• 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

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

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

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

• Wavefront diagrams help visualize the Doppler effect for moving wave sources and stationary observers

• 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

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