to a statement of the now familiar equation ν/ν 0 = V /c. Here V is the relative speed in the line of sight, c is the speed of light, and ν 0 is the light wave’s frequency for sources at rest. Doppler then made two incorrect assumptions: first, that the radiation from stars was largely confined to the visual region of the spectrum, and secondly that the space motions of the stars were frequently a significant fraction of the speed of light. As a consequence, stars normally appear- ing white are seen instead as strongly coloured, either vio- let or red, depending on their approach towards or reces- sion from Earth. Moreover, the brightness of fast-moving stars also depends on their velocity of recession or approach. If this latter parameter is large enough in either direction, then a star would become fainter or even invisible, when the light is shifted entirely out of the visible part of the spec- trum.
eBook - PDF- Vern Ostdiek, Donald Bord(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Vern Ostdiek/University of Michigan Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 224 Chapter 6 Waves and Sound the moving train hears a lower frequency. As a train or a fast car moves by, you hear the sound shift from a higher frequency (pitch) to a lower frequency. The change in the loudness of the sound, which you also hear, is not part of the t t Doppler Effect: it involves a separate process. A similar shift in frequency of sound occurs if you are moving toward a sta u u - a a tionary sound source (Figure 6.19). This Doppler shift happens because the speed of the wave relative to you is higher than that when you are not moving. The wave fronts approach you with a speed equal to the wave speed plus your speed. Because the wavelength is not affected, the equation v 5 f H9261 tells us that the frequency of the wave is increased in proportion to the speed of the wave relative to you. By the same reasoning, when one is moving away from the sound source, the frequency is reduced. The Doppler Effect occurs for both sound and light and is routinely taken into account by astronomers. The frequencies of light emitted by stars that are moving toward or away from Earth are shifted. If the speed of the star is known, the original frequencies of the light can be computed. If the frequencies are known instead, the speed of the star can be computed from the amount of the Doppler shift. Such information is essential for determining the motions of stars in our galaxy or of entire galaxies throughout the universe (see the Astronomy Application at the end of this section). Echolocation is the process of using the waves reflected from an object to determine its location. Radar and sonar are two examples. Basic echolocation uses reflection only: a wave is emitted from a point, reflected by an object of some kind, and detected on its return to the original point.
- Jerry D. Wilson, Anthony J. Buffa, Bo Lou(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
350 College Physics Essentials light is shifted toward the blue (short-wavelength) end of the spectrum, producing a Doppler blue shift. The magnitude of the shift is related to the speed of the source. The Doppler shift of light from astronomical objects is thus a vital tool used by astronomers for measuring speeds (and direction of motion) of various astronomical objects. You may have been subjected to a practical application of the Doppler Effect if you have ever been caught speeding in your car by police radar, which uses reflected radio waves (which are long-wavelength light, not sound). If radio waves are reflected from a parked car, the reflected waves return at the same frequency. But for a car that is moving toward a patrol car, the reflected waves arrive at a higher frequency than the emitted waves. Interestingly enough in this case, there is actually a double Doppler shift: As it intercepts and receives the wave, the moving car acts like a moving observer (the first Doppler shift), but then, in reflecting the wave back, the car acts like a moving source (the second Doppler shift). The mag- nitudes of these combined shifts depend on the speed of the car. Comparing the received frequency to the frequency of the source, a computer can quickly calculate and display this speed for the officer. 14.5.2 Sonic Booms Consider a jet plane that can travel at supersonic speeds. As the speed of a moving source of sound approaches the speed of sound, the waves ahead of the source come close together (▶ Figure 14.10a). When a plane is traveling at the speed of sound, the waves can’t outrun it, and they pile up in front. At supersonic speeds, the waves overlap. This overlapping of a large number of waves forms a large pressure ridge, or shock wave. This kind of wave is sometimes called a bow wave because it is analo- gous to the wave produced by the bow of a boat moving through water at a speed greater than the speed of the water waves.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
As in the case of the moving source, we assume that the air is stationary. The observer moves with a speed υ o (“o” stands for “observer”) toward the stationary source and covers a distance υ o t in a time t. During this time, the mov- ing observer encounters all the condensations that he would if he were stationary, plus an additional number. The additional number of condensations encountered is the distance υ o t divided by the distance λ between successive condensations, or υ o t /λ. Thus, the additional number of condensations encountered per second by the moving 16.9 The Doppler Effect 499 observer is υ o /λ. Since a stationary observer would hear a frequency f s emitted by the source, the moving observer hears a greater frequency f o given by f o = f s + υ o _ = f s ( 1 + υ o ___ f s λ ) Using the fact that υ = f s λ, where υ is the speed of sound, we find that Observer moving toward stationary source f o = f s( 1 + υ o _ υ ) (16.13) An observer moving away from a stationary source moves in the same direction as the sound wave and, as a result, intercepts fewer condensations per second than a sta- tionary observer does. In this case, the moving observer hears a smaller frequency f o that is given by Observer moving away from stationary source f o = f s ( 1 − υ o _ υ ) (16.14) It should be noted that the physical mechanism producing the Doppler Effect is different when the source moves and the observer is stationary from when the observer moves and the source is stationary. When the source moves, as in Figure 16.28b, the wavelength of the sound perceived by the observer changes from λ to λʹ. When the wavelength changes, the stationary observer hears a different fre- quency f o than the frequency produced by the source. On the other hand, when the observer moves and the source is stationary, the wavelength λ does not change (see, for example, Figure 16.29).
