The Doppler effect states that a wave receives a higher frequency as the source moves toward the observer and a lower frequency as 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 that 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 is far away from the source, the frequency of the source is (c-v)/λ.
A commonly used example is the sound of a train whistle, which is more piercing than the usual sound when the train is close to the observer. When a train approaches the observer, the sound of the whistle is harsher than normal. 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 every time you take a step, you emit a pulse, so that 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 blueshifted.
In mobile communication, when the mobile station moves toward 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 communications.
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 a 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 height of a sound corresponds to the frequency of the vibration of the pressure exerted by the sound on the tympanic membrane of the ear (high-frequency sounds are shrill, 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, for example 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 source of the wave towards the observer when the front of vs to take a negative sign; before the source of the wave away from the observer when the movement of vs to take a positive sign. From the above formula is easy to know, when the observer and the source of sound close to each other, f1> f; when the observer and the source of sound away from each other. f1 Let the source of sound S, the observer L, respectively, with a speed of Vs, Vl in the static medium along the same straight line with the same direction of motion, the source of sound waves issued by the propagation speed of sound waves in the medium for the V, and Vs is less than the V, Vl is less than the V. When the source is not moving, the source of the sound found that the frequency is f The frequency of the sound wave received by the observer is: f'=(V-Vl)V/[(V-Vs)X]=(V-Vl)f/(V-Vs) So it is obtained that (1) when the observer and the source of the sound wave do not move, Vs=0, Vl=0, from the above formula f'=f (2) when the observer does not move, and the source of the sound wave is close to the observer, the observer receives the frequency f'=f (2) when the observer does not move, and the source of sound is close to the observer, the observer receives the frequency f'=f. (2) When the observer does not move and the sound source approaches the observer, the observer receives a frequency of F=Vf/(V-Vs) Obviously the frequency at this time is greater than the original frequency All the manifestations of the Doppler effect can be obtained from the above equation. [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 moves towards 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 wave) is divided into the following three types: (1) Longitudinal Doppler effect (i.e., the speed 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 speed 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 (general case of Doppler effect): f'=f [(1-β^2)^(1/2)]/(1-βcosθ) where β=v/c, and θ is the angle from the line between the receiver and the wave source to the direction of velocity. 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 sound waves The Doppler effect of sound waves can also be used for medical diagnosis, which is what we usually call color ultrasound. 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. The color Doppler ultrasound (i.e., color ultrasound) has the advantages of two-dimensional ultrasound structural images, but also provides a wealth of information on hemodynamics, the practical application of which has been widely valued and welcomed, and is known as "non-traumatic angiography" in clinical practice. In order to check the motion of the heart and blood vessels, and to understand the speed of blood flow, it can be realized by launching ultrasound. 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, which has a certain value for the diagnosis of cardiovascular diseases, especially for the circulatory process of oxygen supply, atresia ability, the presence of turbulence, vascular atherosclerosis, etc. can provide valuable diagnostic information. Ultrasound Doppler method of diagnosis of the heart process is this: ultrasound oscillator produces a high-frequency equal-amplitude ultrasound signals to stimulate the transmitter transducer probe, generating continuous ultrasound, to the human cardiovascular organs, when the ultrasound beam encounters the movement of the organs and blood vessels, it will produce a Doppler effect, the reflection of the signal for the transducer to accept, according to the difference in frequency between the reflected wave and the transmitter to find out the blood flow velocity, based on the reflected wave and the frequency difference between the transmitter and the transmitter. The speed of blood flow can be determined from the difference between the frequency of the reflected wave and the emitted wave, and the direction of blood flow can be determined from whether the reflected wave is increasing or decreasing in frequency. In order to make the probe easy to align with the measured blood vessels, a plate-shaped double laminated probe is usually used. Traffic police to the vehicle in motion to launch a known frequency of ultrasound at the same time to measure the frequency of the reflected wave, according to how much the frequency of the reflected wave changes will be able to 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. Supplement: The Doppler effect can also be explained by the theory of attenuation of waves traveling in a medium. Waves traveling in a medium have a dispersion phenomenon, where high frequencies move to lower frequencies as the distance increases. 