Doppler effect Doppler effect Doppler effect of water waves Doppler effect is named in honor of the Austrian physicist and mathematician Christian Johann Doppler (Christian Johann Doppler), who first proposed this theory in 1842. The main idea is that the wavelength of an object's radiation varies because of the relative motion of the source and the observer. In front of the moving source, the wave is compressed, the wavelength becomes shorter and the frequency becomes higher (blue shift blue shift); when moving behind the source, the opposite effect occurs. When the motion is behind the source, the opposite effect occurs: the wavelength becomes longer and the frequency becomes lower (red shift). The higher the speed of the source, the greater the effect. Depending on the degree of red (blue) shift of the light waves, the speed of the source in the direction of observation can be calculated. The shifts in the spectral lines of a star show how fast the star is moving in the direction of observation. Unless the speed of the wave source is very close to the speed of light, the degree of Doppler shift is generally small. The Doppler effect is present in all fluctuating phenomena.
[edit]Discovery of the Doppler effect
In 1842 an Austrian mathematician and physicist named Doppler. One day, he was passing the Doppler effect 1 railroad crossings, coincided with a train from his side, he found that the train from far and near when the whistle becomes louder, the pitch becomes sharp, and the train from near and far when the whistle becomes weaker, the pitch becomes lower. He was greatly interested in this physical phenomenon and studied it. It was found that this was due to the relative motion between the source and the observer, so that the frequency of the sound heard by the observer was different from the frequency of the source. This is the phenomenon of frequency shift. Because, when the sound source is in motion relative to the observer, the sound heard by the observer changes. When the source is away from the observer, the wavelength of the sound wave increases, the tone becomes low, when the source is close to the observer, the wavelength of the sound wave decreases, the tone becomes high. The change in pitch is related to the ratio of the relative velocity between the source and the observer to the speed of sound. The larger the ratio, the more significant the change, which is later called the "Doppler effect". Doppler effect The Doppler effect states that waves are received at a higher frequency when the source moves toward the observer, and at a lower frequency when the source moves away from the observer. The same conclusion can be obtained when the observer moves. However, due to the lack of experimental equipment, Doppler did not experimentally verify the effect at that time. A few years later, a group of trumpeters was asked to play on a flatbed truck, and then trained musicians were asked to recognize the change in pitch by ear in order to verify the effect. Assuming the wavelength of the original wave source is λ, the speed of the wave is c, and the speed of the observer is v: when the observer approaches the source the frequency of the source is (c+v)/λ, and if the observer moves away from the source the frequency of the source is (c-v)/λ. An example often used is the sound of a train whistle, which is more piercing than usual when the train approaches the observer. You can hear the change in harshness as the train passes. The same is true for the sirens of police cars and the engines of race cars. If you think of sound waves as pulses emitted at regular intervals, imagine that if you emit a pulse with every step you take, then every pulse before you is closer to you than if you were standing still. The sound source behind you is one step farther away than when you were standing still. Or, the frequency of the pulse before you is higher than usual, and the frequency of the pulse after you is lower than usual. The Doppler effect doesn't just apply to sound waves, it applies to all types of waves, including electromagnetic waves. Scientist Edwin Hubble used the Doppler effect to conclude that the universe is expanding. He found that the light emitted by objects moving away from the Milky Way becomes less frequent, i.e., it moves towards the red end of the spectrum, known as the redshift, and the faster the objects leave the Milky Way the greater the redshift, which indicates that these objects are moving away from the Milky Way. Conversely, if the objects are moving toward the Milky Way, the light will be blue-shifted. In mobile communication, when the mobile station moves towards the base station, the frequency becomes higher, and when it moves away from the base station, the Doppler effect2 frequency becomes lower. Of course, due to the limitation of our moving speed in our daily life, it is impossible to bring very large frequency shift, but this undeniably will bring impact on mobile communication, in order to avoid this impact causing problems in our communication, we have to take various considerations in technology. It also increases the complexity of mobile communication. In