The Doppler effect (or the Doppler shift) is the change in frequency or wavelength of a wave for an observer who is moving relative to the wave source.
 It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
The photoacoustic Doppler effect, as its name implies, is one specific kind of Doppler effect, which occurs when an intensely modulated light wave induces a photoacoustic wave on moving particles with a specific frequency.
 The observed frequency shift is a good indicator of the velocity of the illuminated moving particles.
 A potential biomedical application is measuring blood flow.
In experimental atomic physics, saturated absorption spectroscopy or Doppler-free spectroscopy is a set-up that enables the precise determination of the transition frequency of an atom between its ground state and an optically excited state.
 The accuracy to which these frequencies can be determined is, ideally, limited only by the width of the excited state, which is the inverse of the lifetime of this state.
 However, the samples of atomic gas that are used for that purpose are generally at room temperature, where the measured frequency distribution is highly broadened due to the Doppler effect.
 Saturated absorption spectroscopy allows precise spectroscopy of the atomic levels without having to cool the sample down to temperatures at which the Doppler broadening is no longer relevant (which would be on the order of a few millikelvins).
 It is also used to lock the frequency of a laser to the precise wavelength of an atomic transmission in atomic physics experiments.
According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field.
 As a result of the nature of spacetime, a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer's own frame of reference.
 A clock that is under the influence of a stronger gravitational field than an observer's will also be measured to tick slower than the observer's own clock.
A carotid Doppler machine is a device which can be used to measure blood flow velocities within the cervical carotid arteries, as well as the vertebral arteries and sometimes the subclavian arteries by means of non-invasive ultrasonic scanning in which the Doppler effect is utilized.
 This can be useful for estimating the effects of atherosclerotic disease on the lumen in late stages; specifically the level of compromise in blood flow caused by reduction in arterial lumen diameter.
The relativistic Doppler effect is the change in frequency (and wavelength) of light, caused by the relative motion of the source and the observer (as in the classical Doppler effect), when taking into account effects described by the special theory of relativity.
In the studies of Fourier optics, sound synthesis, stellar interferometry, optical tweezers, and diffractive optical elements (DOEs) it is often important to know the spatial frequency phase of an observed wave source.
 In order to reconstruct this phase the Adaptive-Additive Algorithm (or AA algorithm), which derives from a group of adaptive (input-output) algorithms, can be used.
 The AA algorithm is an iterative algorithm that utilizes the Fourier Transform to calculate an unknown part of a propagating wave, normally the spatial frequency phase (k space).
 This can be done when given the phase’s known counterparts, usually an observed amplitude (position space) and an assumed starting amplitude (k space).
 To find the correct phase the algorithm uses error conversion, or the error between the desired and the theoretical intensities.
 The AA algorithm is currently being implemented by Dr. Wendell Hill III, Alex Robel, V. Kotlyar Soifer, and David G. Grier.
Christian Andreas Doppler ( ; ] ; 29 November 1803 – 17 March 1853) was an Austrian mathematician and physicist.
 He is celebrated for his principle — known as the Doppler effect — that the observed frequency of a wave depends on the relative speed of the source and the observer.
 He used this concept to explain the color of binary stars.
Tissue Doppler echocardiography (Also called Tissue Doppler Imaging - TDI or Tissue Velocity Imaging - TVI) is a Medical ultrasound technology mainly used in Echocardiography that measures the velocity of the heart muscle or myocardium through the phases of one or more heartbeats by the Doppler effect (frequency shift) of the reflected ultrasound.
 The technique is the same as for Doppler echocardiography, measuring flow velocities.
 Tissue signals however, have higher amplitude and lower velocities, and the signals are extracted by using different filter and gain settings.
 Like Doppler flow, Tissue Doppler can be acquired both by spectral analysis (Spectral density estimation) as pulsed Doppler and by the Autocorrelation technique as colour Tissue Doppler (Duplex ultrasonography).
 While pulsed Doppler only acquires the velocity at one point at a time, colour Doppler can acquire simultaneous pixel velocity values across the whole imaging field.
 Pulsed Doppler on the other hand, is more robust against noise, as peak values are measured on top of the spectrum, and are unaffected of the presence of clutter (stationary reverberation noise).
Transcranial Doppler (TCD) and transcranial color Doppler (TCCD) are types of Doppler ultrasonography that measure the velocity of blood flow through the brain's blood vessels by measuring the echoes of ultrasound waves moving transcranially (through the cranium).
 These modes of medical imaging conduct a spectral analysis of the acoustic signals they receive and can therefore be classified as methods of active acoustocerebrography.
 They are used as tests to help diagnose emboli, stenosis, vasospasm from a subarachnoid hemorrhage (bleeding from a ruptured aneurysm), and other problems.
 These relatively quick and inexpensive tests are growing in popularity.
 The tests are effective for detecting sickle cell disease, ischemic cerebrovascular disease, subarachnoid hemorrhage, arteriovenous malformations, and cerebral circulatory arrest.
 The tests are possibly useful for perioperative monitoring and meningeal infection.
 The equipment used for these tests is becoming increasingly portable, making it possible for a clinician to travel to a hospital, to a doctor's office, or to a nursing home for both inpatient and outpatient studies.
 The tests are often used in conjunction with other tests such as MRI, MRA, carotid duplex ultrasound and CT scans.
 The tests are also used for research in cognitive neuroscience (see Functional transcranial Doppler, below).

Question: A carotid Doppler machine uses what change in frequency or wavelength of a wave for an observer who is moving relative to the wave source?
Answer: Doppler effect