Doppler Doppler EffectEffect

Liu Po Shan Memorial CollegeLiu Po Shan Memorial College

Chan Yick NamChan Yick Nam

Warm-up ActivityWarm-up Activity

Watch the movie . . .Watch the movie . . . What is Doppler Effect ?What is Doppler Effect ?

Summary .. 1Summary .. 1

The Doppler effect is . . .The Doppler effect is . . .

the the apparentapparent change in the frequencychange in the frequency

of a wave motion of a wave motion

when there is when there is relative motionrelative motion

between the source and the observer.between the source and the observer.

Moving SourceMoving Source

Watch the Flash Animation . . .Watch the Flash Animation . . . What is the effect when the source is What is the effect when the source is

moving ?moving ?

Moving SourceMoving Source

Moving SourceStationary Source

2 1

vS

Follow-up ActivityFollow-up Activity

Watch the movie again . . .Watch the movie again . . . Explain why the pitch of the whistle Explain why the pitch of the whistle

changes as the car passes the observer ?changes as the car passes the observer ?

Explanation to movie Explanation to movie

Stationary SourceStationary Source

Stationary Source

c = f

= c

f

Moving Source - Change in Moving Source - Change in

Moving Source

2 1

vS

1 = f

c - vS

2 = f

c + vS

Source Moving Toward ObserverSource Moving Toward Observer

1

vS

1 = f

c - vS

f1 = 1

c

O1

f1 = fc - vS

c

The wavelength becomes SMALLER

The observed frequency becomes HIGHER

Source Moving Away From ObserverSource Moving Away From Observer

2

vS

2 = f

c + vS

f2 = 2

c

O2

f2 = fc + vS

c

The wavelength becomes LARGER

The observed frequency becomes LOWER

v

Moving SourceMoving Source

Watch the JAVA animation . . .Watch the JAVA animation . . . What is the change in wavelength What is the change in wavelength

when the speed of the moving when the speed of the moving source changes ?source changes ?

Summary ... 2A Summary ... 2A

The wavelength changes when The wavelength changes when the source moves.the source moves.

The wave speed is independent The wave speed is independent of the speed of source. of the speed of source.

Apparent Frequency – Apparent Frequency – Moving SourceMoving Source

Watch the FLASH animation . . .Watch the FLASH animation . . . Notice the different frequency Notice the different frequency

observed by stationary observers.observed by stationary observers.

Change in Observed FrequencyChange in Observed Frequency

O1

= f - fc - vS

c

f1 = fc - vS

c

f = f1 - f

f = fc - vS

vS

S

If vs << v then ff c

vS

=

Summary ... 2B Summary ... 2B

The apparent frequency observed by the The apparent frequency observed by the observer changes.observer changes.

For the source moving TOWARD the For the source moving TOWARD the observer,observer,

• The wavelength reduces.The wavelength reduces.• The apparent frequency increases.The apparent frequency increases.

For the source moving AWAY the observer,For the source moving AWAY the observer,• The wavelength increases.The wavelength increases.• The apparent frequency decreases.The apparent frequency decreases.

Moving ObserverMoving Observer

Watch the Flash Animation . . .Watch the Flash Animation . . .

How does the frequency of the wave How does the frequency of the wave observed change when the observer is observed change when the observer is moving ?moving ?

Apparent Frequency Apparent Frequency – Moving Observer– Moving Observer

Compared with the stationary observer:Compared with the stationary observer:

– Observer moving Observer moving towardtoward the source the source encounter encounter more waves in each secondmore waves in each second. So the observer . So the observer observes higher frequency. Apparent observes higher frequency. Apparent frequency frequency increasesincreases..

– Similarly, observer moving Similarly, observer moving awayaway from the from the source source encounter less waves in each secondencounter less waves in each second.. Therefore the apparent frequency Therefore the apparent frequency decreasesdecreases..

Apparent Frequency (moving Observer)Apparent Frequency (moving Observer) – Relative Velocity– Relative Velocity

For moving observer, For moving observer,

the apparent frequency results from the the apparent frequency results from the change in change in relative velocityrelative velocity of the sound to of the sound to the observer.the observer.

the wavelength does not change with the the wavelength does not change with the motion of the observer.motion of the observer.

Moving ObserversMoving Observers

vo

f1 = c

c + vo

f2 = c

c - vo

O2

f

f

vo

O1

S

Stationary Source

Change in Observed FrequencyChange in Observed Frequency

= f - fc + vO

c

f1 = fc + vo

c

f = f1 - f

f = fcvo

ff c

vo

= O

Summary ... 3 Summary ... 3

The apparent change in frequency The apparent change in frequency f f is known as the is known as the Doppler ShiftDoppler Shift..

The Doppler shift The Doppler shift increases as the increases as the relative velocityrelative velocity between the source between the source and the observer and the observer increasesincreases. .

The Doppler effect The Doppler effect applies to all formsapplies to all forms of waves.of waves.

Moving Observer and Moving Observer and Moving SourceMoving Source

• Moving Source change in f

• Moving Observer change in relative velocity f

• Moving Source and Observer

change in and relative velocity f

Equations –Equations –Moving Source and ObserverMoving Source and Observer

f vc

vc ' f

s

o

f vc

vc ' f

s

o

f vc

vc ' f

s

o

f vc

vc ' f

s

o

S

vs

O

vo

S

vs

O

vo

O

vo

O

vo

S

vs

S

vs

The observed frequency of wave is increased whenthe source and observer are approaching each other and is decreased when they are receding from each other.

Mathematically,

fvc

vcf

s

o )('

Combining the EquationCombining the Equation

ONE Equation for ALL casesONE Equation for ALL cases

One Equationf

vc

vc ' f

s

o

Method to Assign Sign

Step 1: Assign the direction from source to observer be positive

Step 2: If the velocities are in the positive direction, vo and vs are set to be positive.

If the velocities are in the negative direction vo and vs are set to be negative.

Step 3: Put the values of vo and vs with sign into the above equation