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Analysis of the Doppler Effect

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Assignment 3: topics and themes in physics- oscillations and waves

Doppler radar

A Doppler radar is specialized radar that uses the Doppler Effect to produce data about an objects velocity at a given distance. Doppler radar works by bouncing microwave signals at the desired target and then listening to its reflection, then the returning frequency is analysed to see how it has changed from the original signal. This variation gives direct and highly accurate measurements of the radial component of a target's velocity relative to the radar.

http://www.qrg.northwestern.edu/projects/vss/docs/media/Communications/doppler.gif

The Doppler effect (or Doppler shift), named after the famed Austrian physicist Christian Doppler who proposed it in 1842 and is the difference between the observed frequency and the emitted 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 approaches, passes and dulls from an observer perspective.

When a wave with angular frequency ω and phase velocity c propagates in a medium, an observer moving with velocity v parallel to c experiences a shifted frequency ω1 = ω (1 − v/c) (1). They do not depend on the relative velocity as other circumstances do occur. Electromagnetic waves share with sound the same property which is propagation velocity. It is independent of the motion of the source . Einstein formulated his extended principle of relativity stated that all physical and electromagnetic forces should depend on relative velocities. From this he created Lorentz transformation.

Sound vs. Light

There are three differences between acoustical (sound) and optical (light) Doppler effects: The optical frequency change is not dependent on which is moving -- the source or observer -- either is it affected by the medium through which the waves are moving, but acoustical frequency is affected by these things. Optical frequency changes are affected if the source or observer moves at right angles to the line connecting the source and observer. Observed acoustical changes are not affected in such a situation. Applications of the Doppler phenomenon include the Doppler radar and the measurement by astronomers of the motion and direction of celestial bodies.

Light itself travels at 186,000miles a second (, but unlike sound light doesn’t need to travel through a medium. Whereas sound cannot be transmitted into space light can be in the form of radiation and also a form of energy that is put through a vacuum. The Doppler effect of light is more commonly known as the relativistic Doppler Effect.

The Doppler Effect In Astronomy

In astronomy, the Doppler Effect was originally studied in the visible part of the electromagnetic spectrum. Today we can the see the use of the Doppler shift(Doppler effect) applies to all parts of the electromagnetic spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is red shifted when its wavelength increases, and is blue shifted when its wavelength decreases.

Astronomers use Doppler shifts to calculate very accurately at what rate stars and other astronomical objects are moving towards or away from Earth. For example the spectral lines emitted by hydrogen gas in distant galaxies is often observed to be considerably red shifted. The spectral line emission, normally found at a wavelength of 21 centimetres on Earth, might be observed at 21.1 centimetres instead. This 0.1 centimetre redshift would indicate that the gas is moving away from Earth at over 1,400 kilometres per second (over 880 miles per second).

The red shift

As an object moves further away from an observer the light waves emitted are affected by the Doppler Effect.

In 1923, American astronomer and physicist Edwin Hubble (1889-1953) observed that the light waves from distant galaxies were shifted so much to the red end of the light spectrum that they must be moving away from the Milky Way, the galaxy in which Earth is located, at a high rate.(10)

After these finding he then furthered his studies and came up with a mathematical formula in which he could determine how far away and at what rate they were moving away from the galaxy and how far towards the red shift. This formula is known as Hubble’s constant. From this a lot of other astronomers have come together and came up with the theory that the universe emerged instantly in a sort of bang, therefore coming amount the big bang theory.

spectrum of light with black verticle lines My research of galaxies the sun itself contains helium. This is proven as on the spectrum scale there are black lines in which the sun has absorbed light through the helium

The Doppler Effect in aerospace

http://buphy.bu.edu/~duffy/PY105/34d.GIFSonic booms, usually produced by airplanes passing through the sound barrier, are another example of the Doppler Effect. As a plane approaches the sound barrier the sound waves become increasingly compressed at the front of the plane. Pilots have also reported that they feel noticeable wall or barrier as they approach the speed of sound, and this is due to this intense compression of the sound waves. When the plane reaches the speed of sound, and passes through, it is said to go supersonic. There have been high speed photos taken of high velocity objects such as planes and bullets approaching, then breaking through the sound barrier. In these, the compression of the sound waves are clearly shown in the front, with the concomitant lengthening at the rear, just as would be expected by the Doppler Effect.

http://www.animations.physics.unsw.edu.au/jw/images/jet_cone.jpg

Doppler Navigation System uses the Doppler effect to measure an aircraft's ground speed and heading. The Doppler radar functions by continuous measurement of Doppler shift and converting the measured values to groundspeed and drift angle. In early systems the aircraft's departure point was loaded into a navigation computer, which then converted the aircraft's heading and Doppler ground speed/drift inputs into a continuous display of aircraft position; this was then displayed as latitude and longitude, and/ or as distance to go along track and position left or right of track, in nautical miles. Aircraft navigation systems such as these are very important because they are key in getting the aircraft from one location to another safely. Doppler navigation is also one of the systems that air traffic controllers use, its allows then to see an aircraft’s exact heading, air speed and altitude. This allows the aircraft controller to give keep aircraft in clear and uncongested air which significantly reduces the chances of in air collisions.

