Finding Interference in the Real World (October 2018) by KEIO FUTURELEARN

I would like to propose the Doppler Effect mainly because of its so important applications in our daily life.

To understand the phenomenon, let us consider the following analogy: if we are still on the beach, we see the waves coming suppose every five seconds, then at a certain frequency; if we now enter the water and sail towards the open sea, we go to the waves, then we meet them more frequently (the frequency increases), while if we sail towards the shore, in the same direction of the waves, the frequency with which we meet decreases. Another example of the Doppler effect can be observed by listening to the difference in the sound emitted by the siren of a rescue vehicle when approaching and when departing, or in the whistle of an approaching train first and then away. The effect is all the more evident the faster the vehicle is. The effect is even more evident when the object or source that emits the sound is near an observer.

It is important to note that the frequency of the sound emitted by the source does not change in the reference system integral with the source. Here are the Doppler effect applications in various & very important Technologies:

1-Astronomy: the red shift

The Doppler effect, applied to light waves, is fundamental in radar astronomy. Interpreting it as due to an effective motion of the source, it has been used to measure the speed with which stars and galaxies are approaching or moving away from us, to discover that a seemingly single star is, in reality, a binary star with components very close to each other, and also to measure the rotation speed of stars and galaxies.

2- Radar Doppler The Doppler effect is also used in some forms of radar to measure the speed of detected objects. A radar beam is thrown against a moving object, for example a car, in the case of the radar supplied to the police forces of many countries in the world. If the object is moving away from the radar, each return wave has to travel a greater space than the

previous one to reach the object and go back, so the space between two successive waves is extended, and the frequency of the radio waves changes measurably.

Medicine Application:

The Doppler effect is also used in medicine for the detection of blood flow velocity. This principle is in fact exploited by the Eco-Doppler flowmeters (ADV, or Acoustic Doppler Velocimeter), in which a source of sound waves, generally ultrasound, is appropriately oriented.

Music:

There are musical instruments that exploit the Doppler effect to make particular onomatopoeic effects; as for example the rotating friction drum. For this type of instrument, when the small drum rotates, the listener perceives two frequency peaks modulated progressively and alternately upwards and downwards, due to the above mentioned effect, and this leads to a sound similar to the frog croaking from which the name of the toy instrument.

Satellite Communication:

A measurement of frequency against time produces a Doppler curve. As the satellite passes, the received frequency appears to fall but not in a constant manner. The rate of change starts off slow, is greatest at the time of closest approach and then tails off towards the end of the transit. This is because we are measuring the rate of change in the component of a satellite's velocity along a line joining the satellite to the receiver. The rate of change is greatest at the time of closest approach and, if measured, can be used to determine passing distance.

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