BlogNCERTImportant Topic of Physcis: Doppler Effect

Important Topic of Physcis: Doppler Effect


Assume there is a cheerful bug in the centre of a circular water puddle. The bug shakes its legs on a regular basis to create disturbances that travel through the water. If these disturbances originate at a single point, they will spread outward in all directions from that point. Because each disturbance is travelling in the same medium, they will all travel at the same speed in all directions. As shown in the diagram to the right, the pattern produced by the bug’s shaking would be a series of concentric circles. At the same frequency, these circles would reach the edges of the water puddle. An observer at point A (the puddle’s left edge) would see the disturbances strike the puddle’s edge at the same frequency as an observer at point B. (at the right edge of the puddle). In reality, the frequency with which disturbances reach the puddle’s edge is the same as the bug’s frequency of causing disturbances. If the bug causes disturbances at a rate of 2 per second, each observer will see them approaching at a rate of 2 per second.

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    Assume that our bug is moving to the right across the puddle of water, producing disturbances at the same rate of 2 per second. Because the bug is moving to the right, each successive disturbance comes from a location closer to observer B and farther away from observer A. As a result, each successive disturbance travels a shorter distance before reaching observer B and thus takes less time to reach observer B. As a result, observer B notices that the frequency of arrival of disturbances is greater than the frequency of production of disturbances. Each successive disturbance, on the other hand, must travel a greater distance before reaching observer A. As a result, observer A sees a frequency of arrival that is lower than the frequency at which the disturbances occur. The net effect of the bug’s motion (the source of waves) is that the observer facing the bug observes a frequency greater than 2 disturbances/second, while the observer facing away from the bug observes a frequency less than 2 disturbances/second. The Doppler effect is the name given to this phenomenon.

    The Doppler effect demonstrates that the universe is expanding. The Doppler effect was used by Edwin Hubble to demonstrate that the universe is expanding. Hubble noticed that the light from distant galaxies was shifting to lower frequencies near the red end of the spectrum. When stars or galaxies move away from us, their colours become red-shifted.


    The Doppler effect is detected when the source of waves travels with respect to an observer. The Doppler effect is defined as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers facing the source and an apparent downward shift in frequency for observers facing away from the source. It is important to note that the effect is not caused by a change in the frequency of the source. In the preceding example, the bug is still producing disturbances at a rate of 2 disturbances per second; it just appears to the observer who the bug is approaching that the disturbances are being produced at a rate greater than 2 disturbances/second. The effect is only visible because the distance between observer B and the bug is decreasing while it is increasing between observer A and the bug. The Doppler effect can be seen for any type of wave, such as a water wave, a sound wave, a light wave, and so on. Because of our encounters with sound waves, we are most familiar with the Doppler effect. Perhaps you recall a day when you were driving along the highway and a police car or an emergency vehicle approached you. The pitch of the siren sound (a measure of the frequency of the siren) was as high as the car approached with its siren blasting, and then suddenly dropped after the car passed by. That was the Doppler effect: an apparent frequency shift for a sound wave produced by a moving source.

    When waves propagate through a medium, such as sound waves, the velocity of the observer and the velocity of the source are relative to the medium through which the waves are transmitted. As a result, the total Doppler effect can be caused by the motion of the source, motion of the observer, or motion of the medium. Each of these effects is examined in turn. Only the relative difference in velocity between the observer and the source needs to be considered for waves that do not require a medium, such as electromagnetic waves or gravitational waves. When the relative velocity is greater than the speed of light, a more complicated relativistic Doppler effect occurs.

    Uses of Doppler Effect:

    Many people believe that the Doppler effect only applies to sound waves. It is compatible with all types of waves, including light. We’ve listed a few doppler effect applications below:

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

    Doppler Effect Limitations:

    To observe (see) Doppler’s effect, the velocity of the source, observer, and medium must all be less than the velocity of sound. If their velocity exceeds the velocity of sound, the wave velocity graph becomes distorted, and shocking waves are produced. Its example is a jet plane; when the speed of the jet plane exceeds the speed of sound, the Doppler effect is not observed.

    The Doppler Effect is only applicable when the velocities of the sound source and the observer are much lower than the velocity of sound.

    Both the source and the observer should move in the same straight line.

    Doppler Effect Formula:

    The Doppler effect is the apparent change in the frequency of waves caused by the relative motion of the sound source and the observer. Using the following equation, we can calculate the apparent frequency in the Doppler effect:

    f’=[(V± V 0)/(V±Vs)]×f

    While there is only one Doppler effect equation, the above equation changes depending on the observer or the source of the sound’s velocity. Let us now look at how we can apply the Doppler effect equation in various situations.

    (a) Moving Source Towards a Static Observer:

    Because the observer’s velocity is zero, in this case, V 0 is also zero. Substituting this into the Doppler effect equation above yields the Doppler effect equation when a source is moving towards a stationary observer as:


    (b) Source Moving Away from the Static Observer:

    We can remove V0 from the equation because the observer’s velocity is zero. However, because the source is moving away from the observer this time, its velocity is negative to indicate the direction. As a result, the equation now reads:


    (c) Moving Observer Towards a Stationary Source:

    In this case, Vs equals zero, yielding the following equation:

    f’=(V+V 0/V) ×f

    (d)An Observer Moving Away from a Fixed Source:

    Because the observer is moving away, the observer’s velocity becomes negative.

    So, rather than adding V 0 , we now subtract it because V 0 is negative.

    Also read: Why Students Fail in Exams

    Frequently Asked Questions (FAQs):

    Question: In physics, what is the Doppler Effect?

    Answer: In physics, the Doppler Effect refers to the change in wave frequency caused by the relative motion of a wave source and its observer.

    Question: Who was the first to discover the Doppler Effect?

    Answer: The Doppler effect was discovered in 1842 by Christian Johann Doppler, an Austrian mathematician, and physicist.

    Question: Is the Doppler effect visible in both longitudinal and transverse waves?

    Answer: Doppler effects can be seen in both types of waves. The Doppler effect (longitudinal waves) and the Doppler effect (transverse waves) are well-known phenomena.


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