How does doppler effect occur

The Doppler effect, or Doppler shift, occurs when the movement of an observer relative to a source or vice versa causes a change in wavelength or frequency. Discovered by Austrian physicist Christian Doppler in , this phenomenon is experienced in many different ways, such as when an ambulance passes you by and you hear an audible change in pitch.

The original version of this post was written by Alexandra Foley and published on July 15, It has since been revised with additional details, animations, and an updated version of the featured model. One of the most common ways we experience the Doppler effect in action is the change in pitch caused by either a moving sound source around a stationary observer or a moving observer around a stationary sound source. When the sound source is stationary, the sound that we hear is at the same pitch as the sound emitted from the sound source.

Sound waves propagating from a stationary sound source in a uniform flow this corresponds to the source moving at constant speed. When the sound source moves, the sound we perceive changes. Going back to the ambulance example, when an ambulance drives past us, the siren sounds different than it would if we were standing right next to it. The moving ambulance has a different pitch as it approaches, when it is closest to us, and as it passes us and drives away.

As the ambulance moves toward us, each successive sound wave is emitted from a closer position than that of the previous wave. Because of this change in position, each sound wave takes less time to reach us than the one before. The distance between wave crests the wavelength is thereby reduced, meaning that the perceived frequency of the wave increases and the sound is perceived to be of a higher pitch.

Conversely, as a sound source moves away, waves are emitted from a source that is farther and farther away. Since each disturbance is traveling in the same medium, they would all travel in every direction at the same speed.

The pattern produced by the bug's shaking would be a series of concentric circles as shown in the diagram at the right. These circles would reach the edges of the water puddle at the same frequency. An observer at point A the left edge of the puddle would observe the disturbances to strike the puddle's edge at the same frequency that would be observed by an observer at point B at the right edge of the puddle. In fact, the frequency at which disturbances reach the edge of the puddle would be the same as the frequency at which the bug produces the disturbances.

If the bug produces disturbances at a frequency of 2 per second, then each observer would observe them approaching at a frequency of 2 per second. Now suppose that our bug is moving to the right across the puddle of water and producing disturbances at the same frequency of 2 disturbances per second. Since the bug is moving towards the right, each consecutive disturbance originates from a position that is closer to observer B and farther from observer A. As the ball approaches you, you observe a higher pitch than when the ball is at rest.

And when the ball is thrown away from you, you observe a lower pitch than when the ball is at rest. The Doppler effect is observed because the distance between the source of sound and the observer is changing.

If the source and the observer are approaching, then the distance is decreasing and if the source and the observer are receding, then the distance is increasing. The source of sound always emits the same frequency. Therefore, for the same period of time, the same number of waves must fit between the source and the observer. For these reasons, if the source is moving towards the observer, the observer perceives sound waves reaching him or her at a more frequent rate high pitch. And if the source is moving away from the observer, the observer perceives sound waves reaching him or her at a less frequent rate low pitch.

It is important to note that the effect does not result because of an actual change in the frequency of the source. The source puts out the same frequency; the observer only perceives a different frequency because of the relative motion between them. The Doppler effect is a shift in the apparent or observed frequency and not a shift in the actual frequency at which the source vibrates. The Doppler effect is observed whenever the speed of the source is moving slower than the speed of the waves.

But if the source actually moves at the same speed as or faster than the wave itself can move, a different phenomenon is observed. If a moving source of sound moves at the same speed as sound, then the source will always be at the leading edge of the waves that it produces. The diagram at the right depicts snapshots in time of a variety of wavefronts produced by an aircraft that is moving at the same speed as sound.

A star travelling towards us will appear blue-shifted higher frequency. This phenomenon was what first led Christian Doppler to document his eponymous effect, and ultimately allowed Edwin Hubble in to propose that the universe was expanding when he observed that all galaxies appeared to be red-shifted i.

The Doppler effect has many other interesting applications beyond sound effects and astronomy. A Doppler radar uses reflected microwaves to determine the speed of distant moving objects. It does this by sending out waves with a particular frequency, and then analysing the reflected wave for frequency changes. It is applied in weather observation to characterise cloud movement and weather patterns, and has other applications in aviation and radiology.

Medical imaging also makes use of the Doppler effect to monitor blood flow through vessels in the body. Doppler ultrasound uses high frequency sound waves and lets us measure the speed and direction of blood flow to provide information on blood clots, blocked arteries and cardiac function in adults and developing fetuses. Our understanding of the Doppler effect has allowed us to learn more about the universe we are part of, measure the world around us and look inside our own bodies.

Future development of this knowledge — including how to reverse the Doppler effect — could lead to technology once only read about in science-fiction novels, such as invisibility cloaks. See more Explainer articles on The Conversation.