Brief history on Doppler effect, concept of Doppler, and it application to real world

1. Tuesday session # 2
Tuesday Session # 2
Doppler Effect
Tuesday Session # 2

2. Doppler effect
 What do you observed when you throw a
pebble into a body of water? (apart from the
pebble sinking)
2

3. Doppler effect
 Doppler effect explained why we perceives a
change in frequency when the wave source
approaches or retreat from us…
3

4. Doppler effect
What is doppler effect?
 Doppler effect is the change in frequency of
a wave for an observer moving relative to its
source.
 the observer observes an upward shift in
frequency when the wave source is
approaching,
 And a downward shift in frequency when the
wave source is retreating
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5. Doppler effect
 Doppler effect applies to all waves including;
 Sound waves,
 Light waves
 Water waves
5

6. Doppler effect
 Doppler effect originated in 1842 by an
Austrian physicist “Christian Doppler”.
 Christian Doppler proposed
doppler effect in his article "Über
das farbige Licht der Doppelsterne
und einiger anderer Gestirne des
Himmels”.
 Translated as “On the coloured light of the
binary stars and some other stars of the
heavens”.
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7. Doppler effect
 Doppler’s hypothesis was tested for sound
wave Buys Ballot in 1845.
 Who confirmed that the frequency of sound’s
pitch emitted was higher than frequency
when the sound source approached him,
 and lower than the emitted frequency when
the sound source receded from him.
 Hippolyte Fizeau found similar result in
electromagnectic waves (1848)
 John Scott Russell (1848)
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8. Concept of Doppler effect
 To understand the concept of doppler
effect we must first understand the
following;
 Wavelength
 Frequency
 Velocity
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9. Concept of Doppler effect
Wavelength
 The distance between two successive crest of
a wave, especially points in a sound wave or
electromagnetic wave.
𝝀 =
𝝊
𝒇
 𝜆 – wavelength
 𝜐 – speed
 𝑓 – frequency
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10. Concept of Doppler effect
Frequency
 The number of occurrence
of repeating event per unit
time.
 In terms of waves, it’s the
number of waves passing a
given point per unit time.
 Unit of frequency is hertz
𝒇 =
𝝂
𝝀
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11. Concept of Doppler effect
Velocity
 The speed of something (waves) in a given
direction.
𝐯 =
𝐝
𝐭
 v – velocity
 d - displacement
 t - time
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12. Concept of Doppler effect
Water waves
 Imagine you are swimming with your friend
Paul, and he is constantly bobbing his head
up and down creating a stream of wave
reaching you at the end of the pool.
 You observed that at every 2 second interval
3 wave reaches you.
 As such the frequency of the wave observed
is;
𝒇 =
𝟑
𝟐. 𝟎𝒔
= 𝟏. 𝟓𝐇𝐳
12

13. Concept of Doppler effect
 Now, you decide to call Paul to enlighten him
on your brilliant observation.
 Paul decide to continue his repeated bobbing
as he approaches you. As he did so you
notice something else.
 Instead of 3,you observes 4 waves reaching
you every 2 second.
 The frequency observes is thus;
𝒇 =
𝟒
𝟐. 𝟎𝒔
= 𝟐𝐇𝐳
13

14. Concept of Doppler effect
 You got so excited about your find, that you
were speechless when Paul arrives.
 So Paul decided to return to his previous
position in pool, still continuously bobbing.
 Paul lost focus of his intention and went pass
his original position.
 You detected his lost of focus and decide to
call out to him,
 But before you could you notice another
phenomena.
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15. Concept of Doppler effect
 This time, instead of 3 waves per 2 second,
you observed that only 1 wave reaches you
every 2 second.
 Which implies that the frequency of wave
approaching has decline.
𝒇 =
𝟐
𝟐. 𝟎𝒔
= 𝟏𝐇𝐳
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16. Concept of Doppler effect
Sound waves
 A siren on a police car (source) emitting a uniform
series of sound waves, moving away from the
source in all direction.
 Jason a distance away observing (observer)
 Air stationary (assumption)
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17. Concept of Doppler effect
 Lets take the wavelength to be 𝝀, the
frequency to be 𝒇 and the speed of the sound
to be 𝝂.
 𝝂 𝒐 and 𝝂 𝒔 represent the speed of the observer
and the source respectively.
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18. Concept of doppler effect
 If both the observer and the source were
stationary, he would observe a frequency
equal to that of the source.
𝝂 𝟎 = 𝟎 and 𝝂 𝒔 = 𝟎
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19. Concept of Doppler effect
 If the observer decide to walk towards the
source, the speed of the waves in relation to
the observer;
𝝂′
= 𝝂 + 𝝂 𝒐
 The wavelength 𝝀 however
remains the same.
 Therefore the frequency 𝒇′
observes by the
observer is;
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20. Concept of Doppler effect
𝒇 =
𝝂′
𝝀
𝒇 =
𝒗 + 𝒗 𝒐
𝝀
 Since 𝝀 =
𝝂
𝒇
frequency of observer can be
express as;
𝒇′
=
𝝂+𝝂 𝒐
𝝂
𝒇
 Frequency observed by the observer
increases.
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21. Concept of Doppler effect
 If the observer decided to walk away from
the source, the speed of the waves relative
to the observer is;
𝑣′
= 𝜈 − 𝜈 𝑜
𝑓′
=
𝜈 − 𝜈 𝑜
𝜈
𝑓
 The frequency heard by the observer in this
scenario is decrease.
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22. Concept of Doppler effect
 Now suppose the vehicle (source) is moving
toward to the observer who is at rest.
 Since the source is moving towards the right,
each successive wave is emitted from a
position closer to the observer than the
previous wave.
 As a result, the wave fronts heard by the
observer are closer together
than they would be if the source
were not moving.
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23. Concept of Doppler effect
 The wavelength 𝝀′
therefore measured by the
observer is shorter than the wavelength 𝝀 of
the source.
 For every consecutive wave which last for a
time interval T, the source moves a distance
𝝂 𝒔 𝑻 =
𝝂 𝒔
𝒇
.
 The wavelength is shorten by this amount
23

