Doppler Effect Study Guide


The Doppler effect, also known as Doppler shift, is a phenomenon that occurs when the source of waves moves in relation to an observer. A common practical demonstration of the Doppler Effect is an ambulance crossing you with its siren blaring!



When a moving source emits light waves, the Doppler effect results in a red or blue shift in frequency. The behavior of sound waves and other types of waves is similar (even if they are not identical).

The relative velocity u of the source and observer for motion towards each other determines the Doppler shift for light.




1. Sirens:

A passing emergency vehicle’s siren will begin higher than its stationary pitch, slide down as it passes, and then continue lower as it recedes from the observer.

  1. Apart from these, Radar, Medical imaging and blood flow measurement, Flow measurement, Velocity profile measurement, Satellite communication, Audio, Vibration measurement, Developmental biology, and so on all utilize Doppler’s effect.


  1. The velocity of the source of sound must be less than that of the velocity of sound, i.e., Vs
  2. The velocity of the observer must be less than the velocity of sound, i.e., Vo
  3. If the velocity of sound of the source is greater than that of the velocity of sound, then due to shock waves, the wavefront gets distorted. Consequently, the change in frequency will not be observed by the observer.


  • A small speed gun can accurately assess your vehicle’s speed and is based on the Doppler Effect!
  • A speed gun is a device that police officers use to keep an eye on fast automobiles.
  • It consists mostly of a radio transmitter and receiver.
  • After bouncing off the target item, a speed gun broadcasts a radio wave signal and gets the same signal back.
  • The frequency of the return waves is higher than the frequency of the sent waves while the object is approaching the radar, and it is lower when the item is moving away.
  • The radar speed gun can calculate the speed of the object from which the waves have been bounced back based on this difference.

speed gunSource 


  • Doppler Effect is the phenomenon of motion-related frequency change.
  • You might have heard the changing pitch of the siren of an ambulance that passes with its siren blaring: When an ambulance approaches you, the siren’s pitch sounds higher than when it moves away from you. This change is a common Doppler principle.
  • The Doppler effect is applicable for both light and sound.


Q. Who discovered the Doppler effect?

Christian Andreas Doppler is best known for his groundbreaking idea of the Doppler effect.

Q. How can the Doppler effect be applied to everyday life?

The Doppler effect has a variety of practical applications. Meteorologists, for example, utilize the Doppler effect to track storms in addition to police radar. The Doppler effect was even employed by doctors to diagnose cardiac issues.

Q. How does the Doppler effect prove that the universe is expanding?

This is due to the fact that when the object moves away, the waves spread out wider. As the waves get farther apart, the wavelength lengthens, pushing the spectrum toward the red end of the spectrum. The Doppler effect explains the redshift, which indicates that the cosmos is expanding. 

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  1. Applications and Limitations of Doppler Effect: Accessed 14th April 2022.
  2. Doppler Effect: Accessed 14th April 2022.
  3. Doppler Effect: Accessed 14th April 2022.
  4. Doppler Effect Examples: Accessed 14th April 2022.

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