IB DP Physics: HL复习笔记9.5.1 The Doppler Effect

The Doppler Effect

  • When a source of sound, such as the whistle of a train or the siren of an ambulance, moves away from an observer:
    • It appears to decrease in frequency, i.e. it sounds lower in pitch
    • Although, the source of the sound remains at a constant frequency
  • This frequency change due to the relative motion between a source of sound or light and an observer is known as the Doppler effect (or Doppler shift)
  • When the observer and the source of sound (e.g. ambulance siren) are both stationary:
    • The waves appear to remain at the same frequency for both the observer and the source

7.1.3.1-Doppler-shift-diagram-1

Stationary source and observer

  • When the source starts to move towards the observer, the wavelength of the waves is shortened
    • The sound, therefore, appears at a higher frequency to the observer

7.1.3.1-Doppler-shift-diagram-2

Moving source and stationary observer

  • Notice how the waves are closer together between the source and the observer compared to point P and the source
  • This also works if the source is moving away from the observer
    • If the observer was at point P instead, they would hear the sound at a lower frequency due to the wavelength of the waves broadening
  • The frequency is increased when the source is moving towards the observer
  • The frequency is decreased when the source is moving away from the observer

Worked Example

A cyclist rides a bike ringing their bell past a stationary observer. Which of the following accurately describes the doppler shift caused by the sound of the bell?
WE-Doppler-shift-for-sound-question-image

ANSWER:    D

    • If the cyclist is riding past the observer, the wavelength of sound waves are going to become longer
      • This rules out options A and C
    • A longer wavelength means a lower frequency (from the wave equation)
    • Lower frequency creates a lower sound pitch
      • Therefore, the answer is row D

Representing The Doppler Effect

  • Wavefront diagrams help visualize the Doppler effect for moving wave sources and stationary observers
doppler-effect
Wavefronts are even in a stationary object but are squashed in the direction of the moving wave source

  • A moving object will cause the wavelength, λ, (and frequency) of the waves to change:
    • The wavelength of the waves in front of the source decreases (λ – Δλ) and the frequency increases
    • The wavelength behind the source increases (λ + Δλ) and the frequency decreases
    • This effect is known as the Doppler effect or Doppler shift
  • Note: Δλ means 'change in wavelength'
  • The Doppler shift is observed by all waves including sound and light
  • When the source starts to move towards the observer, the wavelength of the waves is shortened
    • For sound waves, sound, therefore, appears at a higher frequency to the observer
    • For light waves, the light shifts towards blue due to its higher frequency

8.4.1-Light-Doppler-Shift

Representing red-shifted and blue-shifted light

  • When the source starts to move away from the observer, the wavelength of the wave broadens
    • For sound waves, sound therefore appears at a lower frequency to the observer
    • For light waves, the light shifts towards red due to its lower frequency
  • When the source starts to move towards the observer, the wavelength of the wave shortens
    • For sound waves, sound therefore appears at a higher frequency to the observer
    • For light waves, the light shifts towards blue due to its higher frequency
  • This is because red light has a longer wavelength than blue light

 

 

 

 

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