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# What is the Doppler Effect?-Definition, Conditions, And Applications

August 22, 2022 written by Adeel Abbas

The apparent change in the frequency of a wave due to relative motion between source and observer is called the Doppler effect.

## What is the Doppler effect?

This effect was observed by Johann Doppler while he was observing the frequency of light emitted from distant stars. When an observer is standing on a platform, the pitch of the whistle of an approaching train is heard to be higher.

But when the same train moves away, the pitch of the whistle becomes lower. The change in the frequency due to Doppler Effect can be calculated easily if the relative motion between the source and the observer is along a straight line joining them.

## Observer Moves towards Source at Rest:

Suppose V is the velocity of the sound in the medium and the source emits a sound of frequency f and wavelength λ. If both the source and observer are at rest, waves received by the observer in one second are

V=f/λ

Now if observer A moves towards the source with a velocity Uo. The relative velocity of the waves and the observer is increased to (V+Uo). Then the number of waves received per second or changed frequency is given.

fa=f(V+Uo/V)

fa>f so the pitch of the sound is increased.

## When Observer Moves away from the source at Rest:

Let observer B move away from the source at rest with velocity Uo. Then the relative velocity of the waves and observer is (V-Uo). The observer receives waves at a reduced rate. Then the number of waves received per second or changed frequency Fb is given by

fb=f(V-Uo/V)

fb<f so the pitch of the sound is decreased.

## Source Moves towards the Observer at Rest:

If the source is moving towards the observer with speed Uo. Then in one second, the waves are compressed by an amount known as the Doppler shift represented by 𐤃λ.

The compression of waves is due to the fact that the same numbers of waves are contained in a shorter space depending upon the velocity of the source. The wavelength for observer C is

λc=V-Us/f

fc=(V/V-Us)f

fc>f so the pitch of the sound is increased.

## Source Moves away the Observer at Rest:

When the source moves away from the observer at rest with velocity Uo. Then for the observer D, there will be an increase in the wavelength given by

λD=V+Us/f

fD=(V/V+Us)f

fD<f so the pitch of the sound is decreased.

## Applications of Doppler Effect:

There is many applications doppler effect.

• Doppler Effect is also applicable to electromagnetic waves.
• Doppler Effect is used in the working of the Radar system.
• Radar is an acronym. It is derived from radio detection and ranging
• If an airplane approaches the radar, then the wavelength of radio waves received back is shorter and if the airplane moves away then the wavelength is longer than the wavelength of radio waves emitted by radar.
• Similarly, the speed of satellites moving around the earth can also be determined by using the result of Doppler’s effect.

### SONAR

Sonar stands for sound navigation and ranging. The general name of sonic or ultrasonic underwater echo-ranging and echo-sounding system. Sonar is a technique used for detecting the presence of objects underwater by the acoustical (sound) echo.

This depends upon the relative speed between detector and target. Its military applications are the detection and location of submarines, control of antisubmarine weapons, mine hunting, and depth measurement of the sea.

### Speed of stars:

Astronomers use the Doppler Effect to measure the speeds of stars and galaxies. This is done by comparing the line spectrum of light from the star with the light from a laboratory source. Stars moving towards the earth show a blue shift. This is because the wavelength of light emitted by the star is shorter than if the star had been at rest

So spectrum is shifted towards a shorter wavelength towards the blue end of the spectrum. If the stars move away from the earth there is a red The emitted waves have a longer wavelength shift. So spectrum is shifted towards a longer wavelength, ie, towards the red end.

By measuring the relative shift of the spectrum of stars and galaxies, their speeds of the spectrum can be estimated

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