6.4 Chapter summary

6.4 Chapter summary (ESCMV)

Presentation: 27T2

• The Doppler effect is a change in observed frequency due to the relative motion of a source and an observer.

• The following equation can be used to calculate the frequency of the wave according to the observer or listener:

  \[{f}_{L}=\left(\frac{v+{v}_{L}}{v+{v}_{S}}\right){f}_{S}\]

• If the direction of the wave from the listener to the source is chosen as positive, the velocities have the following signs:

  Source moves towards listener	\({v}_{S}\): negative
  Source moves away from listener	\({v}_{S}\): positive
  Listener moves towards source	\({v}_{L}\): positive
  Listener moves away from source	\({v}_{L}\): negative

• 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.