The Doppler effect shows that when the wave source moves towards the observer, the receiving frequency of the wave becomes higher, while when the wave source is far away from the observer, the receiving frequency becomes lower. When the observer moves, the same conclusion can be drawn. However, due to the lack of experimental equipment, Doppler was not experimentally verified at that time. A few years later, a team of trumpeters were invited to play on the flatbed, and then trained musicians were asked to identify the tone changes with their ears to verify the effect. Assume that the wavelength of the original wave source is λ, the wave velocity is C, and the moving speed of the observer is V:

When the observer approaches the wave source, the observed wave source frequency is (C+V)/λ; If the observer is far away from the wave source, the observed wave source frequency is (c-v)/λ.

A common example is the whistle of a train. When the train approaches the observer, its whistle 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.

If sound waves are regarded as pulses sent out at regular intervals, it is conceivable that 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.

The Doppler effect is not only applicable to sound waves, but also to all types of waves, including electromagnetic waves. Edwin Hubble, a scientist, used the Doppler effect to draw the conclusion that the universe is expanding. He found that the frequency of light emitted by celestial bodies far away from the Milky Way becomes lower, that is, it moves to the red end of the spectrum, which is called red shift. The faster the celestial bodies leave the Milky Way, the greater the redshift, indicating that these celestial bodies are far away from the Milky Way. On the other hand, if the celestial body is moving towards the Milky Way, 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 of Doppler effect 2 becomes lower, so the Doppler effect should be fully considered in mobile communication. 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.

In the case of monochrome, the color perceived by our eyes can be interpreted as the frequency of light wave vibration, or the number of times the electromagnetic field changes alternately in 1 second. In the visible region, the lower the efficiency, the more inclined to red, and the higher the frequency, the more inclined to blue-purple. For example, the bright red frequency generated by He-Ne laser is 4.74×10/4 Hz, while the purple frequency of mercury lamp is above 7×10/4 Hz. This principle also applies to sound waves: the feeling of sound level corresponds to the vibration frequency (high-frequency sound is sharp and low-frequency sound is low) at which sound exerts pressure on the eardrum.

If the wave source is fixed, the vibration of the wave received by the fixed receiver is the same as the rhythm of the wave emitted by the wave source: the transmitting frequency is equal to the receiving frequency. The situation is different if the wave source moves relative to the receiver, for example, away from each other. Compared with the receiver, the distance between the two peaks generated by the wave source is longer, so it takes longer for the two upper peaks to reach the receiver. Then when it reaches the receiver, the frequency decreases and the perceived color moves to red (the opposite is true when the wave source is close to the receiver). In order to give readers an idea of the influence of this effect, the Doppler frequency shift is displayed, and the frequency received by the distant light source when its relative speed changes is approximately given. For example, in the red spectral line of the above He-Ne laser, when the wave source speed is equivalent to half the light speed, the receiving frequency drops from 4.74×10/4 Hz to 2.37×10/4 Hz, which greatly drops to the infrared frequency band.

[Edit this paragraph] Doppler effect of sound waves

In our daily life, we all have this experience: when a train with a whistle passes by a Doppler effect 3 observer, he will find that the tone of the train whistle 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 contrary, the tone sounds low. This phenomenon is called Doppler effect, named after the discoverer Christian Andreas Doppler, an Austrian physicist and mathematician who first discovered this effect in 1842. In order to understand this phenomenon, it is necessary to investigate the propagation law of sound waves emitted by whistle when the train approaches at a uniform speed. 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 tone is higher. On the contrary, the train went far away, and the wavelength of sound waves became larger, as if the waves were stretched. Therefore, the sound sounds very low. F 1=(u+v0)/(u-vs)f is obtained by quantitative analysis, 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 propagation velocity of the wave in the static medium. When the observer moves to the wave source, v0 takes the plus sign; 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 and the sound source are far apart. f 1