the case of monochrome, the color perceived by our eyes can be interpreted as the frequency of the vibration of the light waves, or as the number of times that the electromagnetic field alternates in 1 second. In the visible region, the less efficient this is, the more it tends to be red, and the higher the frequency, the more it tends to be blue - violet. For example, the bright red color produced by a helium-neon laser corresponds to a frequency of 4.74 x 10^14 Hz, while the violet color of a mercury lamp corresponds to a frequency of 7 x 10^14 Hz or more. The same principle applies to sound waves: the perception of the highs and lows of a sound corresponds to the frequency of vibration at which the sound exerts pressure on the tympanic membrane of the ear (high-frequency sounds are sharp, low-frequency sounds are muffled). If the source of the wave is stationary, the vibration of the wave received by the immobile receiver is the same as the rhythm of the wave emitted by the source: the emitted frequency is equal to the received frequency. If the wave source is moving relative to the receiver, e.g. away from each other, then the situation is different. The distance between the two peaks produced by the source is lengthened with respect to the receiver, and therefore the time taken for the two upper peaks to reach the receiver is longer. Then the frequency decreases when it reaches the receiver, and the perceived color shifts toward red (the opposite is true if the wave source is closer to the receiver). To give the reader an idea of the magnitude of the effect, the Doppler shift is shown to give an approximation of the frequency received by a source that is moving away as its relative velocity changes. For example, in the red spectral line of the helium--neon laser mentioned above, when the speed of the source is equal to half the speed of light, the received frequency drops from 4.74 x 10^14 Hz to 2.37 x 10^14 Hz, a value that is substantially downshifted into the infrared band.
[edit]The Doppler effect of sound waves
In our daily lives, we all have this experience: when a train with a whistle passes by a certain Doppler effect 3 observer, he will find that the pitch of the train whistle changes from high to low. Why does this happen? This is because the pitch of the whistle is determined by the frequency of the sound waves. If the frequency is high, the whistle sounds high; if it is low, it sounds low. This phenomenon is known as the Doppler effect, and it is named after its discoverer, Christian Doppler, an Austrian physicist and mathematician. He first discovered this effect in 1842. In order to understand this phenomenon, it is necessary to examine the laws that govern the propagation of sound waves emitted by a whistle when a train is approaching at a constant speed . The result is that the wavelength of the sound wave is shortened, as if the wave were compressed. As a result, the number of waves propagating in a given interval of time increases, which is why the observer perceives the sound as becoming higher in pitch; conversely, as the train moves away, the wavelength of the sound wave becomes larger, as if the wave is being stretched. As a result, the sound sounds low . Quantitative analysis yields f1=(u+v0)/(u-vs)f , where vs is the velocity of the wave source relative to the medium, v0 is the velocity of the observer relative to the medium, f represents the intrinsic frequency of the wave source, and u represents the propagation speed of the wave in a stationary medium. When the observer moves toward the wave source, v0 takes a positive sign; when the observer moves away from the wave source (i.e., along the wave source), v0 takes a negative sign. When the wave source toward the observer when the front of vs take a negative sign; before the wave source away from the observer when the movement of vs take a positive sign. From the above formula is easy to know, when the observer and the sound source close to each other, f1> f; when the observer and the sound source far away from each other. f1 [edit]Doppler effect of light waves Light with fluctuating properties also exhibits this effect, which is also known as the Doppler-Fischer effect. The Doppler effect 4 because the French physicist Fischer (1819 ~ 1896) in 1848 independently of the wavelength shift from the stars to explain, pointed out the use of this effect to measure the relative velocity of the stars. Light waves differ from sound waves in that a change in the frequency of a light wave is perceived as a change in color. If the star is moving away from us, the spectral lines of light move in the direction of red light, known as redshift; if the star is moving toward us, the spectral lines of light move in the direction of violet light, known as blueshift. The formula for calculating the Doppler effect of light (electromagnetic waves) is divided into the following three types: (1) Longitudinal Doppler effect (i.e., the velocity of the wave source and the line between the wave source and the receiver *** line): f'=f [(c+v)/(c-v)]^(1/2) where v is the relative velocity of the wave source and the receiver. When