The Doppler effect is one of the remarkable discoveries in the study of the properties of wave phenomena. Its universal nature determines that today, on the basis of this effect, thousands and thousands of the most diverse devices in various fields of human activity work. The phenomenon, which was later named after its discoverer, was discovered by the Austrian physicist Christian Doppler back in the mid-nineteenth century. Doppler measured the properties of the waves that came to the receiver from a moving and stationary source.
If we consider the Doppler effect in its simplest form, it should be noted that this physical phenomenon describes a change in the signal frequency with respect to the displacement of the source of a given signal from the receiver that receives it. For example, a wave that comes from a certain source, and which has some fixed frequency, will be received by the receiver at a different frequency, if during its passage the source and receiver have changed their location relative to each other, that is, have moved. In this case, the frequency indicator will increase or decrease, depending on which side the source is shifted relative to the receiver. Given the Doppler effect, it can be unequivocally stated that if the receiver moves away from the source, the value of the wave frequency decreases. If the receiver approaches the source of wave radiation, then the index of the wave frequency increases. Accordingly, from these patterns it is concluded that if the source and receiver of the wave during its passage have not changed their location, then the value of the wave frequency will remain the same.
Another caveat that characterizes the effect of Doppler is important. To some extent, this property contradicts the laws of the theory of relativity. The fact is that the value of the frequency change is determined not only by whether the receiver and the radiation source are moving or not, but also by what exactly is moving. Measurements showed that the frequency shift, determined by which particular object is moving, is the more noticeable, the smaller the difference in the velocity of the receiver and source from the velocity of the wave. In fact, in cases where there is a manifestation of the Doppler effect, no contradiction with the theory of relativity is detected, because it is not important the relative movement of the receiver and source, but the nature of the movement of the wave in the elastic medium in which it moves.
The Doppler effect exhibits such properties with respect to waves of acoustic origin and to electromagnetic waves, except that in the case of electromagnetic waves, frequency shift phenomena do not depend on whether the source or receiver moves.
How this rather abstract effect is manifested, to see, however, is quite simple. For example, the Doppler effect in acoustics can be seen, or rather, heard, at the moment when, standing in a traffic jam, you hear the signal of a siren passing by a special vehicle. Surely everyone noted the fact that if such a car is approaching - the sound of a siren sounds in one way, high, and when such a car overtakes you - the sound of a siren is lower. This just confirms the presence of a change in the frequency value of the acoustic signal.
Of great importance is the Doppler frequency plays in radar, in relation to electromagnetic waves. Based on this effect, all radar stations and other devices for detecting moving objects in the most diverse branches of human activity work.
Its properties are used in medical technology to determine blood flow; a procedure such as Doppler echocardiography is also widely known. Based on the Doppler effect, navigation support devices for submarines were built; with its use, meteorologists measure the strength of the wind and the speed of movement of cloud masses.
Even astronomy uses the Doppler effect in its measurements. So, by the magnitude of the shift of the spectra of various astronomical objects, their speed of movement in space is determined, in particular, it was on the basis of this effect that a hypothesis was put forward about the expansion of the Universe.