DOPPLER RADAR

The police also utilise the Doppler Effect in the form of Doppler radar to calculate the speed of passing car and to check whether they are obeying the speed limits. This technology is not only used by the police but also by meteorologists.

The change in frequency experienced as a result of the Doppler effect is exactly twice the ratio between the velocity of the target (for instance, a speeding car) and the speed with which the radar pulse is directed toward the target. From this formula, it is possible to determine the velocity of the target when the frequency change and speed of radar propagation are known. The police officer's Doppler radar performs these calculations; then all the officer has to do is pull over the speeder and write a ticket.

The development in Doppler radar has also helped to helped to eliminate aviation crashes associated with microburst. A microburst is a very localized column of sinking air caused by a small and intense downdraft within a thunderstorm. The crash Delta Air Lines Flight 191 , which a microburst was the primary cause for the crash of the Lockheed L1011 Tri-Star was key factor in the push for developing microburst detection system. As a result of the crash, planes are now fitted with Doppler radar as standard. It allows pilots to prepare for microburst and allows then to increase power to the engines to stop the plane from crashing. The Doppler radar system of the plane is usually located in the planes ray dome.

Meteorology

Meteorologists use Doppler radar to track the movement of storm by detecting the direction and velocity of raindrops or hail, for instance, Doppler radar can be used to determine the motion of winds and, thus, to predict weather patterns that will follow in the next minutes or hours. But Doppler radar can do more than simply detect a storm in progress: Doppler technology also aids meteorologists by interpreting wind direction, as an indicator of incoming storms.

Meteorologists use a similar principle to read weather events. In this case, the stationary transmitter is located in a weather station and the moving object being studied is a storm system. This is what happens:

  1. Radio waves are emitted from a weather station at very specific frequency.
  2. The waves are large enough to interact with clouds and other atmospheric objects. The waves strike objects and bounce back toward the station.
  3. If the clouds or precipitation are moving away from the station, the frequency of the waves reflected back decreases. If the clouds or precipitation are moving toward the station, the frequency of the waves reflected back increases.
  4. Computers in the radar electronically convert Doppler shift data about the reflected radio waves into more useful pictures which show wind speeds and direction.

Doppler radars are also a lot more sensitive to movement of targets in general, whether they are moving towards or away from the radar site which things such as birds, insects, or just clouds when they are operated in certain modes. Doppler radars measure a target's velocity, which both is the speed of movement & the direction that it's moving. These types of radars allow meteorologists to see a lot of the in detail movements that go on inside thunderstorms, this then allows the meteorologists to predict how those thunderstorms will behave in the future.

Doppler Echocardiogram

A traditional echocardiogram uses sound waves to produce fairly accurate images of the heart. In this procedure, a radiologist uses a transducer to transmit and receive ultrasound waves, which are reflected when they reach the edge of two structures with different densities. The image produced by an echocardiogram shows the edges of heart structures, but the down side it cannot measure the speed of blood flow as it makes its way through the heart. In a Doppler echocardiogram, sound waves of a certain frequency are transmitted into the heart. The sound waves bounce off blood cells moving through the heart and blood vessels. The movement of these cells, either toward or away from the transmitted waves which then results in a frequency shift that can be measured. This helps cardiologists determine the speed and direction of blood flow in the heart which is the most significant difference between the traditional echocardiogram and a Doppler echocardiogram.

http://www.vetmed.ucdavis.edu/vmth/small_animal/cardio_kittleson/cases/Echocardiograms/Echo%20I/images/388154PS6.jpg

Transverse and longitudinal waves

A transverse wave is a moving wave that consists of oscillations occurring at right angles to the direction of energy transfer. If a transverse wave is moving in the positive x-direction, its oscillations are in up and down directions that lie in the y–z plane. For transverse waves in matter the displacement of the medium is at 90 degrees to the direction of propagation of the wave. A ripple in a pond and a wave on a string are easily visualized forms of transverse waves. Examples of transverse waves include S (secondary) waves, and the motion of the electric (E) and magnetic (M) fields in an electromagnetic plane wave, which both oscillate perpendicularly to each other as well as to the direction of energy transfer.

http://static.ddmcdn.com/gif/noise-canceling-headphone-7.jpg

Conclusion

In conclusion this reports through my many hours of research, talks in detail about the history and the useful applications of it. Aircraft and aerospace engineering as they use Doppler radar is a very interesting and a wide range of applications and I have toughly enjoyed researching about it especially since these are topic areas which i am particularly interested and other practical uses with the speed guns used in the police force and also the sirens on the motor vehicles. The application of the Doppler Effect and its applications in aerospace are a very interesting subject and something which i very much look forward to looking and working on in am going into university.


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