The Doppler effect was named in memory of Christian Johann, who first put forward this theory in 1842.

He believes that when the sound source moves towards the observer, the frequency of sound waves becomes higher, and when the sound source is far away from the observer, the frequency becomes lower. A common example is the train. When the train approaches the observer, its steam will be more harsh than usual. Harsh sound changes can be heard as the train passes by. The same is true: the alarm of police car, the engine sound of racing car.

Think of sound waves as pulses emitted at regular intervals. Imagine if you send out a pulse at every step, then every pulse in front of you is closer to yourself than when you are still. The sound source behind you is a step further than when it is still. In other words, your previous pulse frequency is higher than usual, and your subsequent pulse frequency is lower than usual.

Austrian physicist Doppler was born in 1803, the son of a masons in Salzburg. His parents had expected his son to follow in his father's footsteps, but he was too weak to be a stonemason since he was a child. They accepted the advice of a math professor and asked Doppler to study math at Vienna Institute of Technology. After graduation, Doppler returned to Salzburg to study philosophy, and then went to Vienna University to study advanced mathematics, astronomy and mechanics.

After graduation, Doppler stayed in Vienna University as an assistant professor for four years, worked as an accountant in a factory, then taught in a technical middle school in Prague and worked as a part-time lecturer at Prague Institute of Technology. It was not until 184 1 that he officially became a professor of mathematics at the Institute of Technology. Doppler is a rigorous teacher. He was once investigated by the school because students complained that the exam was too harsh. Heavy academic affairs and heavy pressure make Doppler's health worse and worse, but his scientific achievements make him famous all over the world. 1850 was appointed as the first dean of the School of Physics of Vienna University, but died three years later at the age of 49.

The famous Doppler effect first appeared in a paper published by 1842. 1842, Austrian physicist Doppler took his daughter for a walk along the railway and noticed that when the wave source and the observer moved relatively, the frequency of the wave received by the observer would change. He tried to use this principle to explain the color change of binary stars. Although Doppler mistook light waves for longitudinal waves, the conclusion of Doppler effect was correct. The Doppler effect has only a slight effect on the color of binary stars. At that time, no instrument could measure these changes. But since 1845, some people have experimented with sound waves. They asked some musicians to play music on the train and others to write down the pitches they heard as the train approached and left the platform. The experimental results support the existence of Doppler effect.

Generalized Doppler effect

The Doppler effect is not only applicable to sound waves, but also to all types of waves, including light waves and electromagnetic waves. Edwin Hubble, a scientist, used the Doppler effect to draw the conclusion that the universe is expanding. He found that the light frequency of the distant Milky Way is getting higher and higher, that is, moving to the red end of the spectrum. This is the red Doppler shift, or red shift. If the Milky Way moves towards him, the light will shift blue.

In mobile communication, when the mobile station moves to the base station, the frequency becomes higher, and when it is far away from the base station, the frequency becomes lower, so the "Doppler effect" in mobile communication should be fully considered. Of course, due to the limitation of our moving speed in daily life, it is impossible to bring great frequency shift, but it will undeniably affect mobile communication. In order to avoid this influence causing problems in our communication, we have to consider it in various technologies. But also increases the complexity of mobile communication.

First, the Doppler effect of sound waves.

In our daily life, we all have this experience: when a train whistling passes by the observer, he will find that the tone of the train whistling changes from high to low. Why is this happening? This is because the tone is determined by the different vibration frequencies of sound waves. If the frequency is high, the tone sounds high. On the other hand, the tone sounds very low. This phenomenon is called Doppler effect, named after the discoverer Christian Andreas Doppler (1803- 1853). Doppler is an Austrian physicist and mathematician. He first discovered this effect in 1842. In order to understand this phenomenon, it is necessary to investigate the time when the train approaches at a constant speed. Regularity of sound waves emitted by whistle propagation. As a result, the wavelength of sound waves becomes shorter, as if the waves were compressed. Therefore, the number of waves propagating in a certain time interval increases, which is also the reason why the observer feels the pitch becomes higher. On the contrary, the farther the train goes, the greater the wavelength of sound waves, as if the waves were stretched. Therefore, the sound sounds very low. The quantitative analysis shows that f 1=(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 natural frequency of the wave source, and u represents the wave in the static medium. When the observer is far away from the wave source (that is, along the wave source), v0 takes the negative sign. When the wave source moves towards the observer, vs has a negative sign. When the front wave source deviates from the observer's motion, Vs takes the plus sign. It is easy to know from the above formula that when the distance between the observer and the sound source is close, f1> f; When the observer is far away from the sound source, f1< f.

