Doppler effect and its application

Christian Doppler (1803-1853), observed that the sound received by a listener is different from the original frequency produced when there is relative motion between the source and the receiver.

What is the doppler effect?

That is whenever the listener or source moves the frequency is changed from the original. This is called the Doppler effect.

The relative motion is due to :

The listener moves towards or away from the source. The source (Train, Speaker, Bus) moves towards or away from the listener.

Both the listener and the source move towards or away from each other. The medium moves when both source and listener are at rest.

Application of Doppler Effect

  • Police use to calculate the speed of vehicles inroads.
  • To track the satellites using the frequency of radio waves emitted, as frequency decrease when it passes away from the earth.
  • Radar(Radio Detection and Ranging) uses reflected waves to calculate the distance the object passes by such as airplanes, missiles.
  • Sonar is used to calculate the distance of underwater objects, like submarines, rocks, icebergs, etc.

Doppler Effect in Sound

In Physics, the Doppler effect is defined as the increase or decrease of the frequency of the wave in relation to an observer who is moving relative to the wave source.

Examples of Doppler effect in Sound

  1. Pitch of a siren on a fire engines or ambulance. As the siren passes the observer, the pitch of siren suddenly drops in pitch. Initially the siren is coming towards the observer, then the pitch is high. After passing the observer, the siren is moving away from the observer then the is pitch is lower. This is an indication of the Doppler effect.

Doppler effect in light

The light waves leave the source and reach the observer, caused by the relative motion of the observer and wave source.

Doppler affects the lights produced by other bodies in space. When the body in space is “Blue Shifted“, its light waves are compacted and come towards the observer. If it is “redshifted” the waves of light are spread apart and travel away from the observer.

What are blueshift and redshift?

Redshift and Blueshift is the change in frequency of the light waves when the source is moving towards or away from the observer. When the object moves away from the observer, it is called redshift. Or when the object moves towards the observer, then the light from the sources is called blueshift.

How is the Doppler effect different for light?

The main difference between the doppler effect between sound and light is, in sound the velocity of the observer and source are relative to the medium. But the Doppler effect in light, where only the relative difference in velocity between the observer and source is vital.


When the police move slowly passes the observer, the observer noticed no color change. But when the police car, moves faster passes the observer, the observer notices that the light changes to blue. Moreover, when the polices car moves away from the observer, the color changes to red. This Doppler effect for light works similarly to the doppler effect for sound.

How is the Doppler effect used in astronomy?

Astronomers regularly calculate how quickly the galaxies and stars move away from us (observers) by calculating the range to which the light is “stretched” into the low frequency, red region of the spectrum.


* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
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