The Doppler effect explains why the pitch of an approaching ambulance siren sounds
- A. lower than the true pitch
- B. higher than the true pitch, because the waves reach the observer more frequently
- C. unchanged
- D. louder but of the same pitch
Explanation
Motion towards the observer crowds the wavefronts together, shortening the observed wavelength and raising the frequency, and the pitch drops abruptly as the source passes and begins to recede. The speed of sound in the air is unchanged throughout; only the observed frequency shifts. The same effect on light gives the red shift used to measure the recession of galaxies.