the source is close to the observer, v is positive, known as the "purple shift" or "blue shift"; otherwise v is negative, known as the "red shift". (2) Transverse Doppler effect (i.e., the velocity of the source is perpendicular to the line between the source and the receiver): f'=f (1-β^2)^(1/2) where β=v/c (3) Universal Doppler effect (the general case of the Doppler effect): f'=f [(1-β^2)^(1/2)]/(1-βcosθ) where β=v/c, and θ is the angle of the receiver's line to the source to the velocity direction. direction. The longitudinal and transverse Doppler effects are special cases when θ is taken to be 0 or π/2, respectively [edit]Application of the Doppler effect of acoustic waves The Doppler effect of acoustic waves can also be used for diagnosis of medical science, that is, the color ultrasound that we usually say. Color ultrasound is simply a high-definition black and white B ultrasound plus color Doppler, first of all, ultrasound frequency shift diagnostic method, that is, D ultrasound, this method applies the principle of the Doppler effect, when the sound source and the receiver (that is, the probe and the reflector) when there is a relative motion between the frequency of the echo has changed, this change in frequency is called the shift, D ultrasound, including pulsed Doppler, continuous Doppler, and color Doppler blood flow images. Color Doppler ultrasound generally uses autocorrelation for Doppler signal processing, and the blood flow signals obtained by autocorrelation are color-coded and then superimposed on a two-dimensional image in real time to form a color Doppler ultrasound blood flow image. Color Doppler ultrasound (i.e., color ultrasound) has the advantages of two-dimensional ultrasound structural images and provides rich information on hemodynamics, so its practical application has been widely valued and welcomed, and it is known as "non-traumatic angiography" in clinical practice. In order to examine the motion of the heart and blood vessels and to understand the speed of blood flow, ultrasound can be emitted. Since the blood in the vessels is a moving object, a Doppler effect is generated between the ultrasound source and the blood moving relative to each other. As the blood vessel moves toward the ultrasound source, the wavelength of the reflected wave is compressed, thus increasing in frequency. As the blood vessel moves away from the source, the wavelength of the reflected wave becomes longer and thus decreases in frequency per unit of time inward. The amount of increase or decrease in the frequency of the reflected wave is directly proportional to the speed of blood flow, and thus the flow rate of blood can be determined based on the frequency shift of the ultrasound wave. We know the speed of intravascular blood flow and blood flow rate, which has a certain value in the diagnosis of cardiovascular diseases, in particular, it can provide valuable diagnostic information on the oxygen supply in the circulatory process, the ability to atresia, the presence of turbulence, vascular atherosclerosis, and other valuable diagnostic information. Ultrasound Doppler method of cardiac diagnosis process is like this: ultrasound oscillator produces a high-frequency equal-amplitude ultrasound signals, excitation of the transducer probe, generating continuous ultrasound, to the human body cardiovascular organs, when the ultrasound beam encounters the movement of the organs and blood vessels, it will produce the Doppler effect, the reflected signal for the transducer to accept, according to the frequency of the reflected wave with the frequency of the difference in the transmission of the speed of the blood, according to the reflected wave with the frequency of the transmission of the speed of blood. The direction of blood flow can be determined from the difference between the frequency of the reflected wave and that of the emitted wave. In order to make the probe easy to align with the measured blood vessel, a plate-shaped double laminated probe is usually used. Traffic police to the traveling vehicle transmits the ultrasonic wave with known frequency and at the same time measures the frequency of the reflected wave, according to how much the frequency of the reflected wave changes, you can know the speed of the vehicle. A monitor equipped with a Doppler speedometer, sometimes mounted above the road, takes a picture of the license plate number of the vehicle while measuring the speed, and automatically prints the measured speed on a photograph. Addendum: The Doppler effect can also be explained by the attenuation theory of wave propagation in a medium. Wave propagation in a medium, there will be dispersion phenomenon, with increasing distance, high frequency to low frequency movement. At present, the development direction of ultrasound in the medical field is color ultrasound, let's talk about the characteristics of color ultrasound: color ultrasound is simply a high-definition black-and-white ultrasound plus color Doppler, first of all, ultrasound frequency shift diagnostic method, i.e., D ultrasound, which applies the principle of the Doppler effect, when there is a