Physics > Sound Waves > Doppler Effect
Doppler Effect
Doppler effect or Doppler shift is a phenomenon that is observed whenever the source of
waves is moving with respect to an observer. For example, an ambulance crossing you
with its siren blaring is a common physical demonstration of the Doppler Effect. In this
article, let us understand the intricacies of the Doppler effect in detail.
De ning Doppler Effect
Doppler Effect In Real Life
Doppler Effect Solved Problems
Doppler Effect Uses
Doppler Effect Formula
Doppler Effect in Light
Doppler Effect FAQs
Doppler Effect Explained
Doppler effect is an important phenomenon in various scienti c disciplines, including
planetary science. The Doppler effect or the Doppler shift describes the changes in the
frequency of any sound or light wave produced by a moving source with respect to an
observer.
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Doppler effect in physics is de ned as the increase (or decrease)
in the frequency of sound, light, or other waves as the source and
observer move towards (or away from) each other.
Waves emitted by a source travelling towards an observer gets compressed. In contrast,
waves emitted by a source travelling away from an observer get stretched out. Christian
Johann Doppler rst proposed the Doppler Effect (Doppler Shift) in 1842.
Doppler Effect Examples
Let us imagine the following scenario:
Case 1: Two people A and B, are standing on the road, as shown below in the picture.
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Which person hears the sound of the revving engine with a greater magnitude?
Person A hears the sound of the revving engine with a greater magnitude than person B.
Person B, standing behind the car, receives fewer waves per second (because they’re
spread out), resulting in a low-pitched sound. But, person A who is in front of the car,
receives more of those soundwave ripples per second. As a result, the frequency of the
waves is higher, which means the sound has a higher pitch.
Case 2: Now let us consider the following situations:
Situation 1: How is the pattern of waves formed when you suddenly jump into a pond?
Situation 2: How is the pattern of waves formed when you are walking in a pond?
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The image given below highlights the difference between wave patterns in both
situations.
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The difference in the wave pattern is due to the source’s movement in the second case.
This is what the Doppler effect is. In the Doppler effect, the frequency received by the
observer is higher during the approach, identical when the relative positions are the same,
and keeps lowering on the recession of source.
In this video let’s see how relative motion between the source and the
observer changes the frequency of sound waves giving rise to Doppler
Effect.
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2,86,844
Related Reading:
Characteristics of Sound Waves
Transverse Waves And Longitudinal Waves
Doppler Effect Formula
Doppler effect is the apparent change in the frequency of waves due to the relative motion
between the source of the sound and the observer. We can deduce the apparent frequency
in the Doppler effect using the following equation:
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While there is only one Doppler effect equation, the above equation changes in different
situations depending on the velocities of the observer or the source of the sound. Let us
see below how we can use the equation of the Doppler effect in different situations.
(a) Source Moving Towards the Observer at Rest
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In this case, the observer’s velocity is zero, so vo is equal to zero. Substituting this into the
Doppler effect equation above, we get the equation of the Doppler effect when a source is
moving towards an observer at rest as:
(b) Source Moving Away from the Observer at Rest
Since the velocity of the observer is zero, we can eliminate vo from the equation. But this
time, the source moves away from the observer, so its velocity is negative to indicate the
direction. Hence, the equation now becomes as follows:
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(c) Observer Moving Towards a Stationary Source
In this case, vs will equal to zero, hence we get the following equation:
(d)Observer Moving Away from a Stationary Source
Since the observer is moving away, the velocity of the observer becomes negative. So,
instead of adding vo, we now subtract, since vo is negative.
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Doppler Effect Solved Problems
Two trains A and B are moving towards each other with a speed of 432 km/h. If the
frequency of the whistle emitted by A is 800 Hz, then what is the apparent frequency of the
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whistle heard by the passenger sitting in train B. (The velocity of sound in air is 360 m/s).
Solution:
The source and the observer are moving towards each other, hence.
v+vo
f = f0(
)
v−vs
Converting 432 km/h into m/s we get 120 m/s.
Substituting the values in the equation, we get
f = 800(
360+120
360−120
) = 1600 H z
Apparent frequency f = 1600 Hz
2. A bike rider approaching a vertical wall observes that the frequency of his bike horn
changes from 440 Hz to 480 Hz when it gets re ected from the wall. Find the speed of
the bike if the speed of sound is 330 m/s.
Solution:
Let the bike approach the wall with speed u.
Then the apparent frequency received by the wall can be calculated as
f = f0(
v
v−u
)
(1)
For the re ected wave,
f
′
= f(
v+u
v
)
(2)
Substituting (1) in (2), we get
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f
′
= f0(
v
v−u
) × (
v+u
v
)
Simplifying, we get
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f
′
= (
v+u
v−u
)f0
Substituting the values in the equation, we get
480 = (
480
440
u =
= (
4
92
v+u
v−u
v+u
v−u
)440
)
× 330 = 14.3 m/s
The speed of the bike is 14.3 m/s
Let us see more solved examples in the video given below
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Uses of Doppler Effect
Many mistake the Doppler effect to be applicable only for sound waves. It works with all
types of waves including light. Below, we have listed a few applications of the doppler
effect:
Sirens
Radar
Astronomy
Medical Imaging
Blood Flow Measurement
Satellite Communication
Vibration Measurement
Developmental Biology
Audio
Velocity Pro le Measurement
Doppler Effect Limitations
Doppler Effect is applicable only when the velocities of the source of the sound and
the observer are much less than the velocity of sound.
The motion of both source and the observer should be along the same straight line.
Doppler Effect In Light
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Doppler effect of light can be described as the apparent change in the frequency of the
light observed by the observer due to relative motion between the source of light and the
observer.
For sound waves, however, the equations for the Doppler shift differ markedly depending
on whether it is the source, the observer, or the air, which is moving. Light requires no
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medium, and the Doppler shift for light travelling in a vacuum depends only on the relative
speed of the observer and source.
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
redshift.
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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.
Frequently Asked Questions – FAQs
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What is the Doppler Effect in Physics?
Doppler Effect in physics refers to the change in wave frequency during the relative motion
between a wave source and its observer.
Who discovered the Doppler Effect?
Christian Johann Doppler, an Austrian mathematician and physicist discovered the Doppler
effect in 1842.
Can Doppler effect be observed in both longitudinal and transverse
waves?
Yes, Doppler effects can be observed for both types of waves. Doppler effect in sound
(longitudinal waves) and Doppler effect in light (transverse waves) are well-known
phenomena.
How can the Doppler Effect be applied to everyday life?
A few daily life examples of Doppler effect are: a) When you stand beside a police radar. b)
The Doppler effect is used by meteorologists to track storms. c) Doctors use the Doppler
Effect in hospitals to diagnose heart problems. d) Tra c police make use of doppler effect
a radar gun to check the speed of the oncoming vehicles.
Why is the Doppler Effect used in hospitals?
Why is the Doppler Effect used in hospitals?
Doppler effect in hospitals is used to monitor blood ow. Blood clots are also detected
with the Doppler effect.
How does the Doppler Effect prove that the universe is expanding?
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The Doppler effect is evidence that the universe is expanding. Edwin Hubble used the
Doppler effect to prove that the universe is expanding. Hubble noticed that the light from
distant galaxies was shifted toward lower frequencies to the red end of the spectrum.
When stars or galaxies are moving away from us, we see their colour as red-shifted.
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effect
Put your understanding of this concept to test by
answering a few MCQs. Click ‘Start Quiz’ to begin!
Select the correct answer and click on the “Finish” button
Check your score and answers at the end of the quiz
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