Doppler Effect

Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. The phenomenon is named after an Austrian physicist named Christian Doppler, who described it in the year in 1842.


What is Doppler Effect?

Let us say that there is no medium between a source and an observer. The light source is moving away from the observer. Then the successive waves have to travel a larger distance as compared to the previous waves to reach the observer. Consequently, the time taken by the waves to reach the observer also increases. This results in the change in the frequency of the wave.

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

The Doppler effect is also observed in the sound waves. But since the light waves travel in the vacuum as well and the speed of light is very large, the classical Doppler effect cannot be applied here.

Understanding Doppler Effect In Real Life

To understand the Doppler effect let us imagine the following scene. You are standing beside a road and a police car with its siren turned on, drives by you. What do you notice about the sound? The siren’s sound isn’t so loud when it is at a distance, which then reaches a maximum when it is just beside you, diminishing again as it crosses and moves away from you.

Consider another instance. Two people A and B are standing on the road as shown below in the picture.

Doppler Effect

Which person do you think hears the sound of the revving engine with a greater magnitude? You know that its person A. But why?

This is where we discuss the Doppler Effect or Doppler Shift. To highlight this effect you should understand the difference between the two situations given below.

Situation 1: How are the pattern of waves formed when you suddenly jump into a pond?

Situation 2: How are the pattern of waves formed when you are walking in a pond?

The image given below highlights the difference of wave patterns in both the situations.

Doppler Effect

This difference is because the source of the waves in the second case moves. This is what the Doppler Effect is. It is named after the physicist Christian Doppler who proposed this in the 19th century. The Doppler effect is the change of frequency of a wave emitted as observed by an observer moving relative to the source. In this, the frequency received by the observer is higher during approach, identical when the relative positions are the same, and keeps lowering on recession of source. If both the source and observer are moving, the total Doppler Effect is calculated based on both these motions.

Let us say that light waves travel from a source to an observer. In this case, the wave travels the fixed distance across which the source and the observer are located. But there are cases when either of the two is moving, that is, the source is moving relative to the observer, or vice versa. It is in these scenarios that the Doppler effect comes into the picture.

Doppler Effect Formula

In physics, where the speed of the receiver and the source relative to the medium are lower than the velocity of waves, the relationship between emitted frequency f0 and observed frequency f is given by:

\(f = (\frac{c\pm v_{r}}{c\pm v_{s}}) f_{0}\)

where

c: velocity of waves in the medium.

vr: velocity of the source relative to the medium.

vs: velocity of the receiver relative to the medium.

The frequency decreases if either is moving away from the other.

Applications of Doppler Effect

Some doppler effect applications are provided in the points mentioned below:

  • Sirens
  • Radar
  • Astronomy
  • Medical Imaging
  • Blood Flow Measurement
  • Satellite Communication
  • Vibration Measurement
  • Developmental Biology
  • Audio
  • Velocity Profile Measurement

Doppler Effect In Light

The Doppler effect also affects the light which is emitted by other bodies in space. If the body is “red shifted” the light waves are spread apart, and it is travelling away from us while if it is “blue shifted,” its light waves are compacted and it is coming towards us. The detailed explanation of the doppler effect in light is given below.

Red Shift and Blue Shift:

  • When the light source moves away from the observer, the frequency received by the observer will be less than the frequency transmitted by the source. This causes a shift towards the red end of the visible light spectrum. Astronomers call it as the red shift.
  • When the light source moves towards the observer, the frequency received by the observer will be greater than the frequency transmitted by the source. This causes a shift towards the high-frequency end of the visible light spectrum. Astronomers call it as the blue shift.

Red Shift and Blue Shift

Doppler Effect In Sound

For sound waves which propagate in a medium, the velocity of the source and the observer are relative to the medium in which the waves are transmitted. The total Doppler effect may, therefore, result from motion of the observer, motion of the source, or motion of the medium. These effects are separately analyzed.

Doppler Effect Questions

  1. How is the Doppler effect used in everyday life?
  2. How does the Doppler effect work?
  3. What animals use the Doppler effect?
  4. Does Doppler effect apply to light?
  5. What is the Doppler effect and how does it affect how you hear a sound?
  6. How does the Doppler effect work in space?
  7. How do marine animals communicate under water?
  8. Does the Doppler effect apply to sound?
  9. How does the Doppler shift affect the color of a star?
  10. How do doctors use the Doppler effect?
  11. What are some applications of the Doppler effect?
  12. Does the Doppler effect apply to all waves?

Stay tuned with Byju’s to learn more about the doppler effect derivation, its equations and applications, doppler effect in radar and much more.


Practise This Question

The apparent frequency of the whistle of an engine changes in the ratio of 6:5 as the engine passes a stationary observer. If the velocity of sound is 330 m/s, then the velocity of the engine is