eBook - PDF- T. M. Helliwell, V. V. Sahakian(Authors)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
(2.130) is taken to be positive and it is assumed that the beam does arrive at O . In short, this applies when the distance between the two observers is shrinking. The frequency received is greater than the frequency emitted, ν > ν , known as a blueshift, for obvious reasons. To see the other possibility – i.e., Oand O moving away from each other – we can simply flip the sign of β in this expression: ν ν = 1 − β 1 + β < 1 receding observers. (2.134) Now the distance between the two observers is increasing, and we find that the received frequency is less than the emitted one: we say there has been a redshift . Doppler shifts for light are an extremely useful tool in physics and technology, from determining the speeds of stars in distant galaxies leading to Hubble’s discovery of the expanding universe, to the use of frequency shifts in the GPS for location and navigation. If such special-relativistic effects were not included in GPS navigation, there would be large errors in position measurements. Interestingly, it even turns out to be equally essential for GPS to include additional effects due to earth’s gravity, as contained in Einstein’s general theory of relativity. 98 2 Relativity Minkowski Diagrams A particularly useful way to depict relativistic dynamics involves a visual tool called a Minkowski diagram. Simply put, it is a plot of the trajectory of a particle on a graph where the horizontal axis is one of the spatial directions and the vertical axis represents time, or actually the product c t. Figure 2.8 shows an example. Light cone Particle trajectory An event Light ray Light ray Fig. 2.8 A point on a Minkowski diagram represents an event. A particle’s trajectory appears as a curve with a slope that exceeds unity everywhere. The trajectory of the particle appears as a line moving upward, forward in time. It is sometimes referred to as the worldline of the particle. Light rays appear on such a diagram as straight lines at 45 ◦ , as shown in the figure.
eBook - PDFNumerical Simulations
Applications, Examples and Theory
- Lutz Angermann(Author)
- 2011(Publication Date)
- IntechOpen(Publisher)
For clarity, all photon paths were rotated afterwards as if the photons had emerged on the +X-axis. Photoacoustics has been added to simulate the acoustic response to pulsed light. Frequency modulation is a modality adding extra information using path length-dependent phase delay information. Here we only deal shortly with LDF. As mentioned previously, LDF makes use of the Doppler Effect encountered with scattering of photons in particles when those particles are moving. The principles are shown in Figure 7. Using the definitions of the variables given in that Figure, the Doppler frequency is given by: ( ) 0 D s ω = − • k k v , (10) and with: 1 2 2 .sin k δ θ = k , (11) we find: 1 2 sin .cos D kv f θ α π = . (12) When applied to tissue, frequently the angles θ and α might be considered randomised. This is due to three reasons: -Preceding scattering by non-moving particles might cause the direction of the photons to be randomised upon encountering moving particles Numerical Simulations - Applications, Examples and Theory 160 Fig. 7. Principles of LDF. The particle has a velocity v . Vectors k 0 and k s denote the incoming and scattered light wave vectors, and δ k is the difference vector. -Most important moving particles are blood cells in capillaries. Due to the (more or less) random orientation of the capillaries the velocities will have random directions -Travelling from injection point to detection point, in general the photons will encounter many Doppler scattering effects, with random velocities and orientations All three effects will broaden the Doppler frequency distribution, which ideally would consist of one single peak, to a smooth distribution as in Figure 8. This means that it is not possible to measure the local velocity, but we only may extract information about the averaged velocity over the measuring volume. The averaging concerns the three effects mentioned above.
eBook - ePub- Jim Al-Khalili(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
In 1907, A. A. Michelson became the first American to win the Nobel Prize in physics for an experiment he had carried out in 1887 together with E. W. Morley. It is probably the most famous experiment in the whole of physics. Michelson had invented a device known as an interferometer, which relies on the wave nature of light to measure the time it takes for a light beam to cover a fixed distance. But no one could anticipate just how crucial the results of this experiment would turn out to be. To understand its significance, we need to look at little more carefully at the way waves move.An important fact in physics is that all waves travel at a speed that does not depend on the speed of the source of the waves. Think of the sound of an approaching fast car. The sound waves will reach your ear before the car since they are travelling faster, but their speed has to do with how quickly the vibrating air molecules can transmit them. They do not reach you any quicker by virtue of being ‘pushed’ along by the moving car. What happens instead is that the waves get squashed up to higher frequency and shorter wavelength in front of the car (the Doppler Effect) but the speed of the sound itself doesn’t changed.†Sounds waves travel through air at a speed of 1200 km/hour. This speed is independent of how fast the car is moving. If the car is travelling at 100 km/hour, then the driver would see the sound waves (assuming sound waves could be seen) moving ahead of him at a speed of only 1100 km/hour (1200 minus 100). The faster the car goes the slower the relative speed of the sound waves that the driver sees. But to a stationary observer watching the approaching car, the sound waves always travel at 1200 km/hour no matter how fast the car is moving. If the driver and the stationary observer had an argument about the speed of sound, the driver would have to admit that the speed he observes the waves moving is not their true speed because he too is moving relative to the air molecules.Michelson and Morley applied this principle to light waves. They assumed that the Earth is moving through the ether as it orbits the Sun (about a 100,000 km/hour). Their experiment is a little tricky to describe, so I will not go into the finer details. Suffice it to say that they measured with very high accuracy the time it took light in a laboratory to travel along two paths of equal distance, one in the direction of the Earth’s motion as it orbited the Sun and the other at right angles to it. Sitting in their laboratory on Earth and observing the speed of light, they were like the car driver who would measure the sound waves leaving the car at different speeds depending on what direction he looked. After all, to him the sound waves that were travelling straight upwards would still be moving at 1200 km/hour.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
