The Doppler Effect Lecture Notes

Definition

• Doppler Effect:
  • Apparent change in the frequency of a wave caused by relative motion between the source of the wave and the observer.
  • Explained visually with objects emitting waves (e.g., fire truck sound, star light, duck ripples).

Explanation

• Wave Behavior:
  • Moving object causes wavefronts to bunch up in the direction of movement and spread out behind it.
• Observer Perception:
  • Approaching object: Waves are at a higher frequency (bunched up).
  • Receding object: Waves are at a lower frequency (spread out).

Applications

Sound Waves

• Example: Car horn:
  • As car approaches, sound waves bunch up, heard at a higher pitch.
  • As car moves away, sound waves spread out, heard at a lower pitch.
• Frequency and Pitch:
  • Frequency of sound waves corresponds to pitch.

Light Waves

• Example: Observing stars:
  • Star moving towards observer: Light waves bunch up, observed at a higher frequency.
  • Star moving away: Light waves spread out, observed at a lower frequency.
• Frequency and Color:
  • Higher frequency: Bluer light.
  • Lower frequency: Redder light.
• Redshift:
  • Change in color due to motion of stars.
  • Used by astronomers to determine star movement relative to Earth.

Important Implications

• Expanding Universe:
  • Most celestial objects are moving away from us, supporting the Big Bang theory.
  • Potential future isolation of Earth as universe expands.

Conclusion

• The Doppler effect is a crucial concept in understanding wave behavior and motion.
• It has significant implications in both sound perception and astronomical observations.

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