24. Concept of Doppler effect
 The wavelength 𝜆′
observe is thus
𝝀′
= 𝝀 − 𝜟𝝀 = 𝝀 −
𝝂 𝒔
𝒇
 Since 𝜆 =
𝜈
𝑓
, the observe frequency 𝒇′
is
𝒇′
=
𝝂
𝝀′
=
𝝂
𝝀 − (𝝂 𝒔 𝒇)
=
𝝂
𝝂 𝒇 𝝂 𝒔 𝒇
𝒇′
=
𝝂
𝝂 − 𝝂 𝒔
𝒇
 The observes frequency increases as the
sources is moving toward the observer.
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25. Concept of Doppler effect
 Conversely, if the source is moving away from
the observer, each wave is emitted from a
position farther from the observer than the
previous wave,
 So the arrival time between successive waves
is increased, reducing the frequency.
𝒇′
=
𝝂
𝝂+𝝂 𝒔
𝒇
25

26. Concept of Doppler effect
 The general Doppler effect
𝒇′
=
𝝂 + 𝝂 𝒔
𝝂 − 𝝂 𝒐
𝒇
 This equation applies to all four conditions
mention previously.
 The sign of 𝜈𝑠and 𝜈 𝑜depend on the direction of
the velocity.
 A positive value is used for motion of the
observer or the source toward the other, and a
negative value is used for motion of one away
from the other.
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27. Application of Doppler effect
 In the 1600 years or so since Doppler first
described the wave phenomenon that would
cement his place in history, several practical
applications of the Doppler effect have
emerged to serve society.
 In all of these applications, the same basic
thing is happening.
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28. Application of Doppler Effect
 One application of Doppler effect found in
nature, occurs in bats hunting for their prey.
 Bats navigates it’s flight by emitting whistles
and listening for the echoes.
 When chasing prey, its brain detects a change
in pitch between the emitted whistle, and the
echo it receives.
 This tells the bat the speed of its target, and
the bat adjusts its own speed accordingly.
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29. Application of Doppler Effect
Radar
 The Doppler effect is used in some types of
radar, to measure the speed of detected
objects.
 For example a police officer uses radar guns to
check for speeding vehicles.
 The radar gun emits waves at a particular
frequency, which when strikes the vehicles
bounce back toward the gun.
 The radar gun then measure the frequency of
the returning waves, then eventually determine
the speed.
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30. Application of Doppler effect
 Doppler radar (radar in general) is a form of
technology used not only by law-enforcement
officers, but also by meteorologists.
 Meteorologists utilize Doppler effect to
determine the direction and velocity of
raindrops, wind direction and other weather
events.
 This principle is very crucial as it allows
meteorologist to predict weather pattern
including coming storm etc.
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31. Application of Doppler effect
Medical diagnosis
 Physicians and medical technicians apply Doppler
effect to measure the rate and direction of blood
flow in a patient's body, along with ultra-sound.
 Ultrasound beam are pointed towards an artery,
and the reflected waves exhibit a shift in
frequency, because the blood cells are acting as
moving sources of sound waves.
 An echocardiogram uses sound waves
transmitted by ultrasound to produce images of
the heart (transducer transmit and receive waves,
which are reflected when they reach the edge of
two structures with different densities).
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32. Application of Doppler effect
Flow Measurement
 Instruments such as the laser Doppler velocimeter
(LDV), and Acoustic Doppler Velocimeter (ADV)
have been developed to measure velocities in a
fluid flow.
 A light beam or an ultrasonic acoustic burst is
release by the LDV and ADV respectively,
subsequently measuring the shift in wavelengths
of reflections from particles moving with the flow.
 This technique allows non-intrusive flow
measurements, at high precision and high
frequency.
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33. Application of Doppler effect
Satellite Communication
 Satellite employs Doppler effect in its tracking
technique for determining distance between
satellite and receiver at the time of closest
approach as well as the time itself.
 Approaching satellite increases the frequency
relative to the actual transmission frequency.
 As it retreats, the frequency lowered. At the
time of closest approach, the transmitted and
received frequencies are usually the same.
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34. Application of Doppler effect
 Doppler effect has found its use in several other
area which includes;
 Astronomy
 Vibration measurement
 To Sense Gesture (computer base)
 Audio
 Velocity profile measurement etc.
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35. References
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