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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.

Physical Quantities
QuantityUnit nameUnit symbol
Frequency (\(f\))hertz\(\text{Hz}\)
Speed (\(v\))metres per second\(\text{m·s$^{-1}$}\)

Table 6.1: Units used in Doppler effect.