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While i was in high-school i learn't the Doppler's Effect which if i remember correctly is:

  • The Apparent change in the frequency of sound caused due the relative motion between the sound and the observer.

This phenomenon seems obvious, but what i would like to know is, what use does Doppler Effect have in real life. Why is it useful?

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Your definition of Doppler effect is a bit too narrow : it applies to any wave and not only to the sound. –  Frédéric Grosshans Nov 25 '10 at 19:08

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It can be used to measure speed - that's how police radar guns and speed cameras work. Radar waves from the gun/ camera are reflected off the moving vehicle, and the wavelength is shifted according to the speed of the vehicle relative to the gun/ camera.

In astrophysics, looking at light from distant galaxies, we notice that certain characteristics of the light are shifted in wavelength due to the Doppler effect. This is known as red-shift, as we notice the light is mostly shifted to the longer-wavelength (red) end of the spectrum. This tells us that distant galaxies are moving away from us, which is the primary piece of information that led to the development of the Big Bang theory.

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The main "uses" of the Doppler effect are, as other people have already mentioned, in the measurement of velocities. In addition to police radar, it's used in weather radar. That's why local news stations (at least in the US) are forever touting their "Doppler N" (for large values of N) weather system-- those systems use the Doppler effect to measure the speed of water droplets in clouds, which provides additional information about their composition and nature.

I can also confirm that they use the Doppler effect in ultrasound imaging of the heart, to measure and map out the flow of blood in different regions. Before my daughter was born, they thought she might be at risk for heart problems, so we had a pediatric cardiologist look at it. They have a nifty color-coded display showing blood moving toward the imager as blue, and away from it as red. (That was the second-best part of the whole thing, after the doctor giving her a clean bill of health...)

My own research area is in laser cooling, where we use the Doppler effect to ensure that atoms only interact with light when they're moving toward the laser. If you tune the laser frequency slightly below the frequency the atoms want to absorb, a stationary atom will not interact with the laser, but an atom moving toward the laser will see the frequency shifted up, closer to the resonant frequency for the atom, and will be more likely to absorb light. When an atom absorbs a photon of light, it gets a small "kick" in the direction the photon was headed. If the atom is moving in the opposite direction to the photon (that is, toward the laser), then the "kick" will act to slow the atom down. With the proper arrangement of lasers, you can slow atoms down in any direction they try to move, thus reducing their overall velocity. Since temperature is a measure of the average kinetic energy of the atoms making up a sample, slower velocities mean lower temperatures. You can easily reach microkelvin temperatures this way-- a few millionths of a degree (C) above absolute zero.

There are a lot of complications along the way-- the Doppler effect is both the key to laser cooling and a problem that needs to be overcome in slowing atoms from room temperature-- but that's the basic idea. There's more information, and nifty Java applet video games, at the Physics 2000 site at the University of Colorado.

Laser cooled atoms are the starting point for all manner of experiments in atomic and molecular physics, and quantum optics. The most important technological application at the moment is in the area of atomic clocks, with the best cesium atomic clocks in the world using laser cooled atoms. At various times, there have been suggestions to put atomic clocks in space, potentially as a next generation GPS system, which would improve the already impressive precision of GPS navigation. I'm not sure what the current status of those ideas is, though.

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I haven't worked with it myself, but an important application of the Doppler effect seems to be to use ultrasound to detect the flow direction of blood. Apparently you can do some really awesome heart diagnostics stuff by using Doppler ultrasound to map flow around valves and the such.

Oh, and many weather radar systems use the Doppler effect to determine the movement of weather systems (actually precipitation patterns, I guess?).

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Yes, and Doppler sonography is also used for imaging of other organs to determine the various blood flows into and out of them, not just the heart: brain, liver, kidney, really pretty much anything. It's a fantastic technology that has saved or improved many lives by eliminating the need for exploratory surgery, and allowing necessary surgeries to be much more targeted. –  Mark Beadles Jan 15 '12 at 18:48

I haven't used it much, but you can use it in games development to implement 3D sound.