relative motion between the source of the sound and the receiving body (i.e., the probe and the reflector), the frequency of the echo is altered, and such frequency changes are referred to as frequency shift, D ultrasound includes the pulse, which is the frequency of the echo. This change in frequency is called frequency shift. D ultrasound includes pulsed Doppler, continuous Doppler, and color Doppler flow images. Color Doppler ultrasound generally uses the autocorrelation technique for Doppler signal processing, and the blood flow signals obtained by the Doppler effect of the autocorrelation technique5 are color-coded and superimposed on a 2D image in real time, i.e., a color Doppler ultrasound blood flow image is formed. As a result, color Doppler ultrasound (i.e., color ultrasound) not only has the advantages of two-dimensional ultrasound structural images, but also provides a wealth of information on hemodynamics, and its practical application has been widely valued and welcomed, and it is known as "non-traumatic angiography" in clinical practice. Its main advantages are: ① It can quickly and intuitively display the two-dimensional distribution of blood flow. ②It can show the running direction of blood flow. ③It is good for identifying arteries and veins. ④It is good for recognizing vascular and non-vascular lesions. ⑤ It facilitates the understanding of the nature of blood flow. ⑥ It can facilitate the understanding of the temporal phase and velocity of blood flow. ⑦ It can reliably detect shunts and refluxes. (8) It can quantitatively analyze the origin, width, length, and area of blood flow bundles. However, color ultrasound using the relevant technology is a pulse wave, the speed of the detector is too high, the color of the color flow color will be error, in quantitative analysis is significantly inferior to the spectral Doppler, nowadays color Doppler ultrasound instrument have the function of the spectral Doppler, that is, for the color - dual-function ultrasound. Color Doppler flow mapping (CDF), also known as color Doppler ultrasound imaging (CDI), obtains echo information from the same source as spectral Doppler, and the distribution and direction of blood flow is displayed in two dimensions, with different speeds being distinguished by different colors. Dual-energy Doppler ultrasound systems, that is, B-mode ultrasound images show the location of blood vessels. Doppler measures blood flow, and this combination of B-mode and Doppler systems allows for more precise localization of any given vessel. In the spectral Doppler display, the direction of blood flow is distinguished by the zero baseline. Above the zero baseline indicates flow toward the probe, and below the zero baseline indicates flow away from the probe. In CDI, the direction of blood flow is color-coded, with red or yellow chromatograms indicating blood flow toward the probe (hot) and blue or blue-green chromatograms indicating blood flow away from the probe (cold). 2. Vascular distribution CDI shows the blood flow in the lumen of blood vessels, thus it belongs to the flow channel type of display, and it cannot show the vessel wall and peripheral membrane. 3. Identify the types of blood vessels in cancerous nodules With CDI, the blood vessels of hepatocellular carcinoma nodules can be categorized. Distinguish them as perinodal circumflex vessels, curved vessels to the inner edge of the nodule. The inflow vessels of the nodule, the internal vessels of the nodule and the outflow vessels of the nodule. Clinical application of color ultrasound (a) vascular disease Using 10MHz high-frequency probe can be found in the blood vessels less than 1mm calcification points, for carotid artery sclerotic occlusive disease has a better diagnostic value, but also can be used to use the blood flow probe local magnification to determine the degree of luminal stenosis, embolus whether there is a possibility of dislodgement, whether it produces ulcers, to prevent cerebral embolism occurs. Color ultrasound for all kinds of arteriovenous fistula can be said to be the best diagnostic method, when the probe to the colorful mosaic of ring color spectrum can be diagnosed. For carotid artery aneurysm, abdominal main vein aneurysm, vaso-occlusive vasculitis, chronic venous disease of the lower extremities (including lower extremity varicose veins, the original occurrence of deep venous valve insufficiency of the lower extremities, lower extremity deep venous reflux disorders, thrombophlebitis and venous thrombosis) using color ultrasound of high-definition, local magnification and blood flow spectroscopic investigation can be made a more correct diagnosis. (B) abdominal organs mainly used in the liver and kidney, but for the identification of benign and malignant lesions in the abdominal cavity, gallbladder cancer and large polyps, chronic inflammation differentiation, common bile duct, hepatic artery differentiation, and other diseases have a certain auxiliary diagnostic value. For cirrhosis, color ultrasound can make a better judgment from the size of the lumen of various blood vessels in the liver, the speed and direction of internal