Chapter 6: doppler effect, summary presentation for grade 12 science

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6. Doppler effect
Physics
Grade 12

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Doppler effect with sound
The Doppler effect is the change in the observed frequency of a wave when the source or the
detector moves relative to the transmitting medium.
The following equation can be used to calculate the frequency of the wave according to the
observer or listener:
If the direction of the wave from the listener to the source is chosen as positive, the velocities
have the following signs:
f L=
(v±vL
v±vS
)f S

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Applications
The Doppler effect can be observed in all types of waves, including ultrasound, light and
radiowaves.
Sonography makes use of ultrasound and the Doppler effect to determine the direction of
blood flow.
Light is emitted by stars. Due to the Doppler effect, the frequency of this light decreases and
the stars appear redder than if they were stationary. This is called a red shift and means that
the stars are moving away from the Earth. This is one of the reasons we can conclude that
the Universe is expanding.

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