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., probe and the reflector), the echo of the frequency changes. When there is relative motion between the source and the receiver (i.e., the probe and the reflector), the frequency of the echo changes, and this change in frequency is called frequency shift. Color Doppler ultrasound generally uses autocorrelation for Doppler signal processing, and the blood flow signals obtained by the Doppler effect of autocorrelation5 are color-coded and superimposed on a two-dimensional image in real time, forming a color Doppler ultrasound blood flow image. 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. But the color ultrasound using the relevant technology is pulse wave, the speed of the detector is too high, the color of the color flow color will be error, in the 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 shown in two dimensions, with different velocities 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. 1. Direction of blood flow In a 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 flow is color-coded, with a red or yellow color spectrum indicating flow toward the probe (hot) and a blue or blue-green color spectrum indicating flow away from the probe (cold). 2. Vascular distribution CDI shows blood flow within the lumen of a vessel and is therefore a flow channel type of display, which does not show the vessel wall or periphery. 3. Identify the types of blood vessels in cancerous nodules The blood vessels of hepatocellular carcinoma nodules can be categorized with CDI. Distinguish them as perinodal winding vessels, curved vessels to the inner edge of the nodule. The inflow blood vessels of the nodule, the internal blood vessels of the nodule and the outflow blood vessels of the nodule. The clinical application of color ultrasound (a) vascular disease The use of 10MHz high-frequency probe can be found in the blood vessels less than 1mm calcification points, for the carotid artery atherosclerotic occlusive disease has a better diagnostic value, but also the use of blood flow to detect the local amplification to determine the degree of luminal stenosis, embolus whether there is a possibility of dislodgement, whether to produce an ulcer, to prevent cerebral embolism. Color ultrasound for all kinds of arteriovenous fistula can 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 diseases of the lower extremities (including varicose veins of the lower extremities, primary deep vein valve insufficiency of the lower extremities, lower extremity deep vein reflux obstruction, thrombophlebitis and venous thrombosis), the use of 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 kidneys, but for the abdominal cavity benign and malignant lesions identification, gallbladder cancer and large polyps, chronic inflammation of the difference between the common bile duct, the distinction between the hepatic artery, 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 the internal flow rate 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 probing and blood flow spectrum probing can be utilized to make differential diagnosis. For the differentiation of benign and malignant intrahepatic space-occupying lesions, cysts and all kinds of arteriovenous tumors, it has better diagnostic value, and primary and secondary liver cancer can be differentiated from the internal blood supply. The use of color ultrasound in the kidneys is mainly used for renal vascular lesions, such as renal arteriovenous fistulae, when the clinical manifestations of intermittent, 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. (C) Small organs Among small organs, color ultrasound has obvious diagnostic accuracy compared with black and white ultrasound, mainly for thyroid, breast and eye, in a way, the 10MHz probe does not hit the color flow Doppler has been compared with the ordinary black and white ultrasound 5MHz, the probe is much clearer, and diagnosis and differential diagnosis of the thyroid lesions are mainly based on the internal blood supply of the thyroid gland, among which the image of hyperthyroidism is the most typical, with specificity, and the image of hyperthyroidism is the most typical, with specificity, and the image of hyperthyroidism is the most typical, with specificity. The image of hyperthyroidism is the most typical and specific, which is a "sea of fire sign". Simple goiter has no significant change compared to normal thyroid blood flow. Subacute thyroiditis, Hashimoto's thyroiditis between the two, can be distinguished by this, and through 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 affordability can be done ultrasound to further clarify the diagnosis. Breast ultrasound is mainly used for differential diagnosis of breast fibroma and breast cancer, while the eye mainly has a better diagnostic value for vascular lesions of the eye. (D) prostate and seminal vesicles Because rectal exploration is currently the best method of diagnosis of the prostate, it is specifically proposed here. This method of exploration of the prostate gland is divided into the migratory zone, the central zone, the peripheral zone, and another part of the prostate fibromuscular stromal zone. 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 without abdominal exploration. (E) Obstetrics and gynecology The main advantage of color ultrasound for obstetrics and gynecology is the identification of benign and malignant tumors, and umbilical cord disease, fetal heart disease and placental function assessment, for trophoblastic diseases have a better auxiliary diagnostic value, infertility, pelvic varicose veins through the observation of the blood flow spectroscopy, but also to make a diagnosis difficult to diagnose under black and white ultrasound. The use of vaginal probe has certain advantages over abdominal exploration, and its superiority is mainly reflected in ① the uterine artery, ovarian blood sensitivity, 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 pelvic organ tenderness sites to determine whether there are adhesions in the pelvis.