Doppler Effect:

  • When a car is not moving and its horn sounds, the sound waves we receive are a series of evenly spaced wavefronts.
  • If a car is moving, wavefronts of the sound are no longer evenly spaced.
    • Ahead of the car wavefronts are compressed as the car is moving in the same direction as the wavefronts. This creates a shorter wavelength and a higher frequency.
    • Behind the car wavefronts are more spread out as the car is moving away from the previous wavefronts. This creates a longer wavelength and a lower frequency.