en:lesson10

# Doppler effect

## Theory

Doppler effect - a change in the frequency of the signal, the observed movement of the signal source relative to the receiver. The effect is named after the Austrian physicist Christian Doppler.

The Doppler effect is easily perceived in practice when a car with the siren turned on passes by the observer. Suppose a siren gives out a certain tone, and it does not change. Due to the fact that the car does not move relative to the observer. If the frequency of sound waves increases, and the observer hears a higher tone than actually because of the siren. At the moment when the car will pass by the observer. When the car goes further and is already approaching.

##### Mathematical description of the phenomenon

If the wave source moves relative to the medium, then the distance between the wave crests (wavelength λ) depends on the speed and direction of movement. If the source moves towards the receiver, that is, it catches up with the wave emitted by it, then the wavelength decreases, if it moves away, the wavelength increases:

where ω0 is the angular frequency with which the source emits waves, c is the speed of wave propagation in the medium, v is the velocity of the wave source relative to the medium (positive if the source approaches the receiver and negative if it moves away).

Frequency recorded by a fixed receiver

Similarly, if the receiver moves towards the waves, it registers their crests more often and vice versa. For a fixed source and a moving receiver

where u is the receiver velocity relative to the medium (positive if it moves towards the source).

Substituting the frequency ω from formula (1) instead of ω0 in formula (2), we obtain the formula for the general case:

## Observation of the Doppler effect

We will observe the Doppler effect using the example of receiving a telegraph code from satellites. Get ready for work and configure the necessary software as described in Lesson 09.

Wait for a “good” satellite flight at least 30 ° above the horizon.

Output the sound to the headphones, and hear a change in the tone of the signal from a moving satellite.

Turn on all the items in the Zoom FFT section.

The Auto Spectrum graph clearly shows how the frequency of a passing satellite changes. 