# Doppler Effect Liu Po Shan Memorial College Chan Yick Nam.

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Doppler Effect Liu Po Shan Memorial College Chan Yick Nam

Warm-up Activity  Watch the movie...  What is Doppler Effect ?

Summary.. 1 The Doppler effect is... the apparent change in the frequency of a wave motion when there is relative motion between the source and the observer.

Moving Source  Watch the Flash Animation...  What is the effect when the source is moving ?

Moving Source Stationary Source 2 1 vSvS

Follow-up Activity  Watch the movie again...  Explain why the pitch of the whistle changes as the car passes the observer ?

Explanation to movie

Stationary Source c = f = c f

Moving Source - Change in Moving Source - Change in Moving Source 2 1 vSvS 1 = f c - v S 2 = f c + v S

Source Moving Toward Observer 1 vSvS 1 = f c - v S f 1 = 1 c O1O1 f c - v S c The wavelength becomes SMALLER The observed frequency becomes HIGHER

Source Moving Away From Observer 2 vSvS 2 = f c + v S f 2 = 2 c O2O2 f c + v S c The wavelength becomes LARGER The observed frequency becomes LOWER v

Moving Source Watch the JAVA animation... Watch the JAVA animation... What is the change in wavelength when the speed of the moving source changes ? What is the change in wavelength when the speed of the moving source changes ?

Summary... 2A The wavelength changes when the source moves. The wavelength changes when the source moves. The wave speed is independent of the speed of source. The wave speed is independent of the speed of source.

Apparent Frequency – Moving Source  Watch the FLASH animation...  Notice the different frequency observed by stationary observers.

Change in Observed Frequency O1O1 = f - f c - v S c f 1 = f c - v S c  f = f 1 - f  f = f c - v S vSvS S If v s << v then ff f c vSvS =

Summary... 2B The apparent frequency observed by the observer changes. The apparent frequency observed by the observer changes. For the source moving TOWARD the observer, For the source moving TOWARD the 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 Observer Watch the Flash Animation... How does the frequency of the wave observed change when the observer is moving ?

Apparent Frequency – Moving Observer Compared with the stationary observer: –Observer moving toward the source encounter more waves in each second. So the observer observes higher frequency. Apparent frequency increases. –Similarly, observer moving away from the source encounter less waves in each second. Therefore the apparent frequency decreases.

Apparent Frequency (moving Observer) – Relative Velocity For moving observer, the apparent frequency results from the change in relative velocity of the sound to the observer. the apparent frequency results from the change in relative velocity of the sound to the observer. the wavelength does not change with the motion of the observer.

Moving Observers vovo f 1 = c c + v o f 2 = c c - v o O2O2 f f vovo O1O1 S Stationary Source

Change in Observed Frequency = f - f c + v O c f 1 = f c + v o c  f = f 1 - f  f = f c vovo ff f c vovo = O

Summary... 3 The apparent change in frequency  f is known as the Doppler Shift. The apparent change in frequency  f is known as the Doppler Shift. The Doppler shift increases as the relative velocity between the source and the observer increases. The Doppler shift increases as the relative velocity between the source and the observer increases. The Doppler effect applies to all forms of waves. The Doppler effect applies to all forms of waves.

Moving Observer and Moving Source Moving Source  change in   f Moving Observer  change in relative velocity   f Moving Source and Observer  change in and relative velocity   f

Equations – Moving Source and Observer S vsvs O vovo S vsvs O vovo O vovo O vovo S vsvs S vsvs

The observed frequency of wave is increased when the source and observer are approaching each other and is decreased when they are receding from each other. Mathematically, Combining the Equation

ONE Equation for ALL cases One Equation 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, v o and v s are set to be positive. If the velocities are in the negative direction v o and v s are set to be negative. Step 3: Put the values of v o and v s with sign into the above equation

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