The Doppler effect · TheLucidSTEM

Interactive Simulator · The Doppler effect

Bunch the Wavefronts Ahead

Move the source and watch the circular wavefronts crowd together in front and spread out behind. The observed frequency rises on approach and falls on recession.

Mission Predict the observed frequency for an observer the source is approaching. Streak 0Best 0

Emitted frequency fₛ 400 Hz

Source speed vₛ 30 m/s

As the source approaches an observer, the observed frequency is:

wave speed v (sound in air)340 m/s

ahead: fₒ = fₛv/(v − vₛ)–

behind: fₒ = fₛv/(v + vₛ)–

Approaching, the wavefronts bunch up so more arrive each second and the observed frequency rises; receding, they spread out and it falls.