flow and the establishment of collateral circulation. For nodular sclerosis and diffuse hepatocellular carcinoma, which are difficult to be distinguished by black and white ultrasound, high-frequency exploration and blood flow spectrum exploration can be utilized to make differential diagnosis. For the differentiation of benign and malignant space-occupying lesions in the liver, cysts and various kinds of arteriovenous tumors, it has better diagnostic value, and primary liver cancer and secondary liver cancer can be differentiated by internal blood supply. Color ultrasound used in the kidney is mainly used for renal vascular lesions, such as the aforementioned renal arteriovenous fistula, when the clinical manifestations of intervals, painless hematuria can not find the cause of the strong indications. For renal artery stenosis, one of the common causes of secondary hypertension, ultrasound can basically make a definitive diagnosis, with a diagnostic accuracy of 98.6% and a sensitivity of 100% when the blood flow velocity at the stenosis is detected to be greater than 150 cm/s. On the other hand, it is also the differential diagnosis of renal cancer, renal pelvis metastatic cancer and benign tumors. （Among the small organs, color ultrasound has obvious diagnostic accuracy compared with black and white ultrasound, mainly for thyroid, breast and eye, in a way, 10MHz probe without color flow Doppler has been much clearer than ordinary black and white ultrasound with 5MHz, and the probe is much clearer, and the diagnosis and differential diagnosis of thyroid pathology is mainly based on the blood supply of the thyroid gland, among which, the image of hyperthyroidism is the most typical, with specificity, and a "sea of fire sign" for one. "sea of fire sign". Simple goiter, on the other hand, shows no significant changes compared to normal thyroid blood flow. Subacute thyroiditis, Hashimoto's thyroiditis between the two, can be used to distinguish between the nodules and the surrounding blood flow can be well differentiated nodular goiter, thyroid adenoma and thyroid cancer, so it is recommended that the diagnosis of the thyroid gland is not too clear, the patient has a certain financial ability to do ultrasound to further clarify the diagnosis. Breast ultrasound is mainly used for breast fibroma and breast cancer differential diagnosis, while the eye mainly has better diagnostic value for vascular lesions of the eye. (D) Prostate and seminal vesicles Because rectal exploration is the best method to diagnose prostate at present, it is specially proposed here. In this method, the prostate gland is divided into the migratory zone, the central zone, the peripheral zone, and a part of the fibromuscular stroma of the prostate. The migratory zone includes both sides of the periurethral sphincter and the abdomen, which is the origin of 100% of BPH, while the migratory zone accounts for only 5% of the size of the prostate in normal subjects. The central area is around the ejaculatory ducts and the tip wall points to the seminal mound, while the peripheral area includes the posterior part of the prostate and the tips on both sides, which is the origin of 70-80% of cancers, and the apical peritoneum is bookish or even disappeared, forming an anatomically weak area, which is a common metastatic channel of cancers and a key area of biopsy of the prostate. Rectal examination has good diagnostic value for various prostate and seminal vesicle diseases, and when combined with prostate biopsy, the diagnosis is basically clear, and prostate diseases, especially prostate cancer, are on the rise in China, and the incidence of prostate cancer in Europe and the United States is even ranked behind lung cancer as the second highest incidence of cancer, and abdominal examination of the prostate is basically unable to make a diagnosis, so it is recommended that the clinic use more rectal Therefore, it is recommended to utilize rectal ultrasound to diagnose prostate diseases more often in clinical practice, so that rectal exploration can be used to diagnose prostate diseases without abdominal exploration. (E) Obstetrics and gynecology The main advantages of color ultrasound for obstetrics and gynecology lie in the identification of benign and malignant tumors and the evaluation of umbilical cord disease, fetal heart disease and placenta function, and it has a better auxiliary diagnostic value for trophoblastic diseases, and it can also make a diagnosis difficult to be made under the black-and-white ultrasound for the infertility and the pelvic varicose veins through the observation of blood flow spectrum. The use of vaginal probe has certain advantages over abdominal exploration, and its superiority is mainly reflected in ① the sensitivity of uterine arteries, ovarian blood flow, high display rate. ②Shorten the examination time and obtain accurate Doppler spectrum. ③No need to fill the bladder. ④It is not disturbed by obesity, abdominal scars, or bowel insufflation. ⑤The activity of the tip of the probe is used to search for the pelvic organ tenderness site to determine whether there are adhesions in the pelvis.