# Doppler effect

Doppler effect . It owes its name to Christian Doppler , who originated the idea in 1842 . It is the increase or decrease in the frequency of a sound wave when the source that produces it and the person that picks it up move away from each other or approach each other.

## Summary

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• 1 Explanation of the phenomenon
• 1 Typical example
• 1.1 Everyday examples
• 2 Confirmation of theory
• 1 Application in practice
• 3 Source

## Explanation of the phenomenon

The doppler effect was discovered in 1842 by Christian Johan Doppler is an effect of wave physics that occurs when a moving source emits waves.

He thought that sound waves could get closer to each other, if the sound source moved in the direction of the receiver, just as the waves would move away, if the sound source moved away from the receiver.

### Typical example

When a train is in motion, its whistle blows. As it passes, a change in hue can be heard on the whistle. This is also the case with sirens in police cars and race cars. Think of sound waves as pulses that are emitted at regular intervals.

Imagine that every time you walk, you emit a pulse. Each pulsation in front of you represents one more step that brings you closer, whereas, if you were standing without moving, each pulsation behind you would represent a step that moves you away. In other words, the pulse rate in front of you is higher than normal, and the pulse rate behind you is less than normal.

#### Everyday examples

An observer located in front of the source will observe how the frequency of the waves is greater than that actually emitted, while an observer behind the source will observe a greater distance between the wave fronts and therefore a lower frequency. Ever when listening to the sound of a vehicle siren; it is remembered how the sound changes as the mobile approaches, and especially the change in the tone, at the moment that has just passed. Those who travel in the car do not observe this change.

## Theory confirmation

In June 1845 a meteorologist from Utrech, Christoph HDBallot , confirmed the Dopplert principle during the train journey from Utrecht to Amsterdam , using a locomotive to make his observations. He placed a group of musicians on a railway and instructed them to play the same musical note while another group of musicians, at the train station, recorded the musical note they heard as the train approached and moved away from them successively. A cumbersome but brilliant idea. It is curious that the classic example to illustrate the Doppler effect is the whistle of a moving locomotive approaching and then moving away from a stationary observer; He did not take this fact into account in his experiment.

Then the French physicist Armand Hippolyte L. Fizeau in the year 1848 , determined that the celestial bodies that approach the Earth are seen in blue and those that move away are seen in red . This, in general terms, means that light waves, when they approach the observer, go towards the ultraviolet end of the spectrum and when they move away, they approach the infrared end of the spectrum, that is, that their waves, like that the voiced ones get louder when they get closer and lower when they get farther apart.

### Application in practice

His original ideas were the germ of others that would have to be developed later. Thus, its principle was used in medicine in the 20th century . He did this hand in hand with another principle of acoustics, ultrasound.

Ultrasound examination is well known today. Its use during pregnancy is practically universal since it is innocuous and very reliable.

The Doppler technique is based on a device that emits ultrasound (sounds whose frequency is greater than 20,000 Hz, that is, they are above the limit of human hearing). When an object is placed in front of the apparatus, the ultrasounds collide with the structure under study and return to the apparatus, where a computer interprets them and transforms each ultrasound into a luminous point. This operation repeated millions of times, gives rise to millions of luminous points that, together, form an image. Ultrasound advances, therefore, according to the principles of mechanical waves, that is, they undergo phenomena of attenuation, dispersion and reflection (“rebound”) depending on the physical properties of the structures they encounter.

One of the first publications on the use of ultrasound in medicine is considered to have been made by KT Dussite in 1942 . In 1955, Scottish physician Ian Donald , a doctor who during the Second World War worked for the Royal Air Force (RAF) on radar and sonar matters , associated with a technician named Tom Brown of the Kelvin & Hughes Company for Scientific Instruments, began to work on the development of ultrasound. Three years later he demonstrated the utility of the new technique in identifying an ovarian mass in a patient misdiagnosed with inoperable cancer.

In 1964 , Callagan and his collaborators applied the Doppler principle to research on fetal blood flow, which allowed its study in detail.

Pourcelot from France in the 1960s also contributed his work on blood flow, leading to the development of the first Doppler equipment for monitoring the cardiovascular system of astronauts in space.

In that period, the American pediatrician and physiologist Robert F. Rushmer , investigated instruments that allowed him to evaluate cardiovascular functions in animals without the need to operate. He was interested in determining cardiovascular dimensions, intravascular pressures, and blood flow using bloodless techniques. Three members of their team of technicians, Dean Franklin , Dick Ellis and Donald Baker managed to develop a multichannel transit-time “ Flowmeter ” that allowed detecting the flow in a blood vessel by means of the Doppler by making a sound wave impact on the red blood cells. moving inside the vessel and then pick up the echo signal returned by those same cells.

In 1965 , the first commercial application of Doppler technology received the name of Doptone, a device that allowed the auscultation of the fetal heartbeat.