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Sounds interesting. Do you have any references for that? –  student May 12 '11 at 20:45
    
@student I will dig some out, I have my notes from University on it but I can have a look for some stuff similar online... –  ing0 May 13 '11 at 14:57
    
here is an example from Microsoft of the use of Doppler pitch shifting to simulate objects moving toward or away from the player. –  Mark Beadles Jan 15 '12 at 18:59

Others have answered about real-life applications of Doppler effect, but you have many others. For example, in quantum optics, it is at the heart of Doppler cooling, which is used to cool down some dilute atom clouds down to a few hundred microkelvins, and then use them for some fundamental physics experiment. If you really want to have a practical application, then this technique is used since the 1990's to improve the precision of atomic clocks, because the movement of the atoms induce a Doppler-shift of the frequency used as reference for this clocks. It's a way to fight Doppler-effect with Doppler-effect ;-)

The idea is to use the Doppler shift to create on each a force which is proportional to its speed of each atom in order to slow it down. This cools the atomic cloud down because the temperature is proportional to the average kinetic energy. This kind of speed dependence is achieved using detuned lasers.

If you shine a laser to an atom with a frequency tuned to an electronic transition, the atom will absorb photons from the laser beam and feel a strong force. If the laser is detuned towards the red, it can be "retuned" through a Doppler shift when the atom moves towards the beam. As a consequence, the more speed the atom has towards the laser, the more it "feels" the laser, and the stronger the force is. Putting 2 lasers in each direction of the space (i.e. six laser) allows then, in the volume where the laser overlap, to create a force proportional to the speed of the atom. Every atom which enters this zone is slowed down to a very small speed, hence the name "optical molasse" given to these set-ups.

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there is also 'Doppler effect' ecography in hospitals , but i do not know how it uses Doppler effect but here is a sanitary application of Doppler effect.

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Chronic cerebrospinal venous insufficiency (CCSVI), or the pathological restriction of venous vessel discharge from the CNS has been proposed by Zamboni, et al, as having a correlative relationship to Multiple Sclerosis. From a clinical perspective, it has been demonstrated that the narrowed jugular veins in an MS patient, once widened, do affect the presenting symptoms of MS and the overall health of the patient. It has also been noted that these same veins once treated, restenose after a time in the majority of cases. Why the veins restenose is speculative. One insight, developed through practical observation, suggests that there are gaps in the therapy protocol as it is currently practiced. In general, CCSVI therapy has focused on directly treating the venous system and the stenosed veins. Several other factors that would naturally affect vein recovery have received much less consideration. As to treatment for CCSVI, it should be noted that no meaningful aftercare protocol based on evidence has been considered by the main proponents of the ‘liberation’ therapy (neck venoplasty). In fact, in all of the clinics or hospitals examined for this study, patients weren’t required to stay in the clinical setting any longer than a few hours post-procedure in most cases. Even though it has been observed to be therapeutically useful by some of the main early practitioners of the ‘liberation’ therapy, follow-up, supportive care for recovering patients post-operatively has not seriously been considered to be part of the treatment protocol. To date, follow-up care has primarily centered on when vein re-imaging should be done post-venoplasty. The fact is, by that time, most patients have restenosed (or partially restenosed) and the follow-up Doppler testing is simply detecting restenosis and retrograde flow in veins that are very much deteriorated due to scarring left by the initial procedure. This article discusses a variable approach as to a combination of safe and effective interventional therapies that have been observed to result in enduring venous drainage of the CNS to offset the destructive effects of inflammation and neurodegeneration, and to regenerate disease damaged tissue.

As stated, it has been observed that a number of presenting symptoms of MS almost completely vanish as soon as the jugulars are widened and the flows equalize in most MS patients. Where a small number of MS patients have received no immediate benefit from the ‘liberation’ procedure, flows in subject samples have been shown not to have equalized post-procedure in these patients and therefore even a very small retrograde blood flow back to the CNS can offset the therapeutic benefits. Furthermore once the obstructed veins are further examined for hemodynamic obstruction and widened at the point of occlusion in those patients to allow full drainage, the presenting symptoms of MS retreat. This noted observation along with the large number of MS patients who have CCSVI establish a clear association of vein disease with MS, although it is clearly not the disease ‘trigger’.For more information please visit this link.

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To my knowledge,following are some uses of Doppler effect:

  1. Measurement of the speed of approaching automobiles.

  2. Ships are equipped with an instrument called SONAR which works on the principle of Doppler effect. It detects the pressence of under water rocks and submarines...

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