The Doppler effect shows that when the wave source moves towards the observer, the frequency of the wave becomes higher, while when the wave source is far away from the observer, the frequency becomes lower. When the observer moves, the same conclusion can be drawn. 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:
The observed frequency of the wave source is (v+c)/λ when the observer is close to the wave source, and (v-c)/λ when the observer is far away from the wave source.
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 light waves and electromagnetic waves. The scientist Hubble Edwin Hubble used the Doppler effect to conclude 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 farther away the celestial bodies are, 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 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 let readers know the influence of this effect, Figure 4 shows the Doppler shift, which roughly gives the frequency received by the distant light source when the relative speed changes. Taking the red spectral line of the He-Ne laser as an example, when the wave source speed is half the speed of light (see the dotted line in the figure), the receiving frequency drops from 4.74×10/4 Hz to 4.74×10/4 Hz, which greatly drops to the infrared frequency band.
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 whistle changes from high to high.
Lower it. 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.
Just high; On the contrary, the tone sounds low. This phenomenon is called Doppler effect, which is based on the discoverer Christian Andreas Doppler.
Doppler, 1803- 1853), an Austrian physicist and mathematician, first discovered this effect in 1842.
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, just like
The wave is compressed. Therefore, the number of waves propagating in a certain time interval increases, which is why the observer feels the pitch becomes higher. On the contrary,
As the train goes away, the wavelength of sound waves becomes larger, as if the waves are stretched. Therefore, the sound sounds very low. 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 is the natural frequency of the wave source, and u is the wave.
Propagation velocity in 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 is negative.
No, when the wave source moves to the observer, vs is preceded by 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 observer interacts with the sound source,
When approaching, f1> f; When the observer and the sound source are far apart. f 1