Nhân tiện giới thiệu hiêu ứng Doppler với bà con, 1 thuật ngữ khá quen thuộc, bài viết từ wikipedia
The Doppler effect (or Doppler shift), named after 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 the waves. It is commonly heard when a vehicle sounding a siren (còi xe) approaches, passes and recedes (nhỏ dần) from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.
For waves that propagate (truyền) in a medium (trung gian), such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered.
Applications
A stationary microphone records moving police sirens at different pitches (cao độ) depending on their relative direction.
Sirens
The
siren on a passing
emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer (nhà thiên văn học)
John Dobson explained the effect thus:
- "The reason the siren slides is because it doesn't hit you."
In other words, if the siren approached the observer directly, the pitch would remain constant (as
vs, r is only the radial component) until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial velocity does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity:
where
vs is the velocity of the object (source of waves) with respect to the medium, and θ is the angle between the object's forward velocity and the line of sight from the object to the observer.
Astronomy
Redshift of
spectral lines in the
optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left).
The Doppler effect for
electromagnetic waves such as light is of great use in
astronomy and results in either a so-called
redshift or
blue shift. It has been used to measure the speed at which
stars and
galaxies are approaching or receding from us, that is, the
radial velocity. This is used to detect if an apparently single star is, in reality, a close
binary and even to measure the rotational speed of stars and galaxies.
The use of the Doppler effect for light in
astronomy depends on our knowledge that the
spectra of stars are not continuous. They exhibit
absorption lines at well defined frequencies that are correlated with the energies required to excite
electrons in various
elements from one level to another. The Doppler effect is recognizable in the fact that the absorption lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blue shift and those of a receding astronomical light source exhibit a redshift.
Among the
nearby stars, the largest radial velocities with respect to the
Sun are +308 km/s (
BD-15°4041, also known as LHS 52, 81.7 light-years away) and -260 km/s (
Woolley 9722, also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial velocity means the star is receding from the Sun, negative that it is approaching.
Temperature measurement
Another use of the Doppler effect, which is found mostly in plasma physics and astronomy, is the estimation of the temperature of a gas (or ion temperature in a plasma) which is emitting a
spectral line. Due to the thermal motion of the emitters, the light emitted by each particle can be slightly red- or blue-shifted, and the net effect is a broadening of the line. This line shape is called a
Doppler profile and the width of the line is proportional to the square root of the temperature of the emitting species, allowing a spectral line (with the width dominated by the Doppler broadening) to be used to infer the temperature.
Radar
Main article:
Doppler radar
The Doppler effect is used in some types of
radar, to measure the velocity of detected objects. A radar beam is fired at a moving target — e.g. a motor car, as police use radar to detect speeding motorists — as it approaches or recedes from the radar source. Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gap between each wave increases, increasing the wavelength. In some situations, the radar beam is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength. In either situation, calculations from the Doppler effect accurately determine the car's velocity. Moreover, the
proximity fuze, developed during World War II, relies upon Doppler radar to explode at the correct time, height, distance, etc.
Medical imaging and blood flow measurement
An
echocardiogram (siêu âm tim) can, within certain limits, produce accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the
ultrasound beam should be as parallel to the blood flow as possible. Velocity measurements allow assessment of cardiac valve (van tim) areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves (valvular regurgitation), and calculation of the
cardiac output.
Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.
Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (
when the received signal arrives).
Velocity measurements of blood flow are also used in other fields of
medical ultrasonography, such as
obstetric ultrasonography and
neurology. Velocity measurement of blood flow in arteries and veins based on Doppler effect is an effective tool for diagnosis of vascular (mạch) problems like stenosis (chứng hẹp mạch máu).
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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 an ultrasonic acoustic burst (vỡ thính giác !!! not sure), and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow is computed as a function of the water velocity and face. This technique allows non-intrusive flow measurements, at high precision and high frequency.
Velocity profile measurement
Developed originally for velocity measurements in medical applications (blood flows), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions. Flows can be pulsating (rung động), oscillating(dao động), laminar (phiến mỏng) or turbulent (hỗn loạn), stationary or transient (tạm thời). This technique is fully non-invasive(không lan tràn).
Underwater acoustics
In military applications the Doppler shift of a target is used to ascertain the speed of a
submarine using both passive and active
sonar systems. As a submarine passes by a passive
sonobuoy, the stable frequencies undergo a Doppler shift, and the speed and range from the sonobuoy can be calculated. If the sonar system is mounted on a moving ship or another submarine, then the relative
velocity can be calculated.
Audio
The
Leslie speaker, associated with and predominantly used with the
Hammond B-3 Organ, takes advantage of the Doppler Effect by using an
electric motor to rotate a horn around a speaker continuously, rapidly alternating the received frequency of a keyboard note.
Vibration Measurement
A
Laser Doppler Vibrometer (LDV) is a non-contact method for measuring vibration. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude (biên độ sóng)and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface.
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