Second, the Doppler effect of light waves (including electromagnetic waves)

Fluctuating light will also have this effect, which is also called Doppler-Fizeau effect. Because the French physicist Fizeau (18 19- 1896) independently explained the wavelength deviation from the star in 1848, and pointed out the method of measuring the relative speed of the star by using this effect. The difference between light wave and sound wave is that if the star moves towards us, the spectral line of light moves towards purple light, which is called blue shift.

Widespread application of Doppler effect

A, radar velocimeter

This Doppler effect is also used by radar velocimeter to detect the speed of motor vehicles. Traffic police emit electromagnetic waves of known frequency, usually infrared rays, to moving vehicles, and measure the frequency of reflected waves at the same time, so that the speed of vehicles can be known according to the change of reflected wave frequency. A police car equipped with a Doppler velocimeter sometimes stops at the side of the road, takes a picture of the car number while measuring the speed, and automatically prints the measured speed on the photo.

Second, the application of Doppler effect in medicine

In clinic, the application of Doppler effect is also increasing. In recent years, ultrasonic pulse Doppler tester has developed rapidly. When the sound source or reflection interface moves, for example, when red blood cells flow through the great vessels of the heart, the frequency of the sound scattered from its surface changes, and the direction and speed of blood flow can be known from this frequency shift. For example, when the red blood cells face the probe, the reflected audio frequency increases according to the Doppler principle, and when the red blood cells leave the probe, the reflected audio frequency decreases.

Third, the Doppler phenomenon in cosmology research.

In the 1920s, American astronomer Slipher discovered the red shift of the spectrum for the first time while studying the spectrum emitted by the distant spiral nebula, and realized that the spiral nebula was rapidly leaving the earth. 1929, Hubble summed up the famous Hubble law according to the generalized red shift: the distance v of galaxies is proportional to the distance r from the earth, that is, V = HR, and h is Hubble constant. According to Hubble's law and the subsequent determination of redshifts of more celestial bodies, people think that the universe has been expanding for a long time and the density of matter has been decreasing. It can be inferred that the structure of the universe does not exist before a certain moment, and it can only be the product of evolution. Therefore, in 1948, G. Gamov and his colleagues put forward the big bang universe model. Since the 1960s, the Big Bang model has been widely accepted, so that astronomers call it the "standard model" of the universe.

The Doppler-Fizeau effect makes it possible to study the motion of celestial bodies at any distance from the earth only by analyzing the spectrum of the received light. 1868, the British astronomer W. Huggins measured the apparent speed of Sirius (that is, the speed at which the object left us) in this way, and got the speed value of 46 km/s.

Fourthly, Doppler effect in mobile communication.

In mobile communication, when the mobile station moves to the base station, the frequency becomes higher, and when it is far away from the base station, the frequency becomes lower, so the "Doppler effect" in mobile communication should be fully considered. Of course, due to the limitation of our moving speed in daily life, it is impossible to bring great frequency deviation. In satellite mobile communication, the frequency becomes higher when the plane moves to the satellite and lower when it is far away from the satellite. Moreover, due to the high speed of aircraft, the "Doppler effect" in satellite mobile communication should be fully considered. In order to avoid this influence causing problems in our communication, we must give various technical considerations. But also increases the complexity of mobile communication.

When you stand by and pay attention to the engine sound of a fast-moving car on the highway, you will find that the tone of the sound will become higher when it comes to you (that is, the frequency will become higher) and lower when it leaves you (that is, the frequency will become lower). This phenomenon is called Doppler effect. There is also Doppler effect in the phenomenon of light. When the light source moves towards you quickly, the frequency of light will also increase, which shows that the color of light shifts to the direction of blue light (because the frequency of blue light is high in visible light), that is, the spectrum shifts blue; When the light source leaves you quickly, the frequency of light will decrease, that is, the color of light will shift to red (because the frequency of red light in visible light is low), that is, the spectrum will shift to red.

Before further studying the Doppler effect, let's learn the basic knowledge about waves:

If we throw a small stone into the calm water, ripples will appear on the water and continue to spread forward. At this time, whenever the water surface at the wave source vibrates, a new wave train will be generated on the water surface.

Let the vibration period of the wave source be t, that is, every time the wave source vibrates t times, the distance between two adjacent wave trains on the water surface is VT, where v is the propagation speed of the wave in the water. In physics, we call the distance between the wavelengths of such adjacent wave trains, which is represented by the symbol λ. In this way, the relationship among wave wavelength, wave velocity and vibration period can be expressed as λ=VT (1).

Because it takes t for a wave source to vibrate once, the number of times the wave source vibrates per unit time is 1/t. In physics, the number of times the wave source vibrates per unit time is called the frequency of the wave, which is expressed by f, so its relationship with the period can be expressed as f= 1/T, or T= 1/f (2).

Combining (1) and (2), we can get λ=VT=V/f (3).

This formula is the basic formula for us to discuss problems related to waves. Although it is summarized from the propagation of water waves, it is applicable to all waves.

Experimental research shows that the wave propagation velocity V is a constant for a given medium. Therefore, when a wave propagates in a medium, its wavelength λ is directly proportional to its period (inversely proportional to its frequency). That is, the higher the frequency of the wave, the smaller the period and the shorter the wavelength; Conversely, the lower the frequency, the longer the period and the longer the wavelength of the wave.

For sound waves, the frequency of the sound determines the tone of the sound. That is, the higher the frequency of sound waves, the higher the tone of sound waves, the sharper, thinner and even harsher the sound. According to the above conclusions, the sound source that produces treble vibrates slowly, the vibration period is longer, and the wavelength of the corresponding sound wave is longer. For example, the wavelength of 10000Hz sound wave is1100 of the wavelength of 100Hz sound wave.

In visible light, the frequency of light waves determines the color of colored light. The frequencies from low to high correspond to red, orange, yellow, green, blue, indigo and purple. Red light has the lowest frequency and the longest wavelength. Purple light has the highest frequency but the shortest wavelength.

Let's discuss the Doppler effect of light with the above background knowledge:

Suppose a light source emits a wave train every t, that is, the period of the light source is t, as shown in the figure, when it is stationary, the time interval between two adjacent wave trains is t and the distance interval is λ=cT.

Where c stands for the speed of light.

When the light source leaves the observer at the speed v, the distance that the light source moves in the time between every two adjacent wave trains is VT, so the time required for the next peak to reach the observer increases by VT/c, so the time required for two adjacent peaks to reach the observer is:

T ' = T+VT/c & gt； T

That is, compared with the observer, the period of light wave becomes longer and the frequency becomes lower. According to the relationship between the above frequency and light color, the color of secondary light will shift to red light. In physics, this phenomenon is called redshift.

At this time, the distance between two adjacent wave trains to the observer, that is, the wavelength, becomes λ'=cT+VT.

That is, the wavelength becomes longer. The ratio of these two wavelengths is λ'/λ = t'/t =1+v/c.

That is to say, the wavelength increases by V/C. We call this relative increase red shift, which depends on the distance speed of the light source. Because in general, v

For example, the Virgo galaxy cluster is leaving our galaxy at a speed of about 1000 km/s, so the wavelength of any spectral line in its spectrum is larger than the normal value by a ratio λ'/λ =1+v/c =10000/300000 =

If the light source moves towards the observer, just change V in the above formula to-V. The difference is that there will be a blue shift of light at this time.

According to the moving speed of the light source, we can calculate the offset of light in the spectrum; On the contrary, according to the shift of light in the spectrum, we can also calculate the moving speed of light source relative to us. Knowing this, it is not difficult for us to understand the discovery